libbpf: Fix spelling mistake "libaries" -> "libraries"

[platform/kernel/linux-starfive.git] / tools / lib / bpf / usdt.c

// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <libelf.h>
#include <gelf.h>
#include <unistd.h>
#include <linux/ptrace.h>
#include <linux/kernel.h>

#include "bpf.h"
#include "libbpf.h"
#include "libbpf_common.h"
#include "libbpf_internal.h"
#include "hashmap.h"

/* libbpf's USDT support consists of BPF-side state/code and user-space
 * state/code working together in concert. BPF-side parts are defined in
 * usdt.bpf.h header library. User-space state is encapsulated by struct
 * usdt_manager and all the supporting code centered around usdt_manager.
 *
 * usdt.bpf.h defines two BPF maps that usdt_manager expects: USDT spec map
 * and IP-to-spec-ID map, which is auxiliary map necessary for kernels that
 * don't support BPF cookie (see below). These two maps are implicitly
 * embedded into user's end BPF object file when user's code included
 * usdt.bpf.h. This means that libbpf doesn't do anything special to create
 * these USDT support maps. They are created by normal libbpf logic of
 * instantiating BPF maps when opening and loading BPF object.
 *
 * As such, libbpf is basically unaware of the need to do anything
 * USDT-related until the very first call to bpf_program__attach_usdt(), which
 * can be called by user explicitly or happen automatically during skeleton
 * attach (or, equivalently, through generic bpf_program__attach() call). At
 * this point, libbpf will instantiate and initialize struct usdt_manager and
 * store it in bpf_object. USDT manager is per-BPF object construct, as each
 * independent BPF object might or might not have USDT programs, and thus all
 * the expected USDT-related state. There is no coordination between two
 * bpf_object in parts of USDT attachment, they are oblivious of each other's
 * existence and libbpf is just oblivious, dealing with bpf_object-specific
 * USDT state.
 *
 * Quick crash course on USDTs.
 *
 * From user-space application's point of view, USDT is essentially just
 * a slightly special function call that normally has zero overhead, unless it
 * is being traced by some external entity (e.g, BPF-based tool). Here's how
 * a typical application can trigger USDT probe:
 *
 * #include <sys/sdt.h>  // provided by systemtap-sdt-devel package
 * // folly also provide similar functionality in folly/tracing/StaticTracepoint.h
 *
 * STAP_PROBE3(my_usdt_provider, my_usdt_probe_name, 123, x, &y);
 *
 * USDT is identified by it's <provider-name>:<probe-name> pair of names. Each
 * individual USDT has a fixed number of arguments (3 in the above example)
 * and specifies values of each argument as if it was a function call.
 *
 * USDT call is actually not a function call, but is instead replaced by
 * a single NOP instruction (thus zero overhead, effectively). But in addition
 * to that, those USDT macros generate special SHT_NOTE ELF records in
 * .note.stapsdt ELF section. Here's an example USDT definition as emitted by
 * `readelf -n <binary>`:
 *
 *   stapsdt              0x00000089       NT_STAPSDT (SystemTap probe descriptors)
 *   Provider: test
 *   Name: usdt12
 *   Location: 0x0000000000549df3, Base: 0x00000000008effa4, Semaphore: 0x0000000000a4606e
 *   Arguments: -4@-1204(%rbp) -4@%edi -8@-1216(%rbp) -8@%r8 -4@$5 -8@%r9 8@%rdx 8@%r10 -4@$-9 -2@%cx -2@%ax -1@%sil
 *
 * In this case we have USDT test:usdt12 with 12 arguments.
 *
 * Location and base are offsets used to calculate absolute IP address of that
 * NOP instruction that kernel can replace with an interrupt instruction to
 * trigger instrumentation code (BPF program for all that we care about).
 *
 * Semaphore above is and optional feature. It records an address of a 2-byte
 * refcount variable (normally in '.probes' ELF section) used for signaling if
 * there is anything that is attached to USDT. This is useful for user
 * applications if, for example, they need to prepare some arguments that are
 * passed only to USDTs and preparation is expensive. By checking if USDT is
 * "activated", an application can avoid paying those costs unnecessarily.
 * Recent enough kernel has built-in support for automatically managing this
 * refcount, which libbpf expects and relies on. If USDT is defined without
 * associated semaphore, this value will be zero. See selftests for semaphore
 * examples.
 *
 * Arguments is the most interesting part. This USDT specification string is
 * providing information about all the USDT arguments and their locations. The
 * part before @ sign defined byte size of the argument (1, 2, 4, or 8) and
 * whether the argument is signed or unsigned (negative size means signed).
 * The part after @ sign is assembly-like definition of argument location
 * (see [0] for more details). Technically, assembler can provide some pretty
 * advanced definitions, but libbpf is currently supporting three most common
 * cases:
 *   1) immediate constant, see 5th and 9th args above (-4@$5 and -4@-9);
 *   2) register value, e.g., 8@%rdx, which means "unsigned 8-byte integer
 *      whose value is in register %rdx";
 *   3) memory dereference addressed by register, e.g., -4@-1204(%rbp), which
 *      specifies signed 32-bit integer stored at offset -1204 bytes from
 *      memory address stored in %rbp.
 *
 *   [0] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
 *
 * During attachment, libbpf parses all the relevant USDT specifications and
 * prepares `struct usdt_spec` (USDT spec), which is then provided to BPF-side
 * code through spec map. This allows BPF applications to quickly fetch the
 * actual value at runtime using a simple BPF-side code.
 *
 * With basics out of the way, let's go over less immeditately obvious aspects
 * of supporting USDTs.
 *
 * First, there is no special USDT BPF program type. It is actually just
 * a uprobe BPF program (which for kernel, at least currently, is just a kprobe
 * program, so BPF_PROG_TYPE_KPROBE program type). With the only difference
 * that uprobe is usually attached at the function entry, while USDT will
 * normally will be somewhere inside the function. But it should always be
 * pointing to NOP instruction, which makes such uprobes the fastest uprobe
 * kind.
 *
 * Second, it's important to realize that such STAP_PROBEn(provider, name, ...)
 * macro invocations can end up being inlined many-many times, depending on
 * specifics of each individual user application. So single conceptual USDT
 * (identified by provider:name pair of identifiers) is, generally speaking,
 * multiple uprobe locations (USDT call sites) in different places in user
 * application. Further, again due to inlining, each USDT call site might end
 * up having the same argument #N be located in a different place. In one call
 * site it could be a constant, in another will end up in a register, and in
 * yet another could be some other register or even somewhere on the stack.
 *
 * As such, "attaching to USDT" means (in general case) attaching the same
 * uprobe BPF program to multiple target locations in user application, each
 * potentially having a completely different USDT spec associated with it.
 * To wire all this up together libbpf allocates a unique integer spec ID for
 * each unique USDT spec. Spec IDs are allocated as sequential small integers
 * so that they can be used as keys in array BPF map (for performance reasons).
 * Spec ID allocation and accounting is big part of what usdt_manager is
 * about. This state has to be maintained per-BPF object and coordinate
 * between different USDT attachments within the same BPF object.
 *
 * Spec ID is the key in spec BPF map, value is the actual USDT spec layed out
 * as struct usdt_spec. Each invocation of BPF program at runtime needs to
 * know its associated spec ID. It gets it either through BPF cookie, which
 * libbpf sets to spec ID during attach time, or, if kernel is too old to
 * support BPF cookie, through IP-to-spec-ID map that libbpf maintains in such
 * case. The latter means that some modes of operation can't be supported
 * without BPF cookie. Such mode is attaching to shared library "generically",
 * without specifying target process. In such case, it's impossible to
 * calculate absolute IP addresses for IP-to-spec-ID map, and thus such mode
 * is not supported without BPF cookie support.
 *
 * Note that libbpf is using BPF cookie functionality for its own internal
 * needs, so user itself can't rely on BPF cookie feature. To that end, libbpf
 * provides conceptually equivalent USDT cookie support. It's still u64
 * user-provided value that can be associated with USDT attachment. Note that
 * this will be the same value for all USDT call sites within the same single
 * *logical* USDT attachment. This makes sense because to user attaching to
 * USDT is a single BPF program triggered for singular USDT probe. The fact
 * that this is done at multiple actual locations is a mostly hidden
 * implementation details. This USDT cookie value can be fetched with
 * bpf_usdt_cookie(ctx) API provided by usdt.bpf.h
 *
 * Lastly, while single USDT can have tons of USDT call sites, it doesn't
 * necessarily have that many different USDT specs. It very well might be
 * that 1000 USDT call sites only need 5 different USDT specs, because all the
 * arguments are typically contained in a small set of registers or stack
 * locations. As such, it's wasteful to allocate as many USDT spec IDs as
 * there are USDT call sites. So libbpf tries to be frugal and performs
 * on-the-fly deduplication during a single USDT attachment to only allocate
 * the minimal required amount of unique USDT specs (and thus spec IDs). This
 * is trivially achieved by using USDT spec string (Arguments string from USDT
 * note) as a lookup key in a hashmap. USDT spec string uniquely defines
 * everything about how to fetch USDT arguments, so two USDT call sites
 * sharing USDT spec string can safely share the same USDT spec and spec ID.
 * Note, this spec string deduplication is happening only during the same USDT
 * attachment, so each USDT spec shares the same USDT cookie value. This is
 * not generally true for other USDT attachments within the same BPF object,
 * as even if USDT spec string is the same, USDT cookie value can be
 * different. It was deemed excessive to try to deduplicate across independent
 * USDT attachments by taking into account USDT spec string *and* USDT cookie
 * value, which would complicated spec ID accounting significantly for little
 * gain.
 */

#define USDT_BASE_SEC ".stapsdt.base"
#define USDT_SEMA_SEC ".probes"
#define USDT_NOTE_SEC  ".note.stapsdt"
#define USDT_NOTE_TYPE 3
#define USDT_NOTE_NAME "stapsdt"

/* should match exactly enum __bpf_usdt_arg_type from bpf_usdt.bpf.h */
enum usdt_arg_type {
        USDT_ARG_CONST,
        USDT_ARG_REG,
        USDT_ARG_REG_DEREF,
};

/* should match exactly struct __bpf_usdt_arg_spec from bpf_usdt.bpf.h */
struct usdt_arg_spec {
        __u64 val_off;
        enum usdt_arg_type arg_type;
        short reg_off;
        bool arg_signed;
        char arg_bitshift;
};

/* should match BPF_USDT_MAX_ARG_CNT in usdt.bpf.h */
#define USDT_MAX_ARG_CNT 12

/* should match struct __bpf_usdt_spec from usdt.bpf.h */
struct usdt_spec {
        struct usdt_arg_spec args[USDT_MAX_ARG_CNT];
        __u64 usdt_cookie;
        short arg_cnt;
};

struct usdt_note {
        const char *provider;
        const char *name;
        /* USDT args specification string, e.g.:
         * "-4@%esi -4@-24(%rbp) -4@%ecx 2@%ax 8@%rdx"
         */
        const char *args;
        long loc_addr;
        long base_addr;
        long sema_addr;
};

struct usdt_target {
        long abs_ip;
        long rel_ip;
        long sema_off;
        struct usdt_spec spec;
        const char *spec_str;
};

struct usdt_manager {
        struct bpf_map *specs_map;
        struct bpf_map *ip_to_spec_id_map;

        int *free_spec_ids;
        size_t free_spec_cnt;
        size_t next_free_spec_id;

        bool has_bpf_cookie;
        bool has_sema_refcnt;
};

struct usdt_manager *usdt_manager_new(struct bpf_object *obj)
{
        static const char *ref_ctr_sysfs_path = "/sys/bus/event_source/devices/uprobe/format/ref_ctr_offset";
        struct usdt_manager *man;
        struct bpf_map *specs_map, *ip_to_spec_id_map;

        specs_map = bpf_object__find_map_by_name(obj, "__bpf_usdt_specs");
        ip_to_spec_id_map = bpf_object__find_map_by_name(obj, "__bpf_usdt_ip_to_spec_id");
        if (!specs_map || !ip_to_spec_id_map) {
                pr_warn("usdt: failed to find USDT support BPF maps, did you forget to include bpf/usdt.bpf.h?\n");
                return ERR_PTR(-ESRCH);
        }

        man = calloc(1, sizeof(*man));
        if (!man)
                return ERR_PTR(-ENOMEM);

        man->specs_map = specs_map;
        man->ip_to_spec_id_map = ip_to_spec_id_map;

        /* Detect if BPF cookie is supported for kprobes.
         * We don't need IP-to-ID mapping if we can use BPF cookies.
         * Added in: 7adfc6c9b315 ("bpf: Add bpf_get_attach_cookie() BPF helper to access bpf_cookie value")
         */
        man->has_bpf_cookie = kernel_supports(obj, FEAT_BPF_COOKIE);

        /* Detect kernel support for automatic refcounting of USDT semaphore.
         * If this is not supported, USDTs with semaphores will not be supported.
         * Added in: a6ca88b241d5 ("trace_uprobe: support reference counter in fd-based uprobe")
         */
        man->has_sema_refcnt = access(ref_ctr_sysfs_path, F_OK) == 0;

        return man;
}

void usdt_manager_free(struct usdt_manager *man)
{
        if (IS_ERR_OR_NULL(man))
                return;

        free(man->free_spec_ids);
        free(man);
}

static int sanity_check_usdt_elf(Elf *elf, const char *path)
{
        GElf_Ehdr ehdr;
        int endianness;

        if (elf_kind(elf) != ELF_K_ELF) {
                pr_warn("usdt: unrecognized ELF kind %d for '%s'\n", elf_kind(elf), path);
                return -EBADF;
        }

        switch (gelf_getclass(elf)) {
        case ELFCLASS64:
                if (sizeof(void *) != 8) {
                        pr_warn("usdt: attaching to 64-bit ELF binary '%s' is not supported\n", path);
                        return -EBADF;
                }
                break;
        case ELFCLASS32:
                if (sizeof(void *) != 4) {
                        pr_warn("usdt: attaching to 32-bit ELF binary '%s' is not supported\n", path);
                        return -EBADF;
                }
                break;
        default:
                pr_warn("usdt: unsupported ELF class for '%s'\n", path);
                return -EBADF;
        }

        if (!gelf_getehdr(elf, &ehdr))
                return -EINVAL;

        if (ehdr.e_type != ET_EXEC && ehdr.e_type != ET_DYN) {
                pr_warn("usdt: unsupported type of ELF binary '%s' (%d), only ET_EXEC and ET_DYN are supported\n",
                        path, ehdr.e_type);
                return -EBADF;
        }

#if __BYTE_ORDER == __LITTLE_ENDIAN
        endianness = ELFDATA2LSB;
#elif __BYTE_ORDER == __BIG_ENDIAN
        endianness = ELFDATA2MSB;
#else
# error "Unrecognized __BYTE_ORDER__"
#endif
        if (endianness != ehdr.e_ident[EI_DATA]) {
                pr_warn("usdt: ELF endianness mismatch for '%s'\n", path);
                return -EBADF;
        }

        return 0;
}

static int find_elf_sec_by_name(Elf *elf, const char *sec_name, GElf_Shdr *shdr, Elf_Scn **scn)
{
        Elf_Scn *sec = NULL;
        size_t shstrndx;

        if (elf_getshdrstrndx(elf, &shstrndx))
                return -EINVAL;

        /* check if ELF is corrupted and avoid calling elf_strptr if yes */
        if (!elf_rawdata(elf_getscn(elf, shstrndx), NULL))
                return -EINVAL;

        while ((sec = elf_nextscn(elf, sec)) != NULL) {
                char *name;

                if (!gelf_getshdr(sec, shdr))
                        return -EINVAL;

                name = elf_strptr(elf, shstrndx, shdr->sh_name);
                if (name && strcmp(sec_name, name) == 0) {
                        *scn = sec;
                        return 0;
                }
        }

        return -ENOENT;
}

struct elf_seg {
        long start;
        long end;
        long offset;
        bool is_exec;
};

static int cmp_elf_segs(const void *_a, const void *_b)
{
        const struct elf_seg *a = _a;
        const struct elf_seg *b = _b;

        return a->start < b->start ? -1 : 1;
}

static int parse_elf_segs(Elf *elf, const char *path, struct elf_seg **segs, size_t *seg_cnt)
{
        GElf_Phdr phdr;
        size_t n;
        int i, err;
        struct elf_seg *seg;
        void *tmp;

        *seg_cnt = 0;

        if (elf_getphdrnum(elf, &n)) {
                err = -errno;
                return err;
        }

        for (i = 0; i < n; i++) {
                if (!gelf_getphdr(elf, i, &phdr)) {
                        err = -errno;
                        return err;
                }

                pr_debug("usdt: discovered PHDR #%d in '%s': vaddr 0x%lx memsz 0x%lx offset 0x%lx type 0x%lx flags 0x%lx\n",
                         i, path, (long)phdr.p_vaddr, (long)phdr.p_memsz, (long)phdr.p_offset,
                         (long)phdr.p_type, (long)phdr.p_flags);
                if (phdr.p_type != PT_LOAD)
                        continue;

                tmp = libbpf_reallocarray(*segs, *seg_cnt + 1, sizeof(**segs));
                if (!tmp)
                        return -ENOMEM;

                *segs = tmp;
                seg = *segs + *seg_cnt;
                (*seg_cnt)++;

                seg->start = phdr.p_vaddr;
                seg->end = phdr.p_vaddr + phdr.p_memsz;
                seg->offset = phdr.p_offset;
                seg->is_exec = phdr.p_flags & PF_X;
        }

        if (*seg_cnt == 0) {
                pr_warn("usdt: failed to find PT_LOAD program headers in '%s'\n", path);
                return -ESRCH;
        }

        qsort(*segs, *seg_cnt, sizeof(**segs), cmp_elf_segs);
        return 0;
}

static int parse_lib_segs(int pid, const char *lib_path, struct elf_seg **segs, size_t *seg_cnt)
{
        char path[PATH_MAX], line[PATH_MAX], mode[16];
        size_t seg_start, seg_end, seg_off;
        struct elf_seg *seg;
        int tmp_pid, i, err;
        FILE *f;

        *seg_cnt = 0;

        /* Handle containerized binaries only accessible from
         * /proc/<pid>/root/<path>. They will be reported as just /<path> in
         * /proc/<pid>/maps.
         */
        if (sscanf(lib_path, "/proc/%d/root%s", &tmp_pid, path) == 2 && pid == tmp_pid)
                goto proceed;

        if (!realpath(lib_path, path)) {
                pr_warn("usdt: failed to get absolute path of '%s' (err %d), using path as is...\n",
                        lib_path, -errno);
                strcpy(path, lib_path);
        }

proceed:
        sprintf(line, "/proc/%d/maps", pid);
        f = fopen(line, "r");
        if (!f) {
                err = -errno;
                pr_warn("usdt: failed to open '%s' to get base addr of '%s': %d\n",
                        line, lib_path, err);
                return err;
        }

        /* We need to handle lines with no path at the end:
         *
         * 7f5c6f5d1000-7f5c6f5d3000 rw-p 001c7000 08:04 21238613      /usr/lib64/libc-2.17.so
         * 7f5c6f5d3000-7f5c6f5d8000 rw-p 00000000 00:00 0
         * 7f5c6f5d8000-7f5c6f5d9000 r-xp 00000000 103:01 362990598    /data/users/andriin/linux/tools/bpf/usdt/libhello_usdt.so
         */
        while (fscanf(f, "%zx-%zx %s %zx %*s %*d%[^\n]\n",
                      &seg_start, &seg_end, mode, &seg_off, line) == 5) {
                void *tmp;

                /* to handle no path case (see above) we need to capture line
                 * without skipping any whitespaces. So we need to strip
                 * leading whitespaces manually here
                 */
                i = 0;
                while (isblank(line[i]))
                        i++;
                if (strcmp(line + i, path) != 0)
                        continue;

                pr_debug("usdt: discovered segment for lib '%s': addrs %zx-%zx mode %s offset %zx\n",
                         path, seg_start, seg_end, mode, seg_off);

                /* ignore non-executable sections for shared libs */
                if (mode[2] != 'x')
                        continue;

                tmp = libbpf_reallocarray(*segs, *seg_cnt + 1, sizeof(**segs));
                if (!tmp) {
                        err = -ENOMEM;
                        goto err_out;
                }

                *segs = tmp;
                seg = *segs + *seg_cnt;
                *seg_cnt += 1;

                seg->start = seg_start;
                seg->end = seg_end;
                seg->offset = seg_off;
                seg->is_exec = true;
        }

        if (*seg_cnt == 0) {
                pr_warn("usdt: failed to find '%s' (resolved to '%s') within PID %d memory mappings\n",
                        lib_path, path, pid);
                err = -ESRCH;
                goto err_out;
        }

        qsort(*segs, *seg_cnt, sizeof(**segs), cmp_elf_segs);
        err = 0;
err_out:
        fclose(f);
        return err;
}

static struct elf_seg *find_elf_seg(struct elf_seg *segs, size_t seg_cnt, long addr, bool relative)
{
        struct elf_seg *seg;
        int i;

        if (relative) {
                /* for shared libraries, address is relative offset and thus
                 * should be fall within logical offset-based range of
                 * [offset_start, offset_end)
                 */
                for (i = 0, seg = segs; i < seg_cnt; i++, seg++) {
                        if (seg->offset <= addr && addr < seg->offset + (seg->end - seg->start))
                                return seg;
                }
        } else {
                /* for binaries, address is absolute and thus should be within
                 * absolute address range of [seg_start, seg_end)
                 */
                for (i = 0, seg = segs; i < seg_cnt; i++, seg++) {
                        if (seg->start <= addr && addr < seg->end)
                                return seg;
                }
        }

        return NULL;
}

static int parse_usdt_note(Elf *elf, const char *path, long base_addr,
                           GElf_Nhdr *nhdr, const char *data, size_t name_off, size_t desc_off,
                           struct usdt_note *usdt_note);

static int parse_usdt_spec(struct usdt_spec *spec, const struct usdt_note *note, long usdt_cookie);

static int collect_usdt_targets(struct usdt_manager *man, Elf *elf, const char *path, pid_t pid,
                                const char *usdt_provider, const char *usdt_name, long usdt_cookie,
                                struct usdt_target **out_targets, size_t *out_target_cnt)
{
        size_t off, name_off, desc_off, seg_cnt = 0, lib_seg_cnt = 0, target_cnt = 0;
        struct elf_seg *segs = NULL, *lib_segs = NULL;
        struct usdt_target *targets = NULL, *target;
        long base_addr = 0;
        Elf_Scn *notes_scn, *base_scn;
        GElf_Shdr base_shdr, notes_shdr;
        GElf_Ehdr ehdr;
        GElf_Nhdr nhdr;
        Elf_Data *data;
        int err;

        *out_targets = NULL;
        *out_target_cnt = 0;

        err = find_elf_sec_by_name(elf, USDT_NOTE_SEC, &notes_shdr, &notes_scn);
        if (err) {
                pr_warn("usdt: no USDT notes section (%s) found in '%s'\n", USDT_NOTE_SEC, path);
                return err;
        }

        if (notes_shdr.sh_type != SHT_NOTE || !gelf_getehdr(elf, &ehdr)) {
                pr_warn("usdt: invalid USDT notes section (%s) in '%s'\n", USDT_NOTE_SEC, path);
                return -EINVAL;
        }

        err = parse_elf_segs(elf, path, &segs, &seg_cnt);
        if (err) {
                pr_warn("usdt: failed to process ELF program segments for '%s': %d\n", path, err);
                goto err_out;
        }

        /* .stapsdt.base ELF section is optional, but is used for prelink
         * offset compensation (see a big comment further below)
         */
        if (find_elf_sec_by_name(elf, USDT_BASE_SEC, &base_shdr, &base_scn) == 0)
                base_addr = base_shdr.sh_addr;

        data = elf_getdata(notes_scn, 0);
        off = 0;
        while ((off = gelf_getnote(data, off, &nhdr, &name_off, &desc_off)) > 0) {
                long usdt_abs_ip, usdt_rel_ip, usdt_sema_off = 0;
                struct usdt_note note;
                struct elf_seg *seg = NULL;
                void *tmp;

                err = parse_usdt_note(elf, path, base_addr, &nhdr,
                                      data->d_buf, name_off, desc_off, &note);
                if (err)
                        goto err_out;

                if (strcmp(note.provider, usdt_provider) != 0 || strcmp(note.name, usdt_name) != 0)
                        continue;

                /* We need to compensate "prelink effect". See [0] for details,
                 * relevant parts quoted here:
                 *
                 * Each SDT probe also expands into a non-allocated ELF note. You can
                 * find this by looking at SHT_NOTE sections and decoding the format;
                 * see below for details. Because the note is non-allocated, it means
                 * there is no runtime cost, and also preserved in both stripped files
                 * and .debug files.
                 *
                 * However, this means that prelink won't adjust the note's contents
                 * for address offsets. Instead, this is done via the .stapsdt.base
                 * section. This is a special section that is added to the text. We
                 * will only ever have one of these sections in a final link and it
                 * will only ever be one byte long. Nothing about this section itself
                 * matters, we just use it as a marker to detect prelink address
                 * adjustments.
                 *
                 * Each probe note records the link-time address of the .stapsdt.base
                 * section alongside the probe PC address. The decoder compares the
                 * base address stored in the note with the .stapsdt.base section's
                 * sh_addr. Initially these are the same, but the section header will
                 * be adjusted by prelink. So the decoder applies the difference to
                 * the probe PC address to get the correct prelinked PC address; the
                 * same adjustment is applied to the semaphore address, if any.
                 *
                 *   [0] https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
                 */
                usdt_rel_ip = usdt_abs_ip = note.loc_addr;
                if (base_addr) {
                        usdt_abs_ip += base_addr - note.base_addr;
                        usdt_rel_ip += base_addr - note.base_addr;
                }

                if (ehdr.e_type == ET_EXEC) {
                        /* When attaching uprobes (which what USDTs basically
                         * are) kernel expects a relative IP to be specified,
                         * so if we are attaching to an executable ELF binary
                         * (i.e., not a shared library), we need to calculate
                         * proper relative IP based on ELF's load address
                         */
                        seg = find_elf_seg(segs, seg_cnt, usdt_abs_ip, false /* relative */);
                        if (!seg) {
                                err = -ESRCH;
                                pr_warn("usdt: failed to find ELF program segment for '%s:%s' in '%s' at IP 0x%lx\n",
                                        usdt_provider, usdt_name, path, usdt_abs_ip);
                                goto err_out;
                        }
                        if (!seg->is_exec) {
                                err = -ESRCH;
                                pr_warn("usdt: matched ELF binary '%s' segment [0x%lx, 0x%lx) for '%s:%s' at IP 0x%lx is not executable\n",
                                        path, seg->start, seg->end, usdt_provider, usdt_name,
                                        usdt_abs_ip);
                                goto err_out;
                        }

                        usdt_rel_ip = usdt_abs_ip - (seg->start - seg->offset);
                } else if (!man->has_bpf_cookie) { /* ehdr.e_type == ET_DYN */
                        /* If we don't have BPF cookie support but need to
                         * attach to a shared library, we'll need to know and
                         * record absolute addresses of attach points due to
                         * the need to lookup USDT spec by absolute IP of
                         * triggered uprobe. Doing this resolution is only
                         * possible when we have a specific PID of the process
                         * that's using specified shared library. BPF cookie
                         * removes the absolute address limitation as we don't
                         * need to do this lookup (we just use BPF cookie as
                         * an index of USDT spec), so for newer kernels with
                         * BPF cookie support libbpf supports USDT attachment
                         * to shared libraries with no PID filter.
                         */
                        if (pid < 0) {
                                pr_warn("usdt: attaching to shared libraries without specific PID is not supported on current kernel\n");
                                err = -ENOTSUP;
                                goto err_out;
                        }

                        /* lib_segs are lazily initialized only if necessary */
                        if (lib_seg_cnt == 0) {
                                err = parse_lib_segs(pid, path, &lib_segs, &lib_seg_cnt);
                                if (err) {
                                        pr_warn("usdt: failed to get memory segments in PID %d for shared library '%s': %d\n",
                                                pid, path, err);
                                        goto err_out;
                                }
                        }

                        seg = find_elf_seg(lib_segs, lib_seg_cnt, usdt_rel_ip, true /* relative */);
                        if (!seg) {
                                err = -ESRCH;
                                pr_warn("usdt: failed to find shared lib memory segment for '%s:%s' in '%s' at relative IP 0x%lx\n",
                                        usdt_provider, usdt_name, path, usdt_rel_ip);
                                goto err_out;
                        }

                        usdt_abs_ip = seg->start + (usdt_rel_ip - seg->offset);
                }

                pr_debug("usdt: probe for '%s:%s' in %s '%s': addr 0x%lx base 0x%lx (resolved abs_ip 0x%lx rel_ip 0x%lx) args '%s' in segment [0x%lx, 0x%lx) at offset 0x%lx\n",
                         usdt_provider, usdt_name, ehdr.e_type == ET_EXEC ? "exec" : "lib ", path,
                         note.loc_addr, note.base_addr, usdt_abs_ip, usdt_rel_ip, note.args,
                         seg ? seg->start : 0, seg ? seg->end : 0, seg ? seg->offset : 0);

                /* Adjust semaphore address to be a relative offset */
                if (note.sema_addr) {
                        if (!man->has_sema_refcnt) {
                                pr_warn("usdt: kernel doesn't support USDT semaphore refcounting for '%s:%s' in '%s'\n",
                                        usdt_provider, usdt_name, path);
                                err = -ENOTSUP;
                                goto err_out;
                        }

                        seg = find_elf_seg(segs, seg_cnt, note.sema_addr, false /* relative */);
                        if (!seg) {
                                err = -ESRCH;
                                pr_warn("usdt: failed to find ELF loadable segment with semaphore of '%s:%s' in '%s' at 0x%lx\n",
                                        usdt_provider, usdt_name, path, note.sema_addr);
                                goto err_out;
                        }
                        if (seg->is_exec) {
                                err = -ESRCH;
                                pr_warn("usdt: matched ELF binary '%s' segment [0x%lx, 0x%lx] for semaphore of '%s:%s' at 0x%lx is executable\n",
                                        path, seg->start, seg->end, usdt_provider, usdt_name,
                                        note.sema_addr);
                                goto err_out;
                        }

                        usdt_sema_off = note.sema_addr - (seg->start - seg->offset);

                        pr_debug("usdt: sema  for '%s:%s' in %s '%s': addr 0x%lx base 0x%lx (resolved 0x%lx) in segment [0x%lx, 0x%lx] at offset 0x%lx\n",
                                 usdt_provider, usdt_name, ehdr.e_type == ET_EXEC ? "exec" : "lib ",
                                 path, note.sema_addr, note.base_addr, usdt_sema_off,
                                 seg->start, seg->end, seg->offset);
                }

                /* Record adjusted addresses and offsets and parse USDT spec */
                tmp = libbpf_reallocarray(targets, target_cnt + 1, sizeof(*targets));
                if (!tmp) {
                        err = -ENOMEM;
                        goto err_out;
                }
                targets = tmp;

                target = &targets[target_cnt];
                memset(target, 0, sizeof(*target));

                target->abs_ip = usdt_abs_ip;
                target->rel_ip = usdt_rel_ip;
                target->sema_off = usdt_sema_off;

                /* notes->args references strings from Elf itself, so they can
                 * be referenced safely until elf_end() call
                 */
                target->spec_str = note.args;

                err = parse_usdt_spec(&target->spec, &note, usdt_cookie);
                if (err)
                        goto err_out;

                target_cnt++;
        }

        *out_targets = targets;
        *out_target_cnt = target_cnt;
        err = target_cnt;

err_out:
        free(segs);
        free(lib_segs);
        if (err < 0)
                free(targets);
        return err;
}

struct bpf_link_usdt {
        struct bpf_link link;

        struct usdt_manager *usdt_man;

        size_t spec_cnt;
        int *spec_ids;

        size_t uprobe_cnt;
        struct {
                long abs_ip;
                struct bpf_link *link;
        } *uprobes;
};

static int bpf_link_usdt_detach(struct bpf_link *link)
{
        struct bpf_link_usdt *usdt_link = container_of(link, struct bpf_link_usdt, link);
        struct usdt_manager *man = usdt_link->usdt_man;
        int i;

        for (i = 0; i < usdt_link->uprobe_cnt; i++) {
                /* detach underlying uprobe link */
                bpf_link__destroy(usdt_link->uprobes[i].link);
                /* there is no need to update specs map because it will be
                 * unconditionally overwritten on subsequent USDT attaches,
                 * but if BPF cookies are not used we need to remove entry
                 * from ip_to_spec_id map, otherwise we'll run into false
                 * conflicting IP errors
                 */
                if (!man->has_bpf_cookie) {
                        /* not much we can do about errors here */
                        (void)bpf_map_delete_elem(bpf_map__fd(man->ip_to_spec_id_map),
                                                  &usdt_link->uprobes[i].abs_ip);
                }
        }

        /* try to return the list of previously used spec IDs to usdt_manager
         * for future reuse for subsequent USDT attaches
         */
        if (!man->free_spec_ids) {
                /* if there were no free spec IDs yet, just transfer our IDs */
                man->free_spec_ids = usdt_link->spec_ids;
                man->free_spec_cnt = usdt_link->spec_cnt;
                usdt_link->spec_ids = NULL;
        } else {
                /* otherwise concat IDs */
                size_t new_cnt = man->free_spec_cnt + usdt_link->spec_cnt;
                int *new_free_ids;

                new_free_ids = libbpf_reallocarray(man->free_spec_ids, new_cnt,
                                                   sizeof(*new_free_ids));
                /* If we couldn't resize free_spec_ids, we'll just leak
                 * a bunch of free IDs; this is very unlikely to happen and if
                 * system is so exausted on memory, it's the least of user's
                 * concerns, probably.
                 * So just do our best here to return those IDs to usdt_manager.
                 */
                if (new_free_ids) {
                        memcpy(new_free_ids + man->free_spec_cnt, usdt_link->spec_ids,
                               usdt_link->spec_cnt * sizeof(*usdt_link->spec_ids));
                        man->free_spec_ids = new_free_ids;
                        man->free_spec_cnt = new_cnt;
                }
        }

        return 0;
}

static void bpf_link_usdt_dealloc(struct bpf_link *link)
{
        struct bpf_link_usdt *usdt_link = container_of(link, struct bpf_link_usdt, link);

        free(usdt_link->spec_ids);
        free(usdt_link->uprobes);
        free(usdt_link);
}

static size_t specs_hash_fn(const void *key, void *ctx)
{
        const char *s = key;

        return str_hash(s);
}

static bool specs_equal_fn(const void *key1, const void *key2, void *ctx)
{
        const char *s1 = key1;
        const char *s2 = key2;

        return strcmp(s1, s2) == 0;
}

static int allocate_spec_id(struct usdt_manager *man, struct hashmap *specs_hash,
                            struct bpf_link_usdt *link, struct usdt_target *target,
                            int *spec_id, bool *is_new)
{
        void *tmp;
        int err;

        /* check if we already allocated spec ID for this spec string */
        if (hashmap__find(specs_hash, target->spec_str, &tmp)) {
                *spec_id = (long)tmp;
                *is_new = false;
                return 0;
        }

        /* otherwise it's a new ID that needs to be set up in specs map and
         * returned back to usdt_manager when USDT link is detached
         */
        tmp = libbpf_reallocarray(link->spec_ids, link->spec_cnt + 1, sizeof(*link->spec_ids));
        if (!tmp)
                return -ENOMEM;
        link->spec_ids = tmp;

        /* get next free spec ID, giving preference to free list, if not empty */
        if (man->free_spec_cnt) {
                *spec_id = man->free_spec_ids[man->free_spec_cnt - 1];

                /* cache spec ID for current spec string for future lookups */
                err = hashmap__add(specs_hash, target->spec_str, (void *)(long)*spec_id);
                if (err)
                         return err;

                man->free_spec_cnt--;
        } else {
                /* don't allocate spec ID bigger than what fits in specs map */
                if (man->next_free_spec_id >= bpf_map__max_entries(man->specs_map))
                        return -E2BIG;

                *spec_id = man->next_free_spec_id;

                /* cache spec ID for current spec string for future lookups */
                err = hashmap__add(specs_hash, target->spec_str, (void *)(long)*spec_id);
                if (err)
                         return err;

                man->next_free_spec_id++;
        }

        /* remember new spec ID in the link for later return back to free list on detach */
        link->spec_ids[link->spec_cnt] = *spec_id;
        link->spec_cnt++;
        *is_new = true;
        return 0;
}

struct bpf_link *usdt_manager_attach_usdt(struct usdt_manager *man, const struct bpf_program *prog,
                                          pid_t pid, const char *path,
                                          const char *usdt_provider, const char *usdt_name,
                                          long usdt_cookie)
{
        int i, fd, err, spec_map_fd, ip_map_fd;
        LIBBPF_OPTS(bpf_uprobe_opts, opts);
        struct hashmap *specs_hash = NULL;
        struct bpf_link_usdt *link = NULL;
        struct usdt_target *targets = NULL;
        size_t target_cnt;
        Elf *elf;

        spec_map_fd = bpf_map__fd(man->specs_map);
        ip_map_fd = bpf_map__fd(man->ip_to_spec_id_map);

        /* TODO: perform path resolution similar to uprobe's */
        fd = open(path, O_RDONLY);
        if (fd < 0) {
                err = -errno;
                pr_warn("usdt: failed to open ELF binary '%s': %d\n", path, err);
                return libbpf_err_ptr(err);
        }

        elf = elf_begin(fd, ELF_C_READ_MMAP, NULL);
        if (!elf) {
                err = -EBADF;
                pr_warn("usdt: failed to parse ELF binary '%s': %s\n", path, elf_errmsg(-1));
                goto err_out;
        }

        err = sanity_check_usdt_elf(elf, path);
        if (err)
                goto err_out;

        /* normalize PID filter */
        if (pid < 0)
                pid = -1;
        else if (pid == 0)
                pid = getpid();

        /* discover USDT in given binary, optionally limiting
         * activations to a given PID, if pid > 0
         */
        err = collect_usdt_targets(man, elf, path, pid, usdt_provider, usdt_name,
                                   usdt_cookie, &targets, &target_cnt);
        if (err <= 0) {
                err = (err == 0) ? -ENOENT : err;
                goto err_out;
        }

        specs_hash = hashmap__new(specs_hash_fn, specs_equal_fn, NULL);
        if (IS_ERR(specs_hash)) {
                err = PTR_ERR(specs_hash);
                goto err_out;
        }

        link = calloc(1, sizeof(*link));
        if (!link) {
                err = -ENOMEM;
                goto err_out;
        }

        link->usdt_man = man;
        link->link.detach = &bpf_link_usdt_detach;
        link->link.dealloc = &bpf_link_usdt_dealloc;

        link->uprobes = calloc(target_cnt, sizeof(*link->uprobes));
        if (!link->uprobes) {
                err = -ENOMEM;
                goto err_out;
        }

        for (i = 0; i < target_cnt; i++) {
                struct usdt_target *target = &targets[i];
                struct bpf_link *uprobe_link;
                bool is_new;
                int spec_id;

                /* Spec ID can be either reused or newly allocated. If it is
                 * newly allocated, we'll need to fill out spec map, otherwise
                 * entire spec should be valid and can be just used by a new
                 * uprobe. We reuse spec when USDT arg spec is identical. We
                 * also never share specs between two different USDT
                 * attachments ("links"), so all the reused specs already
                 * share USDT cookie value implicitly.
                 */
                err = allocate_spec_id(man, specs_hash, link, target, &spec_id, &is_new);
                if (err)
                        goto err_out;

                if (is_new && bpf_map_update_elem(spec_map_fd, &spec_id, &target->spec, BPF_ANY)) {
                        err = -errno;
                        pr_warn("usdt: failed to set USDT spec #%d for '%s:%s' in '%s': %d\n",
                                spec_id, usdt_provider, usdt_name, path, err);
                        goto err_out;
                }
                if (!man->has_bpf_cookie &&
                    bpf_map_update_elem(ip_map_fd, &target->abs_ip, &spec_id, BPF_NOEXIST)) {
                        err = -errno;
                        if (err == -EEXIST) {
                                pr_warn("usdt: IP collision detected for spec #%d for '%s:%s' in '%s'\n",
                                        spec_id, usdt_provider, usdt_name, path);
                        } else {
                                pr_warn("usdt: failed to map IP 0x%lx to spec #%d for '%s:%s' in '%s': %d\n",
                                        target->abs_ip, spec_id, usdt_provider, usdt_name,
                                        path, err);
                        }
                        goto err_out;
                }

                opts.ref_ctr_offset = target->sema_off;
                opts.bpf_cookie = man->has_bpf_cookie ? spec_id : 0;
                uprobe_link = bpf_program__attach_uprobe_opts(prog, pid, path,
                                                              target->rel_ip, &opts);
                err = libbpf_get_error(uprobe_link);
                if (err) {
                        pr_warn("usdt: failed to attach uprobe #%d for '%s:%s' in '%s': %d\n",
                                i, usdt_provider, usdt_name, path, err);
                        goto err_out;
                }

                link->uprobes[i].link = uprobe_link;
                link->uprobes[i].abs_ip = target->abs_ip;
                link->uprobe_cnt++;
        }

        free(targets);
        hashmap__free(specs_hash);
        elf_end(elf);
        close(fd);

        return &link->link;

err_out:
        bpf_link__destroy(&link->link);

        free(targets);
        hashmap__free(specs_hash);
        if (elf)
                elf_end(elf);
        close(fd);
        return libbpf_err_ptr(err);
}

/* Parse out USDT ELF note from '.note.stapsdt' section.
 * Logic inspired by perf's code.
 */
static int parse_usdt_note(Elf *elf, const char *path, long base_addr,
                           GElf_Nhdr *nhdr, const char *data, size_t name_off, size_t desc_off,
                           struct usdt_note *note)
{
        const char *provider, *name, *args;
        long addrs[3];
        size_t len;

        /* sanity check USDT note name and type first */
        if (strncmp(data + name_off, USDT_NOTE_NAME, nhdr->n_namesz) != 0)
                return -EINVAL;
        if (nhdr->n_type != USDT_NOTE_TYPE)
                return -EINVAL;

        /* sanity check USDT note contents ("description" in ELF terminology) */
        len = nhdr->n_descsz;
        data = data + desc_off;

        /* +3 is the very minimum required to store three empty strings */
        if (len < sizeof(addrs) + 3)
                return -EINVAL;

        /* get location, base, and semaphore addrs */
        memcpy(&addrs, data, sizeof(addrs));

        /* parse string fields: provider, name, args */
        provider = data + sizeof(addrs);

        name = (const char *)memchr(provider, '\0', data + len - provider);
        if (!name) /* non-zero-terminated provider */
                return -EINVAL;
        name++;
        if (name >= data + len || *name == '\0') /* missing or empty name */
                return -EINVAL;

        args = memchr(name, '\0', data + len - name);
        if (!args) /* non-zero-terminated name */
                return -EINVAL;
        ++args;
        if (args >= data + len) /* missing arguments spec */
                return -EINVAL;

        note->provider = provider;
        note->name = name;
        if (*args == '\0' || *args == ':')
                note->args = "";
        else
                note->args = args;
        note->loc_addr = addrs[0];
        note->base_addr = addrs[1];
        note->sema_addr = addrs[2];

        return 0;
}

static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec *arg);

static int parse_usdt_spec(struct usdt_spec *spec, const struct usdt_note *note, long usdt_cookie)
{
        const char *s;
        int len;

        spec->usdt_cookie = usdt_cookie;
        spec->arg_cnt = 0;

        s = note->args;
        while (s[0]) {
                if (spec->arg_cnt >= USDT_MAX_ARG_CNT) {
                        pr_warn("usdt: too many USDT arguments (> %d) for '%s:%s' with args spec '%s'\n",
                                USDT_MAX_ARG_CNT, note->provider, note->name, note->args);
                        return -E2BIG;
                }

                len = parse_usdt_arg(s, spec->arg_cnt, &spec->args[spec->arg_cnt]);
                if (len < 0)
                        return len;

                s += len;
                spec->arg_cnt++;
        }

        return 0;
}

/* Architecture-specific logic for parsing USDT argument location specs */

#if defined(__x86_64__) || defined(__i386__)

static int calc_pt_regs_off(const char *reg_name)
{
        static struct {
                const char *names[4];
                size_t pt_regs_off;
        } reg_map[] = {
#if __x86_64__
#define reg_off(reg64, reg32) offsetof(struct pt_regs, reg64)
#else
#define reg_off(reg64, reg32) offsetof(struct pt_regs, reg32)
#endif
                { {"rip", "eip", "", ""}, reg_off(rip, eip) },
                { {"rax", "eax", "ax", "al"}, reg_off(rax, eax) },
                { {"rbx", "ebx", "bx", "bl"}, reg_off(rbx, ebx) },
                { {"rcx", "ecx", "cx", "cl"}, reg_off(rcx, ecx) },
                { {"rdx", "edx", "dx", "dl"}, reg_off(rdx, edx) },
                { {"rsi", "esi", "si", "sil"}, reg_off(rsi, esi) },
                { {"rdi", "edi", "di", "dil"}, reg_off(rdi, edi) },
                { {"rbp", "ebp", "bp", "bpl"}, reg_off(rbp, ebp) },
                { {"rsp", "esp", "sp", "spl"}, reg_off(rsp, esp) },
#undef reg_off
#if __x86_64__
                { {"r8", "r8d", "r8w", "r8b"}, offsetof(struct pt_regs, r8) },
                { {"r9", "r9d", "r9w", "r9b"}, offsetof(struct pt_regs, r9) },
                { {"r10", "r10d", "r10w", "r10b"}, offsetof(struct pt_regs, r10) },
                { {"r11", "r11d", "r11w", "r11b"}, offsetof(struct pt_regs, r11) },
                { {"r12", "r12d", "r12w", "r12b"}, offsetof(struct pt_regs, r12) },
                { {"r13", "r13d", "r13w", "r13b"}, offsetof(struct pt_regs, r13) },
                { {"r14", "r14d", "r14w", "r14b"}, offsetof(struct pt_regs, r14) },
                { {"r15", "r15d", "r15w", "r15b"}, offsetof(struct pt_regs, r15) },
#endif
        };
        int i, j;

        for (i = 0; i < ARRAY_SIZE(reg_map); i++) {
                for (j = 0; j < ARRAY_SIZE(reg_map[i].names); j++) {
                        if (strcmp(reg_name, reg_map[i].names[j]) == 0)
                                return reg_map[i].pt_regs_off;
                }
        }

        pr_warn("usdt: unrecognized register '%s'\n", reg_name);
        return -ENOENT;
}

static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec *arg)
{
        char *reg_name = NULL;
        int arg_sz, len, reg_off;
        long off;

        if (sscanf(arg_str, " %d @ %ld ( %%%m[^)] ) %n", &arg_sz, &off, &reg_name, &len) == 3) {
                /* Memory dereference case, e.g., -4@-20(%rbp) */
                arg->arg_type = USDT_ARG_REG_DEREF;
                arg->val_off = off;
                reg_off = calc_pt_regs_off(reg_name);
                free(reg_name);
                if (reg_off < 0)
                        return reg_off;
                arg->reg_off = reg_off;
        } else if (sscanf(arg_str, " %d @ %%%ms %n", &arg_sz, &reg_name, &len) == 2) {
                /* Register read case, e.g., -4@%eax */
                arg->arg_type = USDT_ARG_REG;
                arg->val_off = 0;

                reg_off = calc_pt_regs_off(reg_name);
                free(reg_name);
                if (reg_off < 0)
                        return reg_off;
                arg->reg_off = reg_off;
        } else if (sscanf(arg_str, " %d @ $%ld %n", &arg_sz, &off, &len) == 2) {
                /* Constant value case, e.g., 4@$71 */
                arg->arg_type = USDT_ARG_CONST;
                arg->val_off = off;
                arg->reg_off = 0;
        } else {
                pr_warn("usdt: unrecognized arg #%d spec '%s'\n", arg_num, arg_str);
                return -EINVAL;
        }

        arg->arg_signed = arg_sz < 0;
        if (arg_sz < 0)
                arg_sz = -arg_sz;

        switch (arg_sz) {
        case 1: case 2: case 4: case 8:
                arg->arg_bitshift = 64 - arg_sz * 8;
                break;
        default:
                pr_warn("usdt: unsupported arg #%d (spec '%s') size: %d\n",
                        arg_num, arg_str, arg_sz);
                return -EINVAL;
        }

        return len;
}

#else

static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec *arg)
{
        pr_warn("usdt: libbpf doesn't support USDTs on current architecture\n");
        return -ENOTSUP;
}

#endif