perf unwind: Fix segbase for ld.lld linked objects

segbase is the address of .eh_frame_hdr and table_data is segbase plus
the header size. find_proc_info computes segbase as `map->start +
segbase - map->pgoff` which is wrong when

* .eh_frame_hdr and .text are in different PT_LOAD program headers
* and their p_vaddr difference does not equal their p_offset difference

Since 10.0, ld.lld's default --rosegment -z noseparate-code layout has
such R and RX PT_LOAD program headers.

    ld.lld (default) => perf report fails to unwind `perf record
    --call-graph dwarf` recorded data
    ld.lld --no-rosegment => ok (trivial, no R PT_LOAD)
    ld.lld -z separate-code => ok but by luck: there are two PT_LOAD but
    their p_vaddr difference equals p_offset difference

    ld.bfd -z noseparate-code => ok (trivial, no R PT_LOAD)
    ld.bfd -z separate-code (default for Linux/x86) => ok but by luck:
    there are two PT_LOAD but their p_vaddr difference equals p_offset
    difference

To fix the issue, compute segbase as dso's base address plus
PT_GNU_EH_FRAME's p_vaddr. The base address is computed by iterating
over all dso-associated maps and then subtract the first PT_LOAD p_vaddr
(the minimum guaranteed by generic ABI) from the minimum address.

In libunwind, find_proc_info transitively called by unw_step is cached,
so the iteration overhead is acceptable.

Reported-by: Sebastian Ullrich <sebasti@nullri.ch>
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Fangrui Song <maskray@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: llvm@lists.linux.dev
Link: https://github.com/ClangBuiltLinux/linux/issues/1646
Link: https://lore.kernel.org/r/20220527182039.673248-1-maskray@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
index 3a9fd4d..97047a1 100644 (file)
--- a/tools/perf/util/dso.h
+++ b/tools/perf/util/dso.h
@@ -196,7 +196,9 @@ struct dso {
                u32              status_seen;
                u64              file_size;
                struct list_head open_entry;
+               u64              elf_base_addr;
                u64              debug_frame_offset;
+               u64              eh_frame_hdr_addr;
                u64              eh_frame_hdr_offset;
        } data;
        /* bpf prog information */

diff --git a/tools/perf/util/unwind-libunwind-local.c b/tools/perf/util/unwind-libunwind-local.c
index 41e29fc..3762269 100644 (file)
--- a/tools/perf/util/unwind-libunwind-local.c
+++ b/tools/perf/util/unwind-libunwind-local.c
@@ -169,29 +169,63 @@ static int __dw_read_encoded_value(u8 **p, u8 *end, u64 *val,
        __v;                                                    \
        })
 
-static u64 elf_section_offset(int fd, const char *name)
+static int elf_section_address_and_offset(int fd, const char *name, u64 *address, u64 *offset)
 {
        Elf *elf;
        GElf_Ehdr ehdr;
        GElf_Shdr shdr;
-       u64 offset = 0;
+       int ret;
 
        elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
        if (elf == NULL)
+               return -1;
+
+       if (gelf_getehdr(elf, &ehdr) == NULL)
+               goto out_err;
+
+       if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
+               goto out_err;
+
+       *address = shdr.sh_addr;
+       *offset = shdr.sh_offset;
+       ret = 0;
+out_err:
+       elf_end(elf);
+       return ret;
+}
+
+#ifndef NO_LIBUNWIND_DEBUG_FRAME
+static u64 elf_section_offset(int fd, const char *name)
+{
+       u64 address, offset;
+
+       if (elf_section_address_and_offset(fd, name, &address, &offset))
                return 0;
 
-       do {
-               if (gelf_getehdr(elf, &ehdr) == NULL)
-                       break;
+       return offset;
+}
+#endif
 
-               if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
-                       break;
+static u64 elf_base_address(int fd)
+{
+       Elf *elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
+       GElf_Phdr phdr;
+       u64 retval = 0;
+       size_t i, phdrnum = 0;
 
-               offset = shdr.sh_offset;
-       } while (0);
+       if (elf == NULL)
+               return 0;
+       (void)elf_getphdrnum(elf, &phdrnum);
+       /* PT_LOAD segments are sorted by p_vaddr, so the first has the minimum p_vaddr. */
+       for (i = 0; i < phdrnum; i++) {
+               if (gelf_getphdr(elf, i, &phdr) && phdr.p_type == PT_LOAD) {
+                       retval = phdr.p_vaddr & -getpagesize();
+                       break;
+               }
+       }
 
        elf_end(elf);
-       return offset;
+       return retval;
 }
 
 #ifndef NO_LIBUNWIND_DEBUG_FRAME
@@ -248,8 +282,7 @@ struct eh_frame_hdr {
 } __packed;
 
 static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
-                              u64 offset, u64 *table_data, u64 *segbase,
-                              u64 *fde_count)
+                              u64 offset, u64 *table_data_offset, u64 *fde_count)
 {
        struct eh_frame_hdr hdr;
        u8 *enc = (u8 *) &hdr.enc;
@@ -265,35 +298,47 @@ static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
        dw_read_encoded_value(enc, end, hdr.eh_frame_ptr_enc);
 
        *fde_count  = dw_read_encoded_value(enc, end, hdr.fde_count_enc);
-       *segbase    = offset;
-       *table_data = (enc - (u8 *) &hdr) + offset;
+       *table_data_offset = enc - (u8 *) &hdr;
        return 0;
 }
 
-static int read_unwind_spec_eh_frame(struct dso *dso, struct machine *machine,
+static int read_unwind_spec_eh_frame(struct dso *dso, struct unwind_info *ui,
                                     u64 *table_data, u64 *segbase,
                                     u64 *fde_count)
 {
-       int ret = -EINVAL, fd;
-       u64 offset = dso->data.eh_frame_hdr_offset;
+       struct map *map;
+       u64 base_addr = UINT64_MAX;
+       int ret, fd;
 
-       if (offset == 0) {
-               fd = dso__data_get_fd(dso, machine);
+       if (dso->data.eh_frame_hdr_offset == 0) {
+               fd = dso__data_get_fd(dso, ui->machine);
                if (fd < 0)
                        return -EINVAL;
 
                /* Check the .eh_frame section for unwinding info */
-               offset = elf_section_offset(fd, ".eh_frame_hdr");
-               dso->data.eh_frame_hdr_offset = offset;
+               ret = elf_section_address_and_offset(fd, ".eh_frame_hdr",
+                                                    &dso->data.eh_frame_hdr_addr,
+                                                    &dso->data.eh_frame_hdr_offset);
+               dso->data.elf_base_addr = elf_base_address(fd);
                dso__data_put_fd(dso);
+               if (ret || dso->data.eh_frame_hdr_offset == 0)
+                       return -EINVAL;
        }
 
-       if (offset)
-               ret = unwind_spec_ehframe(dso, machine, offset,
-                                         table_data, segbase,
-                                         fde_count);
-
-       return ret;
+       maps__for_each_entry(ui->thread->maps, map) {
+               if (map->dso == dso && map->start < base_addr)
+                       base_addr = map->start;
+       }
+       base_addr -= dso->data.elf_base_addr;
+       /* Address of .eh_frame_hdr */
+       *segbase = base_addr + dso->data.eh_frame_hdr_addr;
+       ret = unwind_spec_ehframe(dso, ui->machine, dso->data.eh_frame_hdr_offset,
+                                  table_data, fde_count);
+       if (ret)
+               return ret;
+       /* binary_search_table offset plus .eh_frame_hdr address */
+       *table_data += *segbase;
+       return 0;
 }
 
 #ifndef NO_LIBUNWIND_DEBUG_FRAME
@@ -388,14 +433,14 @@ find_proc_info(unw_addr_space_t as, unw_word_t ip, unw_proc_info_t *pi,
        pr_debug("unwind: find_proc_info dso %s\n", map->dso->name);
 
        /* Check the .eh_frame section for unwinding info */
-       if (!read_unwind_spec_eh_frame(map->dso, ui->machine,
+       if (!read_unwind_spec_eh_frame(map->dso, ui,
                                       &table_data, &segbase, &fde_count)) {
                memset(&di, 0, sizeof(di));
                di.format   = UNW_INFO_FORMAT_REMOTE_TABLE;
                di.start_ip = map->start;
                di.end_ip   = map->end;
-               di.u.rti.segbase    = map->start + segbase - map->pgoff;
-               di.u.rti.table_data = map->start + table_data - map->pgoff;
+               di.u.rti.segbase    = segbase;
+               di.u.rti.table_data = table_data;
                di.u.rti.table_len  = fde_count * sizeof(struct table_entry)
                                      / sizeof(unw_word_t);
                ret = dwarf_search_unwind_table(as, ip, &di, pi,