libbpf: Add support for field size relocations

Add bpf_core_field_size() macro, capturing a relocation against field size.
Adjust bits of internal libbpf relocation logic to allow capturing size
relocations of various field types: arrays, structs/unions, enums, etc.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191101222810.1246166-4-andriin@fb.com

diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
index 0935cd6..11461b2 100644 (file)
--- a/tools/lib/bpf/bpf_core_read.h
+++ b/tools/lib/bpf/bpf_core_read.h
@@ -97,6 +97,13 @@ enum bpf_field_info_kind {
        __builtin_preserve_field_info(field, BPF_FIELD_EXISTS)
 
 /*
+ * Convenience macro to get byte size of a field. Works for integers,
+ * struct/unions, pointers, arrays, and enums.
+ */
+#define bpf_core_field_size(field)                                         \
+       __builtin_preserve_field_info(field, BPF_FIELD_BYTE_SIZE)
+
+/*
  * bpf_core_read() abstracts away bpf_probe_read() call and captures offset
  * relocation for source address using __builtin_preserve_access_index()
  * built-in, provided by Clang.

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 482af74..be4af95 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -2704,8 +2704,10 @@ err_out:
 /* Check two types for compatibility, skipping const/volatile/restrict and
  * typedefs, to ensure we are relocating compatible entities:
  *   - any two STRUCTs/UNIONs are compatible and can be mixed;
- *   - any two FWDs are compatible;
+ *   - any two FWDs are compatible, if their names match (modulo flavor suffix);
  *   - any two PTRs are always compatible;
+ *   - for ENUMs, names should be the same (ignoring flavor suffix) or at
+ *     least one of enums should be anonymous;
  *   - for ENUMs, check sizes, names are ignored;
  *   - for INT, size and signedness are ignored;
  *   - for ARRAY, dimensionality is ignored, element types are checked for
@@ -2733,11 +2735,23 @@ recur:
                return 0;
 
        switch (btf_kind(local_type)) {
-       case BTF_KIND_FWD:
        case BTF_KIND_PTR:
                return 1;
-       case BTF_KIND_ENUM:
-               return local_type->size == targ_type->size;
+       case BTF_KIND_FWD:
+       case BTF_KIND_ENUM: {
+               const char *local_name, *targ_name;
+               size_t local_len, targ_len;
+
+               local_name = btf__name_by_offset(local_btf,
+                                                local_type->name_off);
+               targ_name = btf__name_by_offset(targ_btf, targ_type->name_off);
+               local_len = bpf_core_essential_name_len(local_name);
+               targ_len = bpf_core_essential_name_len(targ_name);
+               /* one of them is anonymous or both w/ same flavor-less names */
+               return local_len == 0 || targ_len == 0 ||
+                      (local_len == targ_len &&
+                       strncmp(local_name, targ_name, local_len) == 0);
+       }
        case BTF_KIND_INT:
                /* just reject deprecated bitfield-like integers; all other
                 * integers are by default compatible between each other
@@ -2926,16 +2940,23 @@ static int bpf_core_calc_field_relo(const struct bpf_program *prog,
        const struct btf_member *m;
        const struct btf_type *mt;
        bool bitfield;
+       __s64 sz;
 
        /* a[n] accessor needs special handling */
        if (!acc->name) {
-               if (relo->kind != BPF_FIELD_BYTE_OFFSET) {
-                       pr_warn("prog '%s': relo %d at insn #%d can't be applied to array access'\n",
+               if (relo->kind == BPF_FIELD_BYTE_OFFSET) {
+                       *val = spec->bit_offset / 8;
+               } else if (relo->kind == BPF_FIELD_BYTE_SIZE) {
+                       sz = btf__resolve_size(spec->btf, acc->type_id);
+                       if (sz < 0)
+                               return -EINVAL;
+                       *val = sz;
+               } else {
+                       pr_warn("prog '%s': relo %d at insn #%d can't be applied to array access\n",
                                bpf_program__title(prog, false),
                                relo->kind, relo->insn_off / 8);
                        return -EINVAL;
                }
-               *val = spec->bit_offset / 8;
                if (validate)
                        *validate = true;
                return 0;
@@ -2963,7 +2984,10 @@ static int bpf_core_calc_field_relo(const struct bpf_program *prog,
                        byte_off = bit_off / 8 / byte_sz * byte_sz;
                }
        } else {
-               byte_sz = mt->size;
+               sz = btf__resolve_size(spec->btf, m->type);
+               if (sz < 0)
+                       return -EINVAL;
+               byte_sz = sz;
                byte_off = spec->bit_offset / 8;
                bit_sz = byte_sz * 8;
        }