Re: [PATCHv2 bpf-next 08/24] libbpf: Add elf_find_multi_func_offset function

Andrii Nakryiko
2023-06-23T20:40:48Z

On Tue, Jun 20, 2023 at 1:37 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding elf_find_multi_func_offset function that looks up
> offsets for symbols specified in syms array argument.
>
> Offsets are returned in allocated array with the 'cnt' size,
> that needs to be released by the caller.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> tools/lib/bpf/libbpf.c | 112 ++++++++++++++++++++++++++++++++
> tools/lib/bpf/libbpf_internal.h | 2 +
> 2 files changed, 114 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 30d9e3b69114..1c310b718961 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -11053,6 +11053,118 @@ static long elf_find_func_offset(Elf *elf, const char *binary_path, const char *
> return ret;
> }
>
> +struct elf_symbol_offset {
> + const char *name;
> + unsigned long offset;
> + int bind;
> + int idx;
> +};
> +
> +static int cmp_func_offset(const void *_a, const void *_b)
> +{
> + const struct elf_symbol_offset *a = _a;
> + const struct elf_symbol_offset *b = _b;
> +
> + return strcmp(a->name, b->name);
> +}
> +
> +static int
> +__elf_find_multi_func_offset(Elf *elf, const char *binary_path, int cnt,
> + const char **syms, unsigned long **poffsets)
> +{
> + int sh_types[2] = { SHT_DYNSYM, SHT_SYMTAB };
> + struct elf_symbol_offset *func_offs;
> + int err = 0, i, idx, cnt_done = 0;
> + unsigned long *offsets = NULL;
> +
> + func_offs = calloc(cnt, sizeof(*func_offs));
> + if (!func_offs)
> + return -ENOMEM;
> +
> + for (i = 0; i < cnt; i++) {
> + func_offs[i].name = syms[i];
> + func_offs[i].idx = i;
> + }
> +
> + qsort(func_offs, cnt, sizeof(*func_offs), cmp_func_offset);
> +
> + for (i = 0; i < ARRAY_SIZE(sh_types); i++) {
> + struct elf_symbol_iter iter;
> + struct elf_symbol *sym;
> +
> + if (elf_symbol_iter_new(&iter, elf, binary_path, sh_types[i]))

a bit lax handling of initialization errors here, let's be a bit more
strict here?

> + continue;
> +
> + while ((sym = elf_symbol_iter_next(&iter))) {
> + struct elf_symbol_offset *fo, tmp = {
> + .name = sym->name,
> + };
> +
> + fo = bsearch(&tmp, func_offs, cnt, sizeof(*func_offs),
> + cmp_func_offset);
> + if (!fo)
> + continue;
> +
> + if (fo->offset > 0) {
> + /* same offset, no problem */
> + if (fo->offset == sym->offset)
> + continue;
> + /* handle multiple matches */
> + if (fo->bind != STB_WEAK && sym->bind != STB_WEAK) {
> + /* Only accept one non-weak bind. */
> + pr_warn("elf: ambiguous match for '%s', '%s' in '%s'\n",
> + sym->name, fo->name, binary_path);
> + err = -LIBBPF_ERRNO__FORMAT;
> + goto out;
> + } else if (sym->bind == STB_WEAK) {
> + /* already have a non-weak bind, and
> + * this is a weak bind, so ignore.
> + */
> + continue;
> + }
> + }
> + if (!fo->offset)
> + cnt_done++;
> + fo->offset = sym->offset;
> + fo->bind = sym->bind;
> + }
> + }
> +
> + if (cnt != cnt_done) {
> + err = -ENOENT;
> + goto out;
> + }
> + offsets = calloc(cnt, sizeof(*offsets));

you can allocate it at the very beginning and fill it out based on
fo->idx, there is no need to store offset in elf_symbol_offset

> + if (!offsets) {
> + err = -ENOMEM;
> + goto out;
> + }
> + for (i = 0; i < cnt; i++) {
> + idx = func_offs[i].idx;
> + offsets[idx] = func_offs[i].offset;
> + }
> +
> +out:
> + *poffsets = offsets;
> + free(func_offs);
> + return err;
> +}
> +
> +int elf_find_multi_func_offset(const char *binary_path, int cnt,
> + const char **syms, unsigned long **poffsets)
> +{
> + struct elf_fd elf_fd = {};

do you need to initialize this struct?

> + long ret = -ENOENT;

same here, you always override ret, so no need to init it?

> +
> + ret = open_elf(binary_path, &elf_fd);
> + if (ret)
> + return ret;
> +
> + ret = __elf_find_multi_func_offset(elf_fd.elf, binary_path, cnt, syms, poffsets);

is there a point of having elf_find_multi_func_offset and
__elf_find_multi_func_offset separately? can you please combine?

> + close_elf(&elf_fd);
> + return ret;
> +}
> +
> /* Find offset of function name in ELF object specified by path. "name" matches
> * symbol name or name@@LIB for library functions.
> */
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index e4d05662a96c..13d5c12fbd0b 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -577,4 +577,6 @@ static inline bool is_pow_of_2(size_t x)
> #define PROG_LOAD_ATTEMPTS 5
> int sys_bpf_prog_load(union bpf_attr *attr, unsigned int size, int attempts);
>
> +int elf_find_multi_func_offset(const char *binary_path, int cnt,
> + const char **syms, unsigned long **poffsets);
> #endif /* __LIBBPF_LIBBPF_INTERNAL_H */
> --
> 2.41.0
>