On Sun, 2024-02-04 at 11:40 -0700, Daniel Xu wrote:
> This commit teaches pahole to parse symbols in .BTF_ids section in
> vmlinux and discover exported kfuncs. Pahole then takes the list of
> kfuncs and injects a BTF_KIND_DECL_TAG for each kfunc.
>
> Example of encoding:
>
> $ bpftool btf dump file .tmp_vmlinux.btf | rg "DECL_TAG 'bpf_kfunc'" | wc -l
> 121
>
> $ bpftool btf dump file .tmp_vmlinux.btf | rg 56337
> [56337] FUNC 'bpf_ct_change_timeout' type_id=56336 linkage=static
> [127861] DECL_TAG 'bpf_kfunc' type_id=56337 component_idx=-1
>
> This enables downstream users and tools to dynamically discover which
> kfuncs are available on a system by parsing vmlinux or module BTF, both
> available in /sys/kernel/btf.
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
I've tested this patch-set using kernel built both with clang and gcc,
on current bpf-next master (2d9a925d0fbf), both times get 124 kfunc definitions.
Tested-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
Two nitpicks below.
[...]
> +static char *get_func_name(const char *sym)
> +{
> + char *func, *end;
> +
> + if (strncmp(sym, BTF_ID_FUNC_PFX, sizeof(BTF_ID_FUNC_PFX) - 1))
> + return NULL;
> +
> + /* Strip prefix */
> + func = strdup(sym + sizeof(BTF_ID_FUNC_PFX) - 1);
> +
> + /* Strip suffix */
> + end = strrchr(func, '_');
> + if (!end || *(end - 1) != '_') {
Nit: this would do out of bounds access on malformed input
"__BTF_ID__func___"
> + free(func);
> + return NULL;
> + }
> + *(end - 1) = '\0';
> +
> + return func;
> +}
[...]
> +static int btf_encoder__tag_kfuncs(struct btf_encoder *encoder)
> +{
[...]
> + elf = elf_begin(fd, ELF_C_READ, NULL);
> + if (elf == NULL) {
> + elf_error("Cannot update ELF file");
> + goto out;
> + }
> +
> + /* Location symbol table and .BTF_ids sections */
> + elf_getshdrstrndx(elf, &strndx);
Nit: in theory elf_getshdrstrndx() could fail and strndx would remain
uninitialized.
[...]
