On Mon, Sep 02, 2024 at 08:58:01AM +0200, Viktor Malik wrote:
> It is possible to create multiple BPF programs sharing the same
> instructions using the compiler `__attribute__((alias("...")))`:
>
> int BPF_PROG(prog)
> {
> [...]
> }
> int prog_alias() __attribute__((alias("prog")));
>
> This may be convenient when creating multiple programs with the same
> instruction set attached to different events (such as bpftrace does).
>
> One problem in this situation is that Clang doesn't generate a BTF entry
> for `prog_alias` which makes libbpf linker fail when processing such a
> BPF object.
this might not solve all the issues, but could we change pahole to
generate BTF FUNC for alias function symbols?
jirka
>
> This commits adds support for that by finding another symbol at the same
> address for which a BTF entry exists and using that entry in the linker.
> This allows to use the linker (e.g. via `bpftool gen object ...`) on BPF
> objects containing aliases.
>
> Note that this won't be sufficient for most programs as we also need to
> add support for handling relocations in the aliased programs. This will
> be added by the following commit.
>
> Signed-off-by: Viktor Malik <vmalik@xxxxxxxxxx>
> ---
> tools/lib/bpf/linker.c | 68 +++++++++++++++++++++++-------------------
> 1 file changed, 38 insertions(+), 30 deletions(-)
>
> diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
> index 9cd3d4109788..5ebc9ff1246e 100644
> --- a/tools/lib/bpf/linker.c
> +++ b/tools/lib/bpf/linker.c
> @@ -1688,6 +1688,34 @@ static bool btf_is_non_static(const struct btf_type *t)
> || (btf_is_func(t) && btf_func_linkage(t) != BTF_FUNC_STATIC);
> }
>
> +static Elf64_Sym *find_sym_by_name(struct src_obj *obj, size_t sec_idx,
> + int sym_type, const char *sym_name)
> +{
> + struct src_sec *symtab = &obj->secs[obj->symtab_sec_idx];
> + Elf64_Sym *sym = symtab->data->d_buf;
> + int i, n = symtab->shdr->sh_size / symtab->shdr->sh_entsize;
> + int str_sec_idx = symtab->shdr->sh_link;
> + const char *name;
> +
> + for (i = 0; i < n; i++, sym++) {
> + if (sym->st_shndx != sec_idx)
> + continue;
> + if (ELF64_ST_TYPE(sym->st_info) != sym_type)
> + continue;
> +
> + name = elf_strptr(obj->elf, str_sec_idx, sym->st_name);
> + if (!name)
> + return NULL;
> +
> + if (strcmp(sym_name, name) != 0)
> + continue;
> +
> + return sym;
> + }
> +
> + return NULL;
> +}
> +
> static int find_glob_sym_btf(struct src_obj *obj, Elf64_Sym *sym, const char *sym_name,
> int *out_btf_sec_id, int *out_btf_id)
> {
> @@ -1695,6 +1723,7 @@ static int find_glob_sym_btf(struct src_obj *obj, Elf64_Sym *sym, const char *sy
> const struct btf_type *t;
> const struct btf_var_secinfo *vi;
> const char *name;
> + Elf64_Sym *s;
>
> if (!obj->btf) {
> pr_warn("failed to find BTF info for object '%s'\n", obj->filename);
> @@ -1710,8 +1739,15 @@ static int find_glob_sym_btf(struct src_obj *obj, Elf64_Sym *sym, const char *sy
> */
> if (btf_is_non_static(t)) {
> name = btf__str_by_offset(obj->btf, t->name_off);
> - if (strcmp(name, sym_name) != 0)
> - continue;
> + if (strcmp(name, sym_name) != 0) {
> + /* the symbol that we look for may not have BTF as it may
> + * be an alias of another symbol; we check if this is
> + * the original symbol and if so, we use its BTF id
> + */
> + s = find_sym_by_name(obj, sym->st_shndx, STT_FUNC, name);
> + if (!s || s->st_value != sym->st_value)
> + continue;
> + }
>
> /* remember and still try to find DATASEC */
> btf_id = i;
> @@ -2132,34 +2168,6 @@ static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *ob
> return 0;
> }
>
> -static Elf64_Sym *find_sym_by_name(struct src_obj *obj, size_t sec_idx,
> - int sym_type, const char *sym_name)
> -{
> - struct src_sec *symtab = &obj->secs[obj->symtab_sec_idx];
> - Elf64_Sym *sym = symtab->data->d_buf;
> - int i, n = symtab->shdr->sh_size / symtab->shdr->sh_entsize;
> - int str_sec_idx = symtab->shdr->sh_link;
> - const char *name;
> -
> - for (i = 0; i < n; i++, sym++) {
> - if (sym->st_shndx != sec_idx)
> - continue;
> - if (ELF64_ST_TYPE(sym->st_info) != sym_type)
> - continue;
> -
> - name = elf_strptr(obj->elf, str_sec_idx, sym->st_name);
> - if (!name)
> - return NULL;
> -
> - if (strcmp(sym_name, name) != 0)
> - continue;
> -
> - return sym;
> - }
> -
> - return NULL;
> -}
> -
> static int linker_fixup_btf(struct src_obj *obj)
> {
> const char *sec_name;
> --
> 2.46.0
>
