On Thu, Dec 5, 2024 at 5:59 AM Quentin Monnet <qmo@xxxxxxxxxx> wrote:
>
> Libelf functions do not set errno on failure. Instead, it relies on its
> internal _elf_errno value, that can be retrieved via elf_errno (or the
> corresponding message via elf_errmsg()). From "man libelf":
>
> If a libelf function encounters an error it will set an internal
> error code that can be retrieved with elf_errno. Each thread
> maintains its own separate error code. The meaning of each error
> code can be determined with elf_errmsg, which returns a string
> describing the error.
>
> As a consequence, libbpf should not return -errno when a function from
> libelf fails, because an empty value will not be interpreted as an error
> and won't prevent the program to stop. This is visible in
> bpf_linker__add_file(), for example, where we call a succession of
> functions that rely on libelf:
>
> err = err ?: linker_load_obj_file(linker, filename, opts, &obj);
> err = err ?: linker_append_sec_data(linker, &obj);
> err = err ?: linker_append_elf_syms(linker, &obj);
> err = err ?: linker_append_elf_relos(linker, &obj);
> err = err ?: linker_append_btf(linker, &obj);
> err = err ?: linker_append_btf_ext(linker, &obj);
>
> If the object file that we try to process is not, in fact, a correct
> object file, linker_load_obj_file() may fail with errno not being set,
> and return 0. In this case we attempt to run linker_append_elf_sysms()
> and may segfault.
>
> This can happen (and was discovered) with bpftool:
>
> $ bpftool gen object output.o sample_ret0.bpf.c
> libbpf: failed to get ELF header for sample_ret0.bpf.c: invalid `Elf' handle
> zsh: segmentation fault (core dumped) bpftool gen object output.o sample_ret0.bpf.c
>
> Fix the issue by returning a non-null error code (-EINVAL) when libelf
> functions fail.
>
> Fixes: faf6ed321cf6 ("libbpf: Add BPF static linker APIs")
> Signed-off-by: Quentin Monnet <qmo@xxxxxxxxxx>
> ---
> tools/lib/bpf/linker.c | 22 ++++++++--------------
> 1 file changed, 8 insertions(+), 14 deletions(-)
>
Ok, so *this* is the real issue with SIGSEGV that we were trying to
"prevent" by file path comparison in that bpftool-specific patch,
right? LGTM, I'll apply to bpf-next.
> diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
> index cf71d149fe26..e56ba6e67451 100644
> --- a/tools/lib/bpf/linker.c
> +++ b/tools/lib/bpf/linker.c
> @@ -566,17 +566,15 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
> }
> obj->elf = elf_begin(obj->fd, ELF_C_READ_MMAP, NULL);
> if (!obj->elf) {
> - err = -errno;
> pr_warn_elf("failed to parse ELF file '%s'", filename);
> - return err;
> + return -EINVAL;
> }
>
> /* Sanity check ELF file high-level properties */
> ehdr = elf64_getehdr(obj->elf);
> if (!ehdr) {
> - err = -errno;
> pr_warn_elf("failed to get ELF header for %s", filename);
> - return err;
> + return -EINVAL;
> }
>
> /* Linker output endianness set by first input object */
> @@ -606,9 +604,8 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
> }
>
> if (elf_getshdrstrndx(obj->elf, &obj->shstrs_sec_idx)) {
> - err = -errno;
> pr_warn_elf("failed to get SHSTRTAB section index for %s", filename);
> - return err;
> + return -EINVAL;
> }
>
> scn = NULL;
> @@ -618,26 +615,23 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
>
> shdr = elf64_getshdr(scn);
> if (!shdr) {
> - err = -errno;
> pr_warn_elf("failed to get section #%zu header for %s",
> sec_idx, filename);
> - return err;
> + return -EINVAL;
> }
>
> sec_name = elf_strptr(obj->elf, obj->shstrs_sec_idx, shdr->sh_name);
> if (!sec_name) {
> - err = -errno;
> pr_warn_elf("failed to get section #%zu name for %s",
> sec_idx, filename);
> - return err;
> + return -EINVAL;
> }
>
> data = elf_getdata(scn, 0);
> if (!data) {
> - err = -errno;
> pr_warn_elf("failed to get section #%zu (%s) data from %s",
> sec_idx, sec_name, filename);
> - return err;
> + return -EINVAL;
> }
>
> sec = add_src_sec(obj, sec_name);
> @@ -2680,14 +2674,14 @@ int bpf_linker__finalize(struct bpf_linker *linker)
>
> /* Finalize ELF layout */
> if (elf_update(linker->elf, ELF_C_NULL) < 0) {
> - err = -errno;
> + err = -EINVAL;
> pr_warn_elf("failed to finalize ELF layout");
> return libbpf_err(err);
> }
>
> /* Write out final ELF contents */
> if (elf_update(linker->elf, ELF_C_WRITE) < 0) {
> - err = -errno;
> + err = -EINVAL;
> pr_warn_elf("failed to write ELF contents");
> return libbpf_err(err);
> }
> --
> 2.43.0
>
