6.12-stable review patch. If anyone has any objections, please let me know.
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From: Quentin Monnet qmo@kernel.org
[ Upstream commit e10500b69c3f3378f3dcfc8c2fe4cdb74fc844f5 ]
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@kernel.org Signed-off-by: Andrii Nakryiko andrii@kernel.org Link: https://lore.kernel.org/bpf/20241205135942.65262-1-qmo@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- tools/lib/bpf/linker.c | 22 ++++++++-------------- 1 file changed, 8 insertions(+), 14 deletions(-)
diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c index 6985ab0f1ca9e..777600822d8e4 100644 --- a/tools/lib/bpf/linker.c +++ b/tools/lib/bpf/linker.c @@ -567,17 +567,15 @@ 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; } if (ehdr->e_ident[EI_DATA] != host_endianness) { err = -EOPNOTSUPP; @@ -593,9 +591,8 @@ 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; @@ -605,26 +602,23 @@ 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); @@ -2635,14 +2629,14 @@ 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); }