Re: [PATCH v2 bpf-next 09/17] libbpf: extend sanity checking ELF symbols with externs validation
From: Yonghong Song <hidden>
Date: 2021-04-22 16:35:24
Also in:
bpf
On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
quoted hunk ↗ jump to hunk
Add logic to validate extern symbols, plus some other minor extra checks, like ELF symbol #0 validation, general symbol visibility and binding validations. Signed-off-by: Andrii Nakryiko <andrii@kernel.org> --- tools/lib/bpf/linker.c | 43 +++++++++++++++++++++++++++++++++--------- 1 file changed, 34 insertions(+), 9 deletions(-)diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c index 1263641e8b97..283249df9831 100644 --- a/tools/lib/bpf/linker.c +++ b/tools/lib/bpf/linker.c@@ -750,14 +750,39 @@ static int linker_sanity_check_elf_symtab(struct src_obj *obj, struct src_sec *s n = sec->shdr->sh_size / sec->shdr->sh_entsize; sym = sec->data->d_buf; for (i = 0; i < n; i++, sym++) { - if (sym->st_shndx - && sym->st_shndx < SHN_LORESERVE - && sym->st_shndx >= obj->sec_cnt) { + int sym_type = ELF64_ST_TYPE(sym->st_info); + int sym_bind = ELF64_ST_BIND(sym->st_info); + + if (i == 0) { + if (sym->st_name != 0 || sym->st_info != 0 + || sym->st_other != 0 || sym->st_shndx != 0 + || sym->st_value != 0 || sym->st_size != 0) { + pr_warn("ELF sym #0 is invalid in %s\n", obj->filename); + return -EINVAL; + } + continue; + }
In ELF file, the first entry of symbol table and section table (index 0)
is invalid/undefined.
Symbol table '.symtab' contains 9 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
Section Headers:
[Nr] Name Type Address Off Size
ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000
00 0 0 0
Instead of validating them, I think we can skip traversal of the index =
0 entry for symbol table and section header table. What do you think?
quoted hunk ↗ jump to hunk
+ if (sym_bind != STB_LOCAL && sym_bind != STB_GLOBAL && sym_bind != STB_WEAK) { + pr_warn("ELF sym #%d is section #%zu has unsupported symbol binding %d\n", + i, sec->sec_idx, sym_bind); + return -EINVAL; + } + if (sym->st_shndx == 0) { + if (sym_type != STT_NOTYPE || sym_bind == STB_LOCAL + || sym->st_value != 0 || sym->st_size != 0) { + pr_warn("ELF sym #%d is invalid extern symbol in %s\n", + i, obj->filename); + + return -EINVAL; + } + continue; + } + if (sym->st_shndx < SHN_LORESERVE && sym->st_shndx >= obj->sec_cnt) { pr_warn("ELF sym #%d in section #%zu points to missing section #%zu in %s\n", i, sec->sec_idx, (size_t)sym->st_shndx, obj->filename); return -EINVAL; } - if (ELF64_ST_TYPE(sym->st_info) == STT_SECTION) { + if (sym_type == STT_SECTION) { if (sym->st_value != 0) return -EINVAL; continue;@@ -1135,16 +1160,16 @@ static int linker_append_elf_syms(struct bpf_linker *linker, struct src_obj *obj size_t dst_sym_idx; int name_off; - /* we already have all-zero initial symbol */ - if (sym->st_name == 0 && sym->st_info == 0 && - sym->st_other == 0 && sym->st_shndx == SHN_UNDEF && - sym->st_value == 0 && sym->st_size ==0) + /* We already validated all-zero symbol #0 and we already + * appended it preventively to the final SYMTAB, so skip it. + */ + if (i == 0) continue; sym_name = elf_strptr(obj->elf, str_sec_idx, sym->st_name); if (!sym_name) { pr_warn("can't fetch symbol name for symbol #%d in '%s'\n", i, obj->filename); - return -1; + return -EINVAL; } if (sym->st_shndx && sym->st_shndx < SHN_LORESERVE) {