libbpf: Extend sanity checking ELF symbols with externs validation

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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210423181348.1801389-10-andrii@kernel.org

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 1263641..b0e0384 100644 (file)
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -750,14 +750,45 @@ 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);
+               int sym_vis = ELF64_ST_VISIBILITY(sym->st_other);
+
+               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;
+               }
+               if (sym_bind != STB_LOCAL && sym_bind != STB_GLOBAL && sym_bind != STB_WEAK) {
+                       pr_warn("ELF sym #%d in section #%zu has unsupported symbol binding %d\n",
+                               i, sec->sec_idx, sym_bind);
+                       return -EINVAL;
+               }
+               if (sym_vis != STV_DEFAULT && sym_vis != STV_HIDDEN) {
+                       pr_warn("ELF sym #%d in section #%zu has unsupported symbol visibility %d\n",
+                               i, sec->sec_idx, sym_vis);
+                       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 +1166,16 @@ 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) {