Re: [PATCH v2 bpf-next 09/17] libbpf: extend sanity checking ELF symbols with externs validation

[Yonghong Song <yhs@xxxxxx>]

On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
> 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@xxxxxxxxxx>
> ---
>   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?

> +		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) {