linux-kernel - Re: [PATCH 1/3] objtool: use gelf_getsymshndx to handle >64k sections

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Date:   Tue, 21 Apr 2020 13:11:47 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] objtool: use gelf_getsymshndx to handle >64k sections

On Tue, Apr 21, 2020 at 11:07:22AM -0700, Sami Tolvanen wrote:
> Currently, objtool fails to load the correct section for symbols when
> the index is greater than SHN_LORESERVE. Use gelf_getsymshndx instead
> of gelf_getsym to handle >64k sections.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  tools/objtool/elf.c | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index 09ddc8f1def3..887445e87380 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -327,12 +327,14 @@ static int read_sections(struct elf *elf)
>  
>  static int read_symbols(struct elf *elf)
>  {
> -	struct section *symtab, *sec;
> +	struct section *symtab, *symtab_shndx, *sec;
>  	struct symbol *sym, *pfunc;
>  	struct list_head *entry;
>  	struct rb_node *pnode;
>  	int symbols_nr, i;
>  	char *coldstr;
> +	Elf_Data *shndx_data = NULL;
> +	Elf32_Word shndx;
>  
>  	symtab = find_section_by_name(elf, ".symtab");
>  	if (!symtab) {
> @@ -340,6 +342,10 @@ static int read_symbols(struct elf *elf)
>  		return -1;
>  	}
>  
> +	symtab_shndx = find_section_by_name(elf, ".symtab_shndx");
> +	if (symtab_shndx)
> +		shndx_data = symtab_shndx->data;
> +
>  	symbols_nr = symtab->sh.sh_size / symtab->sh.sh_entsize;
>  
>  	for (i = 0; i < symbols_nr; i++) {
> @@ -353,8 +359,9 @@ static int read_symbols(struct elf *elf)
>  
>  		sym->idx = i;
>  
> -		if (!gelf_getsym(symtab->data, i, &sym->sym)) {
> -			WARN_ELF("gelf_getsym");
> +		if (!gelf_getsymshndx(symtab->data, shndx_data, i, &sym->sym,
> +				      &shndx)) {
> +			WARN_ELF("gelf_getsymshndx");
>  			goto err;
>  		}
>  
> @@ -368,10 +375,13 @@ static int read_symbols(struct elf *elf)
>  		sym->type = GELF_ST_TYPE(sym->sym.st_info);
>  		sym->bind = GELF_ST_BIND(sym->sym.st_info);
>  
> -		if (sym->sym.st_shndx > SHN_UNDEF &&
> -		    sym->sym.st_shndx < SHN_LORESERVE) {
> -			sym->sec = find_section_by_index(elf,
> -							 sym->sym.st_shndx);
> +		if ((sym->sym.st_shndx > SHN_UNDEF &&
> +		     sym->sym.st_shndx < SHN_LORESERVE) ||
> +		    (shndx_data && sym->sym.st_shndx == SHN_XINDEX)) {
> +			if (sym->sym.st_shndx != SHN_XINDEX)
> +				shndx = sym->sym.st_shndx;
> +
> +			sym->sec = find_section_by_index(elf, shndx);
>  			if (!sym->sec) {
>  				WARN("couldn't find section for symbol %s",
>  				     sym->name);
> -- 
> 2.26.1.301.g55bc3eb7cb9-goog
> 

-- 
Kees Cook