linux-kernel - Re: [PATCH v2] objtool: fix .cold. functions parent symbols search

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Date:   Fri, 9 Nov 2018 11:23:09 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Artem Savkov <asavkov@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] objtool: fix .cold. functions parent symbols search

On Wed, Nov 07, 2018 at 10:45:15PM +0100, Artem Savkov wrote:
> Because find_symbol_by_name() traverses the same lists as read_symbols()
> changing sym->name in place without copying it affects the result of
> find_symbol_by_name() and, in case when ".cold" function precedes it's
> parent in sec->symbol_list, can result in function being considered a
> parent of itself. This leads to function length being set to 0 and other
> consequent side-effects including a segfault in add_switch_table().
> The effects of this bug are only visible when building with
> -ffunction-sections in KCFLAGS.
> 
> Fix by copying the search string instead of modifying it in place.
> 
> Signed-off-by: Artem Savkov <asavkov@...hat.com>

This needs a "Fixes" tag to identify the patch which introduced the bug.

> ---
>  tools/objtool/elf.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index 6dbb9fae0f9d..781c8afb29b9 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -298,6 +298,7 @@ static int read_symbols(struct elf *elf)
>  	/* Create parent/child links for any cold subfunctions */
>  	list_for_each_entry(sec, &elf->sections, list) {
>  		list_for_each_entry(sym, &sec->symbol_list, list) {
> +			char *pname;
>  			if (sym->type != STT_FUNC)
>  				continue;
>  			sym->pfunc = sym->cfunc = sym;
> @@ -305,9 +306,9 @@ static int read_symbols(struct elf *elf)
>  			if (!coldstr)
>  				continue;
>  
> -			coldstr[0] = '\0';
> -			pfunc = find_symbol_by_name(elf, sym->name);
> -			coldstr[0] = '.';
> +			pname = strndup(sym->name, coldstr - sym->name);
> +			pfunc = find_symbol_by_name(elf, pname);
> +			free(pname);
>  
>  			if (!pfunc) {
>  				WARN("%s(): can't find parent function",

strndup()'s return code needs to be checked.

Also, for such a short-lived allocation, I think a stack-allocated
string would be better.

-- 
Josh