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

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Date:   Wed, 7 Nov 2018 11:08:56 -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] objtool: fix .cold. functions parent symbols search

On Wed, Nov 07, 2018 at 03:05:59PM +0100, Artem Savkov wrote:
> The way it is currently done it is possible for read_symbols() to find the
> same symbol as parent for ".cold" functions.

I seem to remember having this discussion for kpatch-build, but I forget
the details.  Can you explain how this can happen (and also add that
detail to the commit message)?  I haven't seen any bug reports, is it
purely theoretical?

> This leads to a bunch of
> complications such as func length being set to 0 and a segfault in
> add_switch_table(). Fix by copying the search string instead of modifying
> it in place.
> 
> Signed-off-by: Artem Savkov <asavkov@...hat.com>
> ---
>  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",
> -- 
> 2.17.2
> 

-- 
Josh