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

Artem Savkov
2018-11-10T12:18:40Z

On Fri, Nov 09, 2018 at 11:23:09AM -0600, Josh Poimboeuf wrote:
> 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@redhat.com>
>
> This needs a "Fixes" tag to identify the patch which introduced the bug.

Ok, will do.

> > ---
> > 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.

Hm, there seems to be no limit on lengths of strings in string table.
What size would you consider reasonable for this stack-allocated string?

--
Regards,
Artem