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

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Date:   Wed, 7 Nov 2018 19:42:46 +0100
From:   Artem Savkov <asavkov@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...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 11:08:56AM -0600, Josh Poimboeuf wrote:
> 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)?

find_symbol_by_name() traverses the same lists as read_symbols and when
we change sym->name in place without copying it it changes in the list
as well. So if child function is before parent in sec->symbol_list the
same function will be returned as "parent". It is hard for me to put it
into words worthy to be included into commit message.

> I haven't seen any bug reports, is it purely theoretical?

No, 4.20-rc1 (actually anything after 4a60aa05a063 "objtool: Support
per-function rodata sections", before that add_switch_table() doesn't
seem to be called for those .cold. funcs) fails to build for mewith
KCFLAGS="-ffunction-sections -fdata-sections" because objtool is
segfaulting.

> > 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

-- 
Regards,
  Artem