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

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Message-ID: <20181107202942.s24ej5mnh4b3mflw@treble>
Date:   Wed, 7 Nov 2018 14:29:42 -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 07:42:46PM +0100, Artem Savkov wrote:
> 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".

Ah, I see.

> It is hard for me to put it into words worthy to be included into
> commit message.

The above description made sense to me, so it sounds like you're on the
right path :-)

> > 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.

If you only see it triggered with -ffunction-sections, that would be
another useful nugget for the commit log.

Also, if it's fixing a regression, be sure to add the 'Fixes' tag.

Thanks!

-- 
Josh