linux-kernel - Re: [PATCH] Revert "scripts/faddr2line: Check only two symbols when calculating symbol size"

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Message-ID: <CAOZ5it0ih05MBMzQxKuD8LYTL-by5qppjj_YXqA6iuyikm0_6w@mail.gmail.com>
Date: Tue, 20 Aug 2024 09:38:46 -0700
From: Brian Johannesmeyer <bjohannesmeyer@...il.com>
To: Will Deacon <will@...nel.org>
Cc: Carlos Llamas <cmllamas@...gle.com>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	linux-kernel@...r.kernel.org, kernel-team@...roid.com, 
	John Stultz <jstultz@...gle.com>
Subject: Re: [PATCH] Revert "scripts/faddr2line: Check only two symbols when
 calculating symbol size"

On Fri, Aug 16, 2024 at 5:50 AM Will Deacon <will@...nel.org> wrote:
>
> On Mon, Aug 12, 2024 at 11:01:20PM +0000, Carlos Llamas wrote:
> > This reverts commit c02904f05ff805d6c0631634d5751ebd338f75ec.
> >
> > Such commit assumed that only two symbols are relevant for the symbol
> > size calculation. However, this can lead to an incorrect symbol size
> > calculation when there are mapping symbols emitted by readelf.
> >
> > For instance, when feeding 'update_irq_load_avg+0x1c/0x1c4', faddr2line
> > might need to processes the following readelf lines:
> >
> >  784284: ffffffc0081cca30   428 FUNC    GLOBAL DEFAULT     2 update_irq_load_avg
> >   87319: ffffffc0081ccb0c     0 NOTYPE  LOCAL  DEFAULT     2 $x.62522
> >   87321: ffffffc0081ccbdc     0 NOTYPE  LOCAL  DEFAULT     2 $x.62524
> >   87323: ffffffc0081ccbe0     0 NOTYPE  LOCAL  DEFAULT     2 $x.62526
> >   87325: ffffffc0081ccbe4     0 NOTYPE  LOCAL  DEFAULT     2 $x.62528
> >   87327: ffffffc0081ccbe8     0 NOTYPE  LOCAL  DEFAULT     2 $x.62530
> >   87329: ffffffc0081ccbec     0 NOTYPE  LOCAL  DEFAULT     2 $x.62532
> >   87331: ffffffc0081ccbf0     0 NOTYPE  LOCAL  DEFAULT     2 $x.62534
> >   87332: ffffffc0081ccbf4     0 NOTYPE  LOCAL  DEFAULT     2 $x.62535
> >  783403: ffffffc0081ccbf4   424 FUNC    GLOBAL DEFAULT     2 sched_pelt_multiplier
> >
> > The symbol size of 'update_irq_load_avg' should be calculated with the
> > address of 'sched_pelt_multiplier', after skipping the mapping symbols
> > seen in between. However, the offending commit cuts the list short and
> > faddr2line incorrectly assumes 'update_irq_load_avg' is the last symbol
> > in the section, resulting in:
> >
> >   $ scripts/faddr2line vmlinux update_irq_load_avg+0x1c/0x1c4
> >   skipping update_irq_load_avg address at 0xffffffc0081cca4c due to size mismatch (0x1c4 != 0x3ff9a59988)
> >   no match for update_irq_load_avg+0x1c/0x1c4
> >
> > After reverting the commit the issue is resolved:
> >
> >   $ scripts/faddr2line vmlinux update_irq_load_avg+0x1c/0x1c4
> >   update_irq_load_avg+0x1c/0x1c4:
> >   cpu_of at kernel/sched/sched.h:1109
> >   (inlined by) update_irq_load_avg at kernel/sched/pelt.c:481
> >
> > Cc: Brian Johannesmeyer <bjohannesmeyer@...il.com>
> > Cc: John Stultz <jstultz@...gle.com>
> > Cc: Will Deacon <will@...nel.org>
> > Signed-off-by: Carlos Llamas <cmllamas@...gle.com>
> > ---
> >  scripts/faddr2line | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
>
> Acked-by: Will Deacon <will@...nel.org>
>
> I'm assuming that Josh will pick this up.
>
> Cheers,
>
> Will

Acked-by: Brian Johannesmeyer <bjohannesmeyer@...il.com>

Good catch. Thanks for the fix.

Just curious: What compiler did you use to build the kernel, where you
hit this error with 'update_irq_load_avg'? I was unable to reproduce
it with LLVM 14. Thanks!

-Brian


-Brian