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Date:   Tue, 25 Jul 2023 18:21:04 +0100
From:   Will Deacon <will@kernel.org>
To:     Josh Poimboeuf <jpoimboe@kernel.org>
Cc:     linux-kernel@vger.kernel.org, kernel-team@android.com,
        John Stultz <jstultz@google.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 1/2] scripts/faddr2line: Ignore non-function symbols in
 readelf output
Message-ID: <20230725172104.GA16719@willie-the-truck>
References: <20230724174517.15736-1-will@kernel.org>
 <20230724174517.15736-2-will@kernel.org>
 <20230724234734.zy67gm674vl3p3wv@treble>
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On Mon, Jul 24, 2023 at 04:47:34PM -0700, Josh Poimboeuf wrote:
> On Mon, Jul 24, 2023 at 06:45:16PM +0100, Will Deacon wrote:
> > Non-function symbols emitted in the readelf output can confuse the
> > 'faddr2line' symbol size calculation, resulting in the erroneous
> > rejection of valid offsets. This is especially prevalent when building
> > an arm64 kernel with CONFIG_CFI_CLANG=y, where most functions are
> > prefixed with a 32-bit data value in a '$d.n' section. For example:
> > 
> > 447538: ffff800080014b80   548 FUNC    GLOBAL DEFAULT    2 do_one_initcall
> >    104: ffff800080014c74     0 NOTYPE  LOCAL  DEFAULT    2 $x.73
> >    106: ffff800080014d30     0 NOTYPE  LOCAL  DEFAULT    2 $x.75
> >    111: ffff800080014da4     0 NOTYPE  LOCAL  DEFAULT    2 $d.78
> >    112: ffff800080014da8     0 NOTYPE  LOCAL  DEFAULT    2 $x.79
> >     36: ffff800080014de0   200 FUNC    LOCAL  DEFAULT    2 run_init_process
> > 
> > Adding a warning to do_one_initcall() results in:
> > 
> >   | WARNING: CPU: 0 PID: 1 at init/main.c:1236 do_one_initcall+0xf4/0x260
> > 
> > Which 'faddr2line' refuses to accept:
> > 
> > $ ./scripts/faddr2line vmlinux do_one_initcall+0xf4/0x260
> > skipping do_one_initcall address at 0xffff800080014c74 due to size mismatch (0x260 != 0x224)
> > no match for do_one_initcall+0xf4/0x260
> > 
> > Filter out entries from readelf that are not "FUNC" type, so that the
> > size of a symbol is calculated as a delta to the next function address.
> 
> Problem is, I think the kernel's symbol printing code prints the nearest
> kallsyms symbol, and there are some valid non-FUNC code symbols.  For
> example, syscall_return_via_sysret.  This patch would cause those valid
> symbols to get missed.

Damn, yes, I can see a few of those in the arm64 vmlinux too. Thanks for
pointing it out.

> I presume these '$x.*' symbols don't end up in System.map?  How do they
> get filtered out?  By the linker maybe?

See the horrible ad-hoc list of ignored symbol types which get regexed
out by scripts/mksysmap.

> We may want to use whatever criteria used there, e.g. have faddr2line
> ignore symbols starting with '$' or so?

My first hack for this did exactly that (ignore symbols starting with '$'),
but then I tried to find ways to make it a bit more general. Lemme have a
crack at factoring out the stuff from mksysmap before I resort back to
checking for the '$' prefix.

Will