Re: [PATCH v4 3/3] scripts/faddr2line: Skip over mapping symbols in output from readelf

From: Will Deacon <will@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: linux-kernel@vger.kernel.org, kernel-team@android.com,
	Masahiro Yamada <masahiroy@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nicolas Schier <nicolas@fjasle.eu>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	John Stultz <jstultz@google.com>,
	linux-kbuild@vger.kernel.org
Subject: Re: [PATCH v4 3/3] scripts/faddr2line: Skip over mapping symbols in output from readelf
Date: Mon, 2 Oct 2023 16:26:45 +0100	[thread overview]
Message-ID: <20231002152644.GA1519@willie-the-truck> (raw)
In-Reply-To: <CAKwvOd=gDX4ebkyHyqr276nrZVuRaoJG9Ptofpq8WjejD3s5AA@mail.gmail.com>

On Mon, Sep 18, 2023 at 08:46:22AM -0700, Nick Desaulniers wrote:
> On Thu, Sep 14, 2023 at 6:12 AM Will Deacon <will@kernel.org> wrote:
> >
> > Mapping 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 these entries from readelf using a shell reimplementation of
> > is_mapping_symbol(), so that the size of a symbol is calculated as a
> > delta to the next symbol present in ksymtab.
> >
> > Cc: Josh Poimboeuf <jpoimboe@kernel.org>
> > Cc: John Stultz <jstultz@google.com>
> > Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  scripts/faddr2line | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/scripts/faddr2line b/scripts/faddr2line
> > index 6b8206802157..20d9b3d37843 100755
> > --- a/scripts/faddr2line
> > +++ b/scripts/faddr2line
> > @@ -179,6 +179,11 @@ __faddr2line() {
> >                         local cur_sym_elf_size=${fields[2]}
> >                         local cur_sym_name=${fields[7]:-}
> >
> > +                       # is_mapping_symbol(cur_sym_name)
> > +                       if [[ ${cur_sym_name} =~ ^((\.L)|(L0)|(\$[adtx](\.|$))) ]]; then
> 
> Thanks for the patch!
> 
> I'm curious about the `|$` in the final part of the regex.  IIUC that
> will match something like
> $a
> Do we have any such symbols without `.<n>` suffixes?

tbh, I just blindly followed the implementation of is_mapping_symbol()
at the time, but Masahiro has since pointed out that it's been
significantly simplified so this regex should get much more manageable
in the next version.

Will