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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 20 Jan 2021 10:57:03 -0800
Message-ID: <CAKwvOd=HNuB8bqJvXEjvYWorika99QX=jKSfHy2hf0NOKrrc8w@mail.gmail.com>
Subject: Re: [PATCH] checkpatch: add warning for avoiding .L prefix symbols in
 assembly files
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Aditya <yashsri421@gmail.com>, LKML <linux-kernel@vger.kernel.org>,
        Lukas Bulwahn <lukas.bulwahn@gmail.com>,
        dwaipayanray1@gmail.com, Mark Brown <broonie@kernel.org>,
        linux-kernel-mentees@lists.linuxfoundation.org,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Joe Perches <joe@perches.com>
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Precedence: bulk

On Wed, Jan 20, 2021 at 10:43 AM Joe Perches <joe@perches.com> wrote:
>
> On Wed, 2021-01-20 at 18:23 +0530, Aditya wrote:
> > On 20/1/21 2:51 pm, Joe Perches wrote:
> > > On Wed, 2021-01-20 at 12:55 +0530, Aditya Srivastava wrote:
> > > > Local symbols prefixed with '.L' do not emit symbol table entries, as
> > > > they have special meaning for the assembler.
> > > >
> > > > '.L' prefixed symbols can be used within a code region, but should be
> > > > avoided for denoting a range of code via 'SYM_*_START/END' annotations.
> > > >
> > > > Add a new check to emit a warning on finding the usage of '.L' symbols
> > > > in '.S' files, if it lies within SYM_*_START/END annotation pair.
> > >
> > > I believe this needs a test for $file as it won't work well on
> > > patches as the SYM_*_START/END may not be in the patch context.
> > >
> > Okay.
> >
> > > Also, is this supposed to work for local labels like '.L<foo>:'?
> > > I don't think a warning should be generated for those.
> > >
> > Yes, currently it will generate warning for all symbols which start
> > with .L and have non- white character symbol following it, if it is
> > lying within SYM_*_START/END annotation pair.
> >
> > Should I reduce the check to \.L_\S+ instead? (please note "_"
> > following ".L")
>
> Use grep first.  That would still match several existing labels.
>
> > Pardon me, I'm not good with assembly :/
>
> Spending time reading docs can help with that.
>
> Mark?  Can you please comment about the below?
>
> I believe the test should be:
>
>         if ($realfile =~ /\.S$/ &&
>             $line =~ /^\+\s*SYM_[A-Z]+_(?:START|END)(?:_[A-Z_]+)?\s*\(\s*\.L/) {
>                 WARN(...);
>         }
>
> so that only this code currently matches:
>
> $ git grep -P '^\s*SYM_[A-Z]+_(?:START|END)(?:_[A-Z_]+)?\s*\(\s*\.L' -- '*.S'
> arch/x86/boot/compressed/head_32.S:SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
> arch/x86/boot/compressed/head_32.S:SYM_FUNC_END(.Lrelocated)
> arch/x86/boot/compressed/head_64.S:SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
> arch/x86/boot/compressed/head_64.S:SYM_FUNC_END(.Lrelocated)
> arch/x86/boot/compressed/head_64.S:SYM_FUNC_START_LOCAL_NOALIGN(.Lpaging_enabled)
> arch/x86/boot/compressed/head_64.S:SYM_FUNC_END(.Lpaging_enabled)
> arch/x86/boot/compressed/head_64.S:SYM_FUNC_START_LOCAL_NOALIGN(.Lno_longmode)
> arch/x86/boot/compressed/head_64.S:SYM_FUNC_END(.Lno_longmode)
> arch/x86/boot/pmjump.S:SYM_FUNC_START_LOCAL_NOALIGN(.Lin_pm32)
> arch/x86/boot/pmjump.S:SYM_FUNC_END(.Lin_pm32)
> arch/x86/entry/entry_64.S:SYM_CODE_START_LOCAL_NOALIGN(.Lbad_gs)
> arch/x86/entry/entry_64.S:SYM_CODE_END(.Lbad_gs)
> arch/x86/lib/copy_user_64.S:SYM_CODE_START_LOCAL(.Lcopy_user_handle_tail)
> arch/x86/lib/copy_user_64.S:SYM_CODE_END(.Lcopy_user_handle_tail)
> arch/x86/lib/getuser.S:SYM_CODE_START_LOCAL(.Lbad_get_user_clac)
> arch/x86/lib/getuser.S:SYM_CODE_END(.Lbad_get_user_clac)
> arch/x86/lib/getuser.S:SYM_CODE_START_LOCAL(.Lbad_get_user_8_clac)
> arch/x86/lib/getuser.S:SYM_CODE_END(.Lbad_get_user_8_clac)
> arch/x86/lib/putuser.S:SYM_CODE_START_LOCAL(.Lbad_put_user_clac)
> arch/x86/lib/putuser.S:SYM_CODE_END(.Lbad_put_user_clac)
> arch/x86/realmode/rm/wakeup_asm.S:SYM_DATA_START_LOCAL(.Lwakeup_idt)
> arch/x86/realmode/rm/wakeup_asm.S:SYM_DATA_END(.Lwakeup_idt)

Josh, I assume objtool does not annotate code under:
arch/x86/boot/
arch/x86/entry/
arch/x86/realmode/
?

The rest, ie
arch/x86/lib/
seem potentially relevant?
-- 
Thanks,
~Nick Desaulniers