linux-kernel - Re: [PATCH] elfnote: mark all .note sections SHF

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Date:   Fri, 3 Apr 2020 17:40:20 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Ilie Halip <ilie.halip@...il.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] elfnote: mark all .note sections SHF_ALLOC

On 2020-04-03, Nathan Chancellor wrote:
>On Fri, Apr 03, 2020 at 03:13:34PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
>> dropping Jeremy; I got bounceback from the email address. Ping for review?
>>
>> On Wed, Mar 25, 2020 at 4:13 PM Nick Desaulniers
>> <ndesaulniers@...gle.com> wrote:
>> >
>> > ELFNOTE_START allows callers to specify flags for .pushsection assembler
>> > directives.  All callsites but ELF_NOTE use "a" for SHF_ALLOC. For
>> > vdso's that explicitly use ELF_NOTE_START and BUILD_SALT, the same
>> > section is specified twice after preprocessing, once with "a" flag, once
>> > without. Example:
>> >
>> > .pushsection .note.Linux, "a", @note ;
>> > .pushsection .note.Linux, "", @note ;
>> >
>> > While GNU as allows this ordering, it warns for the opposite ordering,
>> > making these directives position dependent. We'd prefer not to precisely
>> > match this behavior in Clang's integrated assembler.  Instead, the non
>> > __ASSEMBLY__ definition of ELF_NOTE uses
>> > __attribute__((section(".note.Linux"))) which is created with SHF_ALLOC,
>> > so let's make the __ASSEMBLY__ definition of ELF_NOTE consistent with C
>> > and just always use "a" flag.
>> >
>> > This allows Clang to assemble a working mainline (5.6) kernel via:
>> > $ make CC=clang AS=clang
>> >
>> > Link: https://github.com/ClangBuiltLinux/linux/issues/913
>> > Cc: Jeremy Fitzhardinge <jeremy@...source.com>
>> > Debugged-by: Ilie Halip <ilie.halip@...il.com>
>> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
>Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

I asked on binutils@...rceware.org whether GNU as is willing to change.
https://sourceware.org/pipermail/binutils/2020-March/109997.html
I'll also ping that thread.


Reviewed-by: Fangrui Song <maskray@...gle.com>

>> > ---
>> > Ilie has further treewide cleanups:
>> > https://github.com/ihalip/linux/commits/elfnote
>> > This patch is the simplest to move us forwards.
>> >
>> >  include/linux/elfnote.h | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/include/linux/elfnote.h b/include/linux/elfnote.h
>> > index 594d4e78654f..69b136e4dd2b 100644
>> > --- a/include/linux/elfnote.h
>> > +++ b/include/linux/elfnote.h
>> > @@ -54,7 +54,7 @@
>> >  .popsection                            ;
>> >
>> >  #define ELFNOTE(name, type, desc)              \
>> > -       ELFNOTE_START(name, type, "")           \
>> > +       ELFNOTE_START(name, type, "a")          \
>> >                 desc                    ;       \
>> >         ELFNOTE_END
>> >
>> > --
>> > 2.26.0.rc2.310.g2932bb562d-goog
>> >
>>
>>
>> --
>> Thanks,
>> ~Nick Desaulniers
>
>
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