[PATCH] elfnote: mark all .note sections SHF_ALLOC

[Nick Desaulniers]

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 <jer...@xensource.com>
Debugged-by: Ilie Halip <ilie....@gmail.com>
Signed-off-by: Nick Desaulniers <ndesau...@google.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

[Nick Desaulniers]

dropping Jeremy; I got bounceback from the email address. Ping for review?

--
Thanks,
~Nick Desaulniers

[Nathan Chancellor]

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
> <ndesau...@google.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 <jer...@xensource.com>
> > Debugged-by: Ilie Halip <ilie....@gmail.com>
> > Signed-off-by: Nick Desaulniers <ndesau...@google.com>

Reviewed-by: Nathan Chancellor <natecha...@gmail.com>

[Fangrui Song]

I asked on binu...@sourceware.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 <mas...@google.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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[Gulshan Kumar]

unsubscribe

[Nick Desaulniers]

(bumping for review)

--
Thanks,
~Nick Desaulniers