Re: [PATCH] export.h: reduce __ksymtab_strings string duplication by using "MS" section flags

[Jessica Yu <jeyu at kernel dot org>]

+++ Rasmus Villemoes [25/11/19 10:29 +0100]:
> cc += binutils ML
>
> [on @progbits v %progbits in generic (data only) assembly code]
>
> On 22/11/2019 12.44, Masahiro Yamada wrote:
> > On Fri, Nov 22, 2019 at 1:09 AM Jessica Yu <jeyu@kernel.org> wrote:
> > >
>
> > > I think this would work, and it feels like the more correct solution
> > > especially if arm isn't the only one with the odd progbits char. It
> > > might be overkill if it's just arm that's affected though. I leave it
> > > to Masahiro to see what he prefers.
> >
> >
> > BTW, is there any reason why
> > not use %progbits all the time?
> >
> >
> > include/linux/init.h hard-codes %progbits
> >
> >    #define __INITDATA .section ".init.data","aw",%progbits
> >    #define __INITRODATA .section ".init.rodata","a",%progbits
> >
> >
> > So, my understanding is '%' works for all architectures,
> > but it is better to ask 0-day bot to test it.
>
> It seems that you're absolutely right. The binutils source has code like
>
> +             if (*input_line_pointer == '@' || *input_line_pointer == '%')
> +               {
> +                 ++input_line_pointer;
> +                 if (strncmp (input_line_pointer, "progbits",
> +                              sizeof "progbits" - 1) == 0)
> +                   {
> +                     type = SHT_PROGBITS;
> +                     input_line_pointer += sizeof "progbits" - 1;
> +                   }
> +                 else if (strncmp (input_line_pointer, "nobits",
> +                                   sizeof "nobits" - 1) == 0)
> +                   {
> +                     type = SHT_NOBITS;
> +                     input_line_pointer += sizeof "nobits" - 1;
> +                   }

Yeah, I saw this too while digging. I also came across this commit in
the binutils source:

commit ecc054c097e7ced281d02ef9632eb0261a410b96
Author: Thomas Preud'homme <thomas.preudhomme@arm.com>
Date:   Fri Mar 2 11:51:34 2018 +0000

   [GDB/testsuite] Use %progbits in watch-loc.c

   While using @progbits in .pushsection work on some targets, it does not
   work on arm target where this introduces a comment. This patch replaces
   its use in gdb.dlang/watch-loc.c and gdb.mi/dw2-ref-missing-frame-func.c
   by %progbits which should work on all targets since it is used in
   target-independent elf/section7.s GAS test.

   2018-03-02  Thomas Preud'homme  <thomas.preudhomme@arm.com>

   gdb/testsuite/
           * gdb.dlang/watch-loc.c: Use %progbits instead of @progbits.
           * gdb.mi/dw2-ref-missing-frame-func.c: Likewise.

So that seems to confirm this theory :-) I'm just surprised it isn't
documented anywhere it seems.

> The only reason I thought one needed to do it differently on ARM is this
> from the gas docs:
>
> ===
>   The optional TYPE argument may contain one of the following
> constants:
>
> '@progbits'
>     section contains data
> ...
>   Note on targets where the '@' character is the start of a comment (eg
> ARM) then another character is used instead.  For example the ARM port
> uses the '%' character.
> ===
>
> That doesn't suggest the possibility that % or some other character
> might work on all architectures.
>
> Jessica, can you resend using just %progbits to let kbuild chew on that?
> Please include a comment about the misleading gas documentation.

Yup, sounds good. Thanks!

Jessica