[PATCH] kbuild: linker script do not match C names unless LD_DEAD_CODE_DATA_ELIMINATION is configured

[Nicholas Piggin]

On Tue, 8 Aug 2017 12:42:18 +0900
Masahiro Yamada <yamada.masahiro at socionext.com> wrote:

> Hi Nicholas,
> 
> 2017-08-08 12:19 GMT+09:00 Nicholas Piggin <npiggin at gmail.com>:
> > On Tue, 8 Aug 2017 10:53:56 +0900
> > Masahiro Yamada <yamada.masahiro at socionext.com> wrote:
> >  
> >> Hi Nicholas,
> >>
> >> 2017-07-26 21:46 GMT+09:00 Nicholas Piggin <npiggin at gmail.com>:  
> >> > The .data and .bss sections were modified in the generic linker script to
> >> > pull in sections named .data.<C identifier>, which are generated by gcc with
> >> > -ffunction-sections and -fdata-sections options.
> >> >
> >> > The problem with this pattern is it can also match section names that Linux
> >> > defines explicitly, e.g., .data.unlikely. This can cause Linux sections to
> >> > get moved into the wrong place.
> >> >
> >> > The way to avoid this is to use ".." separators for explicit section names
> >> > (the dot character is valid in a section name but not a C identifier).
> >> > However currently there are sections which don't follow this rule, so for
> >> > now just disable the wild card by default.
> >> >
> >> > Example: http://marc.info/?l=linux-arm-kernel&m=150106824024221&w=2
> >> >
> >> > Cc: <stable at vger.kernel.org> # 4.9
> >> > Fixes: b67067f1176df ("kbuild: allow archs to select link dead code/data elimination")
> >> > Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> >> > ---
> >> >  include/asm-generic/vmlinux.lds.h | 38 ++++++++++++++++++++++++++------------
> >> >  1 file changed, 26 insertions(+), 12 deletions(-)
> >> >
> >> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> >> > index da0be9a8d1de..9623d78f8494 100644
> >> > --- a/include/asm-generic/vmlinux.lds.h
> >> > +++ b/include/asm-generic/vmlinux.lds.h
> >> > @@ -60,6 +60,22 @@
> >> >  #define ALIGN_FUNCTION()  . = ALIGN(8)
> >> >
> >> >  /*
> >> > + * LD_DEAD_CODE_DATA_ELIMINATION option enables -fdata-sections, which
> >> > + * generates .data.identifier sections, which need to be pulled in with
> >> > + * .data. We don't want to pull in .data..other sections, which Linux
> >> > + * has defined. Same for text and bss.
> >> > + */
> >> > +#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
> >> > +#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
> >> > +#define DATA_MAIN .data .data.[0-9a-zA-Z_]*
> >> > +#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
> >> > +#else
> >> > +#define TEXT_MAIN .text
> >> > +#define DATA_MAIN .data
> >> > +#define BSS_MAIN .bss
> >> > +#endif  
> >>
> >>
> >> From the git-log, I think you are planning to rename
> >>   .data.unlikely   ->   .data..unlikey
> >> then the problem will be fixed.
> >>
> >> ("git grep data.unlikely" gave me only 7 instances.)
> >>
> >>
> >> Why is this patch necessary?
> >>
> >>
> >> You are trying to migrate some architectures to DCDE,
> >> so this #ifdef does not solve the problem, I think.  
> >
> > Well basically I want to preserve previous behaviour and avoid
> > regressions as far as possible. Patches to rename existing
> > sections I think would not be so suitable for @stable. And
> > there could be other things we've missed.
> >
> > Yes I would like to ensure all our sections use .. separators
> > which (hopefully). Until then I think this is a minimal fix.
> >  
> 
> 
> After you rename the sections, will you revert this patch?
> 
> I think the #ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
> should not be permanent.

Once we have the code reasonably well used on a number of archs,
I think we should remove the ifdefs, yes we definitely should be
moving toward sharing common code wherever possible.

Thanks,
Nick