Re: [PATCH v2] MIPS: Truncate load-y into 32bit for 32bit kernel

[Fangrui Song]

On 2020-04-07, Nick Desaulniers wrote:
> On Tue, Apr 7, 2020 at 1:07 AM Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx> wrote:
> > LLD failed to link vmlinux with 64bit load address for 32bit ELF
> > while bfd will strip 64bit address into 32bit silently.
> > To fix LLD build, we should truncate load address provided by platform
> > into 32bit for 32bit kernel.
> >
> > Signed-off-by: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>
> > Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
> > Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> >
> > --
> > V2: Take MaskRay's shell magic.
>
> V2 is way too clever, V1 was much more readable.

This is difficult:/
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_04
The POSIX shell is only required to do signed long integer arithmetic.
"signed long" can be 32-bit.

awk may not provide precision more than a double ("... decimal-floating-constant token as specified
by the ISO C standard")

  % gawk 'BEGIN {printf("%x", (0xffffffff80101234 % 0x100000000))}' /dev/null
  80101000

> Can this tag be added to the commit to help us track when and where it lands?
> Link: https://github.com/ClangBuiltLinux/linux/issues/786

And this tag for GNU ld enhancement:

Link: https://sourceware.org/bugzilla/show_bug.cgi?id=25784

> > ---
> >  arch/mips/Makefile | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/mips/Makefile b/arch/mips/Makefile
> > index e1c44aed8156..f8fd3c39fb55 100644
> > --- a/arch/mips/Makefile
> > +++ b/arch/mips/Makefile
> > @@ -286,6 +286,9 @@ ifdef CONFIG_64BIT
> >        $(error CONFIG_CPU_DADDI_WORKAROUNDS unsupported without -msym32)
> >      endif
> >    endif
> > +else
> > +       # Truncate address into 32-bit
> > +       load-y := 0x$(shell echo "$(load-y)" | rev | head -c 8 | rev)
> >  endif
> >
> >  KBUILD_AFLAGS  += $(cflags-y)
> > --
>
> --
> Thanks,
> ~Nick Desaulniers