Re: [PATCH] powerpc: Replace ppc64 DT_RELACOUNT usage with DT_RELASZ/24

[Alexey Kardashevskiy]

On 3/11/22 15:15, Michael Ellerman wrote:
> Fāng-ruì Sòng <maskray@xxxxxxxxxx> writes:
> > On Thu, Mar 10, 2022 at 11:48 AM Nick Desaulniers
> > <ndesaulniers@xxxxxxxxxx> wrote:
> > > On Tue, Mar 8, 2022 at 9:53 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:
> > > > DT_RELACOUNT is an ELF dynamic tag inherited from SunOS indicating the
> > > > number of R_*_RELATIVE relocations. It is optional but {ld.lld,ld.lld}
> > > > -z combreloc always creates it (if non-zero) to slightly speed up glibc
> > > > ld.so relocation resolving by avoiding R_*R_PPC64_RELATIVE type
> > > > comparison. The tag is otherwise nearly unused in the wild and I'd
> > > > recommend that software avoids using it.
> > > >
> > > > lld>=14.0.0 (since commit da0e5b885b25cf4ded0fa89b965dc6979ac02ca9)
> > > > underestimates DT_RELACOUNT for ppc64 when position-independent long
> > > > branch thunks are used. Correcting it needs non-trivial arch-specific
> > > > complexity which I'd prefer to avoid. Since our code always compares the
> > > > relocation type with R_PPC64_RELATIVE, replacing every occurrence of
> > > > DT_RELACOUNT with DT_RELASZ/sizeof(Elf64_Rela)=DT_RELASZ/24 is a correct
> > > > alternative.
> > >
> > > checking that sizeof(Elf64_Rela) == 24, yep: https://godbolt.org/z/bb4aKbo5T
> > >
> > > > DT_RELASZ is in practice bounded by an uint32_t. Dividing x by 24 can be
> > > > implemented as (uint32_t)(x*0xaaaaaaab) >> 4.
> > >
> > > Yep: https://godbolt.org/z/x9445ePPv
> > >
> > > > Link: https://github.com/ClangBuiltLinux/linux/issues/1581
> > > > Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> > > > Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
> > > > ---
> > > >   arch/powerpc/boot/crt0.S       | 28 +++++++++++++++++-----------
> > > >   arch/powerpc/kernel/reloc_64.S | 15 +++++++++------
> > > >   2 files changed, 26 insertions(+), 17 deletions(-)
> ...
>
> > I rebased the patch on
> > git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
> > master and got a conflict.
> > Seems that https://lore.kernel.org/linuxppc-dev/20220309061822.168173-1-aik@xxxxxxxxx/T/#u
> > ("[PATCH kernel v4] powerpc/64: Add UADDR64 relocation support") fixed
> > the issue.
> > It just doesn't change arch/powerpc/boot/crt0.S
>
> Yeah sorry, I applied Alexey's v4 just before I saw your patch arrive on
> the list.
>
> If one of you can rework this so it applies on top that would be great :)


I guess it is me as now I have to add that UARRD64 thing to crt0.S as 
well, don't I?

And also we are giving up on the llvm ld having a bug with RELACOUNT?