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

From: Fāng-ruì Sòng
Date: Thu Mar 10 2022 - 16:08:26 EST

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(-)
> >
> > diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
> > index feadee18e271..1c96ebe7ef1a 100644
> > --- a/arch/powerpc/boot/crt0.S
> > +++ b/arch/powerpc/boot/crt0.S
> > @@ -8,7 +8,7 @@
> > #include "ppc_asm.h"
> >
> > RELA = 7
> > -RELACOUNT = 0x6ffffff9
> > +RELASZ = 8
> >
> > .data
> > /* A procedure descriptor used when booting this as a COFF file.
> > @@ -65,7 +65,7 @@ p_base: mflr r10 /* r10 now points to runtime addr of p_base */
> > subf r11,r11,r12 /* runtime - linktime offset */
> >
> > /* The dynamic section contains a series of tagged entries.
> > - * We need the RELA and RELACOUNT entries. */
> > + * We need the RELA and RELASZ entries. */
> > li r9,0
> > li r0,0
> > 9: lwz r8,0(r12) /* get tag */
> > @@ -75,18 +75,21 @@ p_base: mflr r10 /* r10 now points to runtime addr of p_base */
> > bne 11f
> > lwz r9,4(r12) /* get RELA pointer in r9 */
> > b 12f
> > -11: addis r8,r8,(-RELACOUNT)@ha
> > - cmpwi r8,RELACOUNT@l
> > +11: cmpwi r8,RELASZ
> > bne 12f
> > - lwz r0,4(r12) /* get RELACOUNT value in r0 */
> > + lwz r0,4(r12) /* get RELASZ / 24 in r0 */
> > + lis r8,0xaaaa
> > + ori r8,r8,0xaaab
> > + mulhwu r0,r0,r8
> > + srwi r0,r0,4
>
> Both translation units include arch/powerpc/include/asm/ppc_asm.h,
> which happens to define LOAD_REG_IMMEDIATE. Let's reuse that rather
> than open code lis+ori?
>
> > 12: addi r12,r12,8
> > b 9b
> >
> > /* The relocation section contains a list of relocations.
> > * We now do the R_PPC_RELATIVE ones, which point to words
> > * which need to be initialized with addend + offset.
> > - * The R_PPC_RELATIVE ones come first and there are RELACOUNT
> > - * of them. */
> > + * The R_PPC_RELATIVE ones come first and there are at most
> > + * RELASZ/24 of them. */
> > 10: /* skip relocation if we don't have both */
> > cmpwi r0,0
> > beq 3f
> > @@ -160,14 +163,17 @@ p_base: mflr r10 /* r10 now points to runtime addr of p_base */
> > bne 10f
> > ld r13,8(r11) /* get RELA pointer in r13 */
> > b 11f
> > -10: addis r12,r12,(-RELACOUNT)@ha
> > - cmpdi r12,RELACOUNT@l
> > +10: cmpdi r12,RELASZ
> > bne 11f
> > - ld r8,8(r11) /* get RELACOUNT value in r8 */
> > + ld r8,8(r11) /* get RELASZ / 24 in r8 */
> > + lis r0,0xaaaa
> > + ori r0,r0,0xaaab
> > + mulhwu r8,r8,r0
> > + srwi r8,r8,4
> > 11: addi r11,r11,16
> > b 9b
> > 12:
> > - cmpdi r13,0 /* check we have both RELA and RELACOUNT */
> > + cmpdi r13,0 /* check we have both RELA and RELASZ */
> > cmpdi cr1,r8,0
> > beq 3f
> > beq cr1,3f
> > diff --git a/arch/powerpc/kernel/reloc_64.S b/arch/powerpc/kernel/reloc_64.S
> > index 02d4719bf43a..362be759609f 100644
> > --- a/arch/powerpc/kernel/reloc_64.S
> > +++ b/arch/powerpc/kernel/reloc_64.S
> > @@ -8,7 +8,7 @@
> > #include <asm/ppc_asm.h>
> >
> > RELA = 7
> > -RELACOUNT = 0x6ffffff9
> > +RELASZ = 8
> > R_PPC64_RELATIVE = 22
> >
> > /*
> > @@ -27,7 +27,7 @@ _GLOBAL(relocate)
> > add r10,r10,r12 /* r10 has runtime addr of _stext */
> >
> > /*
> > - * Scan the dynamic section for the RELA and RELACOUNT entries.
> > + * Scan the dynamic section for the RELA and RELASZ entries.
> > */
> > li r7,0
> > li r8,0
> > @@ -38,13 +38,16 @@ _GLOBAL(relocate)
> > bne 2f
> > ld r7,8(r11) /* get RELA pointer in r7 */
> > b 3f
> > -2: addis r6,r6,(-RELACOUNT)@ha
> > - cmpdi r6,RELACOUNT@l
> > +2: cmpdi r6,RELASZ
> > bne 3f
> > - ld r8,8(r11) /* get RELACOUNT value in r8 */
> > + ld r8,8(r11) /* get RELA / 24 in r8 */
> > + lis r0,0xaaaa
> > + ori r0,r0,0xaaab
> > + mulhwu r8,r8,r0
> > + srwi r8,r8,4
> > 3: addi r11,r11,16
> > b 1b
> > -4: cmpdi r7,0 /* check we have both RELA and RELACOUNT */
> > +4: cmpdi r7,0 /* check we have both RELA and RELASZ */
> > cmpdi cr1,r8,0
> > beq 6f
> > beq cr1,6f
> > --
> > 2.35.1.616.g0bdcbb4464-goog
> >
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers
>

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