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

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Mar 8, 2022 at 9:53 PM Fangrui Song <maskray@google.com> 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@kernel.org>
> Signed-off-by: Fangrui Song <maskray@google.com>
> ---
>  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