On Wed, Apr 06, 2022 at 02:01:48PM +1000, Alexey Kardashevskiy wrote:
> So far the RELACOUNT tag from the ELF header was containing the exact
> number of R_PPC_RELATIVE/R_PPC64_RELATIVE relocations. However the LLVM's
> recent change [1] make it equal-or-less than the actual number which
> makes it useless.
>
> This replaces RELACOUNT in zImage loader with a pair of RELASZ and RELAENT.
> The vmlinux relocation code is fixed in commit d79976918852
> ("powerpc/64: Add UADDR64 relocation support").
>
> To make it more future proof, this walks through the entire .rela.dyn
> section instead of assuming that the section is sorter by a relocation
> type. Unlike d79976918852, this does not add unaligned UADDR/UADDR64
> relocations as we are likely not to see those in practice - the zImage
> is small and very arch specific so there is a smaller chance that some
> generic feature (such as PRINK_INDEX) triggers unaligned relocations.
>
> [1] https://github.com/llvm/llvm-project/commit/da0e5b885b25cf4
> Signed-off-by: Alexey Kardashevskiy <a...@ozlabs.ru>
> ---
> Changes:
> v2:
> * s/divd/divwu/ for ppc32
> * updated the commit log
> * named all new labels instead of numbering them
> (s/101f/.Lcheck_for_relaent/ and so on)
> ---
> arch/powerpc/boot/crt0.S | 45 ++++++++++++++++++++++++++--------------
> 1 file changed, 29 insertions(+), 16 deletions(-)
>
> diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
> index feadee18e271..e9306d862f8d 100644
> --- a/arch/powerpc/boot/crt0.S
> +++ b/arch/powerpc/boot/crt0.S
> @@ -8,7 +8,8 @@
> #include "ppc_asm.h"
>
> RELA = 7
> -RELACOUNT = 0x6ffffff9
> +RELASZ = 8
> +RELAENT = 9
>
> .data
> /* A procedure descriptor used when booting this as a COFF file.
> @@ -75,34 +76,39 @@ 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 .Lcheck_for_relaent
> + lwz r0,4(r12) /* get RELASZ value in r0 */
> + b 12f
> +.Lcheck_for_relaent:
> + cmpwi r8,RELAENT
> bne 12f
> - lwz r0,4(r12) /* get RELACOUNT value in r0 */
> + lwz r14,4(r12) /* get RELAENT value in r14 */
> 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. */
> + * which need to be initialized with addend + offset */
> 10: /* skip relocation if we don't have both */
> cmpwi r0,0
> beq 3f
> cmpwi r9,0
> beq 3f
> + cmpwi r14,0
> + beq 3f
>
> add r9,r9,r11 /* Relocate RELA pointer */
> + divwu r0,r0,r14 /* RELASZ / RELAENT */
> mtctr r0
> 2: lbz r0,4+3(r9) /* ELF32_R_INFO(reloc->r_info) */
> cmpwi r0,22 /* R_PPC_RELATIVE */
> - bne 3f
> + bne .Lnext
> lwz r12,0(r9) /* reloc->r_offset */
> lwz r0,8(r9) /* reloc->r_addend */
> add r0,r0,r11
> stwx r0,r11,r12
> - addi r9,r9,12
> +.Lnext: add r9,r9,r14
> bdnz 2b
>
> /* Do a cache flush for our text, in case the loader didn't */
> @@ -160,32 +166,39 @@ 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
> - bne 11f
> - ld r8,8(r11) /* get RELACOUNT value in r8 */
> +10: cmpwi r12,RELASZ
> + bne .Lcheck_for_relaent
> + lwz r8,8(r11) /* get RELASZ pointer in r8 */
> + b 11f
> +.Lcheck_for_relaent:
> + cmpwi r12,RELAENT
> + bne 11f
> + lwz r14,8(r11) /* get RELAENT pointer in r14 */
> 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, RELASZ, RELAENT*/
> cmpdi cr1,r8,0
> beq 3f
> beq cr1,3f
> + cmpdi r14,0
> + beq 3f
>
> /* Calcuate the runtime offset. */
> subf r13,r13,r9
>
> /* Run through the list of relocations and process the
> * R_PPC64_RELATIVE ones. */
> + divd r8,r8,r14 /* RELASZ / RELAENT */
While you are at it, this one should also be divdu.
I really wished IBM had used explicit signed/unsigned indication in the
mnemonics (divds, divdu, divws, divwu) instead. Fortunately very little
assemby code uses these instructions nowadays.
> mtctr r8
> 13: ld r0,8(r9) /* ELF64_R_TYPE(reloc->r_info) */
> cmpdi r0,22 /* R_PPC64_RELATIVE */
> - bne 3f
> + bne .Lnext
> ld r12,0(r9) /* reloc->r_offset */
> ld r0,16(r9) /* reloc->r_addend */
> add r0,r0,r13
> stdx r0,r13,r12
> - addi r9,r9,24
> +.Lnext: add r9,r9,r14
> bdnz 13b
>
> /* Do a cache flush for our text, in case the loader didn't */
> --
> 2.30.2
Cheers,
Gabriel
