[PATCH kernel] powerpc/boot: Stop using RELACOUNT

[Michael Ellerman]

Alexey Kardashevskiy <aik at ozlabs.ru> writes:
> 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 [2].

That's committed so you can say:
  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 [1], this does not add unaligned UADDR/UADDR64 relocations
                ^
                that should be 2?

> as in hardly possible to see those in arch-specific zImage.

I don't quite parse that. Is it true we can never see them in zImage?
Maybe it's true that we don't see them in practice.

> [1] https://github.com/llvm/llvm-project/commit/da0e5b885b25cf4
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git/commit/?h=next&id=d799769188529a
> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
> ---
>  arch/powerpc/boot/crt0.S | 43 +++++++++++++++++++++++++---------------
>  1 file changed, 27 insertions(+), 16 deletions(-)
>
> diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
> index feadee18e271..6ea3417da3b7 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 +76,38 @@ 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 at l
> +11:	cmpwi	r8,RELASZ
> +	bne	111f
> +	lwz	r0,4(r12)       /* get RELASZ value in r0 */
> +	b	12f
> +111:	cmpwi	r8,RELAENT

Can you use named local labels for new labels you introduce?

This could be .Lcheck_for_relaent: perhaps.

>  	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 */
> +	divd    r0,r0,r14       /* RELASZ / RELAENT */

This is in the 32-bit portion isn't it. AFAIK 32-bit CPUs don't
implement divd. I'm not sure why the toolchain allowed it. I would
expect it to trap if run on real 32-bit hardware.

>  	mtctr	r0
>  2:	lbz	r0,4+3(r9)	/* ELF32_R_INFO(reloc->r_info) */
>  	cmpwi	r0,22		/* R_PPC_RELATIVE */
> -	bne	3f
> +	bne	22f
>  	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
> +22:	add	r9,r9,r14
>  	bdnz	2b
>  
>  	/* Do a cache flush for our text, in case the loader didn't */

cheers