Re: [PATCH v3 1/3] powerpc/reloc32: fix corrupted modversion CRCs

[Suzuki K Poulose <Suzuki.Poulose@arm.com>]

On 27/10/16 17:27, Ard Biesheuvel wrote:
> Commit 0e0ed6406e61 ("powerpc/modules: Module CRC relocation fix causes
> perf issues") fixed an issue with relocatable PIE kernels in a way that
> essentially reintroduced the issue again for 32-bit builds.
>
> Since the chosen approach does is not applicable to 32-bit, fix the
> issue by updating the runtime relocation routine to ignore the load
> offset for the interval [__start___kcrctab, __stop___kcrctab_gpl_future),
> which is where the CRCs reside. This ensures that the values of the CRC
> pseudo-symbols are no longer made dependent on the runtime load offset.
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Ard,

These changes look good to me (having originally written the code).

Reviewed-by : Suzuki K Poulose <suzuki.poulose@arm.com>

Cheers
Suzuki

> ---
>  arch/powerpc/kernel/reloc_32.S | 36 +++++++++++++++++---
>  1 file changed, 32 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/kernel/reloc_32.S b/arch/powerpc/kernel/reloc_32.S
> index f366fedb0872..150686b9febb 100644
> --- a/arch/powerpc/kernel/reloc_32.S
> +++ b/arch/powerpc/kernel/reloc_32.S
> @@ -87,12 +87,12 @@ eodyn:				/* End of Dyn Table scan */
>  	 * Work out the current offset from the link time address of .rela
>  	 * section.
>  	 *  cur_offset[r7] = rela.run[r9] - rela.link [r7]
> -	 *  _stext.link[r12] = _stext.run[r10] - cur_offset[r7]
> -	 *  final_offset[r3] = _stext.final[r3] - _stext.link[r12]
> +	 *  _stext.link[r11] = _stext.run[r10] - cur_offset[r7]
> +	 *  final_offset[r3] = _stext.final[r3] - _stext.link[r11]
>  	 */
>  	subf	r7, r7, r9	/* cur_offset */
> -	subf	r12, r7, r10
> -	subf	r3, r12, r3	/* final_offset */
> +	subf	r11, r7, r10
> +	subf	r3, r11, r3	/* final_offset */
>
>  	subf	r8, r6, r8	/* relaz -= relaent */
>  	/*
> @@ -101,6 +101,21 @@ eodyn:				/* End of Dyn Table scan */
>  	 * r13	- points to the symbol table
>  	 */
>
> +#ifdef CONFIG_MODVERSIONS
> +	/*
> +	 * Treat R_PPC_RELATIVE relocations differently when they target the
> +	 * interval [__start___kcrctab, __stop___kcrctab_gpl_future): in this
> +	 * case, the relocated quantities are CRC pseudo-symbols, which should
> +	 * be preserved as-is, rather than be modified to take the runtime
> +	 * offset into account.
> +	 */
> +	lwz	r10, (p_kcrc_start - 0b)(r12)
> +	lwz	r11, (p_kcrc_stop - 0b)(r12)
> +	subf	r12, r7, r12			/* link time addr of 0b */
> +	add	r10, r10, r12
> +	add	r11, r11, r12
> +#endif
> +
>  	/*
>  	 * Check if we have a relocation based on symbol
>  	 * r5 will hold the value of the symbol.
> @@ -135,7 +150,15 @@ get_type:
>  	bne	hi16
>  	lwz	r4, 0(r9)	/* r_offset */
>  	lwz	r0, 8(r9)	/* r_addend */
> +#ifdef CONFIG_MODVERSIONS
> +	cmplw	r4, r10
> +	blt	do_add
> +	cmplw	r4, r11
> +	blt	skip_add
> +do_add:
> +#endif
>  	add	r0, r0, r3	/* final addend */
> +skip_add:
>  	stwx	r0, r4, r7	/* memory[r4+r7]) = (u32)r0 */
>  	b	nxtrela		/* continue */
>
> @@ -207,3 +230,8 @@ p_dyn:		.long	__dynamic_start - 0b
>  p_rela:		.long	__rela_dyn_start - 0b
>  p_sym:		.long	__dynamic_symtab - 0b
>  p_st:		.long	_stext - 0b
> +
> +#ifdef CONFIG_MODVERSIONS
> +p_kcrc_start:	.long	__start___kcrctab - 0b
> +p_kcrc_stop:	.long	__stop___kcrctab_gpl_future - 0b
> +#endif
>