linux-kernel - Re: [PATCH] ARM: move __kuser_cmpxchg{32,64} into .kprobes.text

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Message-ID: <alpine.LFD.2.20.1602171806230.13632@knanqh.ubzr>
Date:	Wed, 17 Feb 2016 18:11:37 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
cc:	Russell King <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: move __kuser_cmpxchg{32,64} into .kprobes.text

On Wed, 17 Feb 2016, Arnd Bergmann wrote:

> When kprobes is used, most of the entry-armv.S file is put into the .kprobes.text
> section rather than .text, but the kuser_cmpxchg64_fixup and kuser_cmpxchg32_fixup
> code is not, which can lead to a link error when extremely large kernels get
> built (typically for randconfig):
> 
> arch/arm/kernel/built-in.o: In function `__dabt_usr':
> :(.kprobes.text+0x47c): relocation truncated to fit: R_ARM_JUMP24 against `.text'
> arch/arm/kernel/built-in.o: In function `__irq_usr':
> :(.kprobes.text+0x4e4): relocation truncated to fit: R_ARM_JUMP24 against `.text'
> arch/arm/kernel/built-in.o: In function `__fiq_usr':
> :(.kprobes.text+0x740): relocation truncated to fit: R_ARM_JUMP24 against `.text'
> 
> This moves the two remaining functions into the same section as the rest,
> to avoid the link error.
> 
> The current behavior has existed for many years and has not caused problems in
> practice except for extreme randconfig builds, so the patch should not need to
> get backported.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Fixes: b49c0f24cf67 ("[ARM] 4659/1: remove possibilities for spurious false negative with __kuser_cmpxchg")
> Fixes: 785d3cd286f0 ("ARM kprobes: prevent some functions involved with kprobes from being probed")

Stylewise it might be worth having something like:

#ifdef CONFIG_KPROBES
#define KPROBE_TEXT  .section .kprobes.text,"ax",%progbits
#else
#define KPROBE_TEXT .text
#endif

and then use that throughout.  Then...

Acked-by: Nicolas Pitre <nico@...aro.org>


> ---
>  arch/arm/kernel/entry-armv.S | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index e2550500486d..030e43a0dce8 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -895,7 +895,11 @@ __kuser_cmpxchg64:				@ 0xffff0f60
>  	rsbs	r0, r3, #0			@ set return val and C flag
>  	ldmfd	sp!, {r4, r5, r6, pc}
>  
> +#ifdef CONFIG_KPROBES
> +	.section	.kprobes.text,"ax",%progbits
> +#else
>  	.text
> +#endif
>  kuser_cmpxchg64_fixup:
>  	@ Called from kuser_cmpxchg_fixup.
>  	@ r4 = address of interrupted insn (must be preserved).
> @@ -953,7 +957,11 @@ __kuser_cmpxchg:				@ 0xffff0fc0
>  	rsbs	r0, r3, #0			@ set return val and C flag
>  	usr_ret	lr
>  
> +#ifdef CONFIG_KPROBES
> +	.section	.kprobes.text,"ax",%progbits
> +#else
>  	.text
> +#endif
>  kuser_cmpxchg32_fixup:
>  	@ Called from kuser_cmpxchg_check macro.
>  	@ r4 = address of interrupted insn (must be preserved).
> -- 
> 2.7.0
> 
>