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

From: Nicolas Pitre
Date: Wed Feb 17 2016 - 18:11:46 EST


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@xxxxxxxx>
> 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@xxxxxxxxxx>


> ---
> 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
>
>