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From: Dave Martin <Dave.Martin@arm.com>
To: Will Deacon <will@kernel.org>
Cc: Tamas Zsoldos <tamas.zsoldos@arm.com>,
	Mark Brown <broonie@kernel.org>,
	kernel-team@android.com, linux-arm-kernel@lists.infradead.org,
	Daniel Kiss <daniel.kiss@arm.com>
Subject: Re: [PATCH 2/3] arm64: vdso: Add a comment to justify the mysterious NOP in sigreturn
Date: Tue, 19 May 2020 14:26:54 +0100	[thread overview]
Message-ID: <20200519132654.GF5031@arm.com> (raw)
In-Reply-To: <20200519121818.14511-3-will@kernel.org>

On Tue, May 19, 2020 at 01:18:17PM +0100, Will Deacon wrote:
> Every so often we have to remind ourselves about the purpose of the
> weird NOP instruction immediately preceding the sigreturn trampoline.
> 
> Add a short comment to state that it exists for some unwinders that
> determine the caller address by subtracting from the return address.
> 
> Cc: Dave Martin <dave.martin@arm.com>
> Cc: Daniel Kiss <daniel.kiss@arm.com>
> Cc: Tamas Zsoldos <tamas.zsoldos@arm.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
>  arch/arm64/kernel/vdso/sigreturn.S | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/vdso/sigreturn.S b/arch/arm64/kernel/vdso/sigreturn.S
> index 83ac284dae79..7853fa9692f6 100644
> --- a/arch/arm64/kernel/vdso/sigreturn.S
> +++ b/arch/arm64/kernel/vdso/sigreturn.S
> @@ -14,7 +14,12 @@
>  
>  	.text
>  
> -	nop
> +/*
> + * This mysterious NOP is required for some unwinders that subtract one from
> + * the return address in order to identify the calling function.
> + * Hack borrowed from arch/powerpc/kernel/vdso64/sigtramp.S.
> + */
> +	nop	// Mysterious NOP

Reviewed-by: Dave Martin <Dave.Martin@arm.com>

>  /*
>   * GDB relies on being able to identify the sigreturn instruction sequence to
>   * unwind from signal handlers. We cannot, therefore, use SYM_FUNC_START()
> -- 
> 2.26.2.761.g0e0b3e54be-goog
> 
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

_______________________________________________
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  reply	other threads:[~2020-05-19 13:27 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-19 12:18 [PATCH 0/3] arm64 sigreturn unwinding fixes Will Deacon
2020-05-19 12:18 ` [PATCH 1/3] arm64: vdso: Don't prefix sigreturn trampoline with a BTI C instruction Will Deacon
2020-05-19 12:38   ` Mark Brown
2020-05-19 13:25     ` Dave Martin
2020-05-19 14:35       ` Mark Brown
2020-05-19 14:55         ` Dave Martin
2020-05-19 15:42           ` Mark Brown
2020-05-20  9:48             ` Dave Martin
2020-05-20 10:46               ` Mark Brown
2020-05-20 11:08                 ` Dave Martin
2020-05-19 15:26     ` Will Deacon
2020-05-19 13:21   ` Dave Martin
2020-05-19 13:29     ` Will Deacon
2020-05-19 12:18 ` [PATCH 2/3] arm64: vdso: Add a comment to justify the mysterious NOP in sigreturn Will Deacon
2020-05-19 13:26   ` Dave Martin [this message]
2020-05-19 12:18 ` [PATCH 3/3] arm64: vdso: Fix CFI directives in sigreturn trampoline Will Deacon
2020-05-19 13:09   ` Dave P Martin
2020-05-19 13:39     ` Will Deacon
2020-05-19 13:55       ` Dave Martin
2020-05-19 15:24         ` Will Deacon
2020-05-19 15:30         ` Daniel Kiss
2020-05-19 15:55           ` Will Deacon

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