linux-kernel - Re: [PATCH] x86_64,entry: Fix RCX for traced syscalls

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Message-ID: <20140626195629.GC5714@two.firstfloor.org>
Date:	Thu, 26 Jun 2014 21:56:29 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH] x86_64,entry: Fix RCX for traced syscalls

> show RCX == RIP even under strace.

If you think it's really worth the extra instruction?

It's not wrong, but it's not clear if it's useful.

-Andi

> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
>  arch/x86/kernel/entry_64.S | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index b25ca96..6624e18 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -143,7 +143,8 @@ ENDPROC(native_usergs_sysret64)
>  	movq \tmp,RSP+\offset(%rsp)
>  	movq $__USER_DS,SS+\offset(%rsp)
>  	movq $__USER_CS,CS+\offset(%rsp)
> -	movq $-1,RCX+\offset(%rsp)
> +	movq RIP+\offset(%rsp),\tmp  /* get rip */
> +	movq \tmp,RCX+\offset(%rsp)  /* copy it to rcx as sysret would do */
>  	movq R11+\offset(%rsp),\tmp  /* get eflags */
>  	movq \tmp,EFLAGS+\offset(%rsp)
>  	.endm
> -- 
> 1.9.3
> 

-- 
ak@...ux.intel.com -- Speaking for myself only.
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