Re: [RFC][PATCH] x86: don't destroy %rbp on kernel-mode faults

[Arjan van de Ven]

Vegard Nossum wrote:
> Hi,
>
> The RFC part of this patch is: Does anybody see why touching %rcx would
> be bad? It certainly looks like %ecx is free. This fixes the stacktrace
> problem I was seeing, and Pekka tested a bootup to userspace. (Pekka also
> did half of the debugging. When will git allow multiple authors for a
> patch? :-))
>
>
> Vegard
>
>
> From b1cbf24fcd05aa5ed2e610c80c06bc519d3188f7 Mon Sep 17 00:00:00 2001
> From: Vegard Nossum <vegard.nossum@xxxxxxxxx>
> Date: Mon, 19 May 2008 21:39:44 +0200
> Subject: [PATCH] x86: don't destroy %rbp on kernel-mode faults
>
> From the code:
>
>     B stepping K8s sometimes report an truncated RIP for IRET exceptions
>     returning to compat mode. Check for these here too.
>
> The code then proceeds to truncate the upper 32 bits of %rbp. This means
> that when do_page_fault() is finally called, its prologue,
>
>     do_page_fault:
>         push %rbp
>         movl %rsp, %rbp
>
> will put the truncated base pointer on the stack. This means that the
> stack tracer will not be able to follow the base-pointer changes and
> will see all subsequent stack frames as unreliable.
>
> This patch changes the code to use a different register (%rcx) for the
> checking and leaves %rbp untouched.
>
> Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxx>
> Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> Signed-off-by: Pekka Enberg <penberg@xxxxxxxxxxxxxx>
> Signed-off-by: Vegard Nossum <vegard.nossum@xxxxxxxxx>

looks good to me; good debugging!

Acked-by: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
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