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

From: Arjan van de Ven
Date: Mon May 19 2008 - 16:23:06 EST

Vegard Nossum wrote:

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? :-))


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,

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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