Messages in this thread Patch in this message | | | Date | Mon, 19 May 2008 22:02:30 +0200 | Subject | [RFC][PATCH] x86: don't destroy %rbp on kernel-mode faults | From | Vegard Nossum <> |
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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@gmail.com> 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@firstfloor.org> Cc: Ingo Molnar <mingo@elte.hu> Cc: Arjan van de Ven <arjan@linux.intel.com> Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi> Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com> --- arch/x86/kernel/entry_64.S | 8 ++++---- 1 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index 1edd9ac..ff53692 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -926,11 +926,11 @@ error_kernelspace: iret run with kernel gs again, so don't set the user space flag. B stepping K8s sometimes report an truncated RIP for IRET exceptions returning to compat mode. Check for these here too. */ - leaq irq_return(%rip),%rbp - cmpq %rbp,RIP(%rsp) + leaq irq_return(%rip),%rcx + cmpq %rcx,RIP(%rsp) je error_swapgs - movl %ebp,%ebp /* zero extend */ - cmpq %rbp,RIP(%rsp) + movl %ecx,%ecx /* zero extend */ + cmpq %rcx,RIP(%rsp) je error_swapgs cmpq $gs_change,RIP(%rsp) je error_swapgs -- 1.5.4.1
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