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Subject[patch] x86_64: stack unwinder crash fix

* Ingo Molnar <> wrote:

> Jan,
> in 2.6.19-rc6 i'm getting frequent unwinder failures, like:

> it's quite an annoyance, i rarely see the unwinder getting a stackdump
> right, without 'falling back' to the inexact backtrace ...

to make matters worse, i also got an unwinder /crash/ while it tried to
dump the stack ...

that particular bug is fixed by the patch below - aligning the stack
pointer solved both the case of corrupted RSP values and corrupted/buggy
dwarf2 debug info.

Linus, please apply, this is a 2.6.19 must-fix i think.


Subject: [patch] x86_64: stack unwinder crash fix
From: Ingo Molnar <>

the new dwarf2 unwinder crashes while trying to dump the stack:

Leftover inexact backtrace:

Unable to handle kernel paging request at ffffffff82800000 RIP:
[<ffffffff8026cf26>] dump_trace+0x35b/0x3d2
PGD 203027 PUD 205027 PMD 0
Oops: 0000 [2] PREEMPT SMP
Modules linked in:
Pid: 30, comm: khelper Not tainted 2.6.19-rc6-rt1 #11
RIP: 0010:[<ffffffff8026cf26>] [<ffffffff8026cf26>] dump_trace+0x35b/0x3d2
RSP: 0000:ffff81003fb9d848 EFLAGS: 00010006
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff805b3520 RDI: 0000000000000000
RBP: ffffffff827ffff9 R08: ffffffff80aad000 R09: 0000000000000005
R10: ffffffff80aae000 R11: ffffffff8037961b R12: ffff81003fb9d858
R13: 0000000000000000 R14: ffffffff80598460 R15: ffffffff80ab1fc0
FS: 0000000000000000(0000) GS:ffffffff806c4200(0000) knlGS:0000000000000000
CS: 0010 DS: 0018 ES: 0018 CR0: 000000008005003b
CR2: ffffffff82800000 CR3: 0000000000201000 CR4: 00000000000006e0

this crash happened because it did not sanitize the dwarf2 data it
got, and got an unaligned stack pointer - which happily walked past
the process stack (and eventually reached the end of kernel memory
and pagefaulted there) due to this naive iteration condition:

HANDLE_STACK (((long) stack & (THREAD_SIZE-1)) != 0);

note that i386 is alot more conservative when it comes to trusting
stack pointers:

static inline int valid_stack_ptr(struct thread_info *tinfo, void *p)
return p > (void *)tinfo &&
p < (void *)tinfo + THREAD_SIZE - 3;

but the x86_64 code did not take this bit of i386 code.

the fix is to align the stack pointer.

Signed-off-by: Ingo Molnar <>
arch/x86_64/kernel/traps.c | 6 ++++++
1 file changed, 6 insertions(+)

Index: linux/arch/x86_64/kernel/traps.c
--- linux.orig/arch/x86_64/kernel/traps.c
+++ linux/arch/x86_64/kernel/traps.c
@@ -290,6 +290,12 @@ void dump_trace(struct task_struct *tsk,
if (tsk && tsk != current)
stack = (unsigned long *)tsk->thread.rsp;
+ /*
+ * Align the stack pointer on word boundary, later loops
+ * rely on that (and corruption / debug info bugs can cause
+ * unaligned values here):
+ */
+ stack = (unsigned long *)((unsigned long)stack & ~(sizeof(long)-1));

* Print function call entries within a stack. 'cond' is the
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