Somehow, CONFIG_PARAVIRT=n convinces gcc to change the
x86_64_start_kernel() prologue from:
0000000000000129 <x86_64_start_kernel>:
129: 55 push %rbp
12a: 48 89 e5 mov %rsp,%rbp
to:
0000000000000124 <x86_64_start_kernel>:
124: 4c 8d 54 24 08 lea 0x8(%rsp),%r10
129: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp
12d: 41 ff 72 f8 pushq -0x8(%r10)
131: 55 push %rbp
132: 48 89 e5 mov %rsp,%rbp
This is an unusual pattern which aligns rsp (though in this case it's
already aligned) and saves the start_cpu() return address again on the
stack before storing the frame pointer.
The unwinder assumes the last stack frame header is at a certain offset,
but the above code breaks that assumption, resulting in the following
warning:
WARNING: kernel stack frame pointer at ffffffff82e03f40 in swapper:0 has bad
value (null)
Fix it by checking for the last task stack frame at the aligned offset
in addition to the normal unaligned offset.
Fixes: acb4608ad186 ("x86/unwind: Create stack frames for saved syscall
registers")
Reported-by: Borislav Petkov <b...@alien8.de>
Signed-off-by: Josh Poimboeuf <jpoim...@redhat.com>
---
arch/x86/kernel/unwind_frame.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index ea7b7f9..33aeaae 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -46,7 +46,14 @@ static bool is_last_task_frame(struct unwind_state *state)
unsigned long bp = (unsigned long)state->bp;
unsigned long regs = (unsigned long)task_pt_regs(state->task);
- return bp == regs - FRAME_HEADER_SIZE;
+ /*
+ * We have to check for the last task frame at two different locations
+ * because gcc can occasionally decide to realign the stack pointer and
+ * change the offset of the stack frame by a word in the prologue of a
+ * function called by head/entry code.
+ */
+ return bp == regs - FRAME_HEADER_SIZE ||
+ bp == regs - FRAME_HEADER_SIZE - sizeof(long);
}
/*
--
2.7.4
