[PATCH 3/4] MIPS: Stacktrace: Fix __usermode() of uninitialised regs

[Matt Redfearn <matt.redfearn@imgtec.com>]

Commit 81a76d7119f6 ("MIPS: Avoid using unwind_stack() with usermode")
added a check if the passed regs are from user mode, and perform a raw
backtrace if so.
When WARN() is invoked, __dump_stack calls show_stack()
with NULL task and stack pointers. This leads show_stack to create a
pt_regs struct on the stack, and initialise it via prepare_frametrace().
When show_backtrace() examines the regs, the value of the status
register checked by user_mode() is unpredictable, depending on the
uninitialised content of the stack. This leads to show_backtrace()
sometimes showing raw backtraces instead of correctly walking the kernel
stack.

Fix this by initialising the contents of the saved status register in
prepare_frametrace().

Fixes: 81a76d7119f6 ("MIPS: Avoid using unwind_stack() with usermode")
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
---

 arch/mips/include/asm/stacktrace.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/mips/include/asm/stacktrace.h b/arch/mips/include/asm/stacktrace.h
index 780ee2c2a2ac..4845945d02a5 100644
--- a/arch/mips/include/asm/stacktrace.h
+++ b/arch/mips/include/asm/stacktrace.h
@@ -1,6 +1,7 @@
 #ifndef _ASM_STACKTRACE_H
 #define _ASM_STACKTRACE_H
 
+#include <asm/asm.h>
 #include <asm/ptrace.h>
 
 #ifdef CONFIG_KALLSYMS
@@ -47,6 +48,8 @@ static __always_inline void prepare_frametrace(struct pt_regs *regs)
 		: "=m" (regs->cp0_epc),
 		"=m" (regs->regs[29]), "=m" (regs->regs[31])
 		: : "memory");
+	/* show_backtrace behaviour depends on user_mode(regs) */
+	regs->cp0_status = read_c0_status();
 }
 
 #endif /* _ASM_STACKTRACE_H */
-- 
2.7.4