x86: use kernel_stack_pointer() in dumpstack.c

The way to obtain a kernel-mode stack pointer from a struct pt_regs in
32-bit mode is "subtle": the stack doesn't actually contain the stack
pointer, but rather the location where it would have been marks the
actual previous stack frame.  For clarity, use kernel_stack_pointer()
instead of coding this weirdness explicitly.

Furthermore, user_mode() is only valid when the process is known to
not run in V86 mode.  Use the safer user_mode_vm() instead.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 2d8a371d4339..b8ce165dde5d 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -268,11 +268,12 @@ int __kprobes __die(const char *str, struct pt_regs *regs, long err)
 
         show_registers(regs);
 #ifdef CONFIG_X86_32
-        sp = (unsigned long) (&regs->sp);
-        savesegment(ss, ss);
-        if (user_mode(regs)) {
+        if (user_mode_vm(regs)) {
                 sp = regs->sp;
                 ss = regs->ss & 0xffff;
+        } else {
+                sp = kernel_stack_pointer(regs);
+                savesegment(ss, ss);
         }
         printk(KERN_EMERG "EIP: [<%08lx>] ", regs->ip);
         print_symbol("%s", regs->ip);