[PATCH v3 09/51] x86/dumpstack: fix x86_32 kernel_stack_pointer() previous stack access

[Josh Poimboeuf]

On x86_32, when an interrupt happens from kernel space, SS and SP aren't
pushed and the existing stack is used.  So pt_regs is effectively two
words shorter, and the previous stack pointer is normally the memory
after the shortened pt_regs, aka '&regs->sp'.

But in the rare case where the interrupt hits right after the stack
pointer has been changed to point to an empty stack, like for example
when call_on_stack() is used, the address immediately after the
shortened pt_regs is no longer on the stack.  In that case, instead of
'&regs->sp', the previous stack pointer should be retrieved from the
beginning of the current stack page.

kernel_stack_pointer() wants to do that, but it forgets to dereference
the pointer.  So instead of returning a pointer to the previous stack,
it returns a pointer to the beginning of the current stack.

Fixes: 0788aa6a23cb ("x86: Prepare removal of previous_esp from i386 
thread_info structure")
Signed-off-by: Josh Poimboeuf <jpoim...@redhat.com>
---
 arch/x86/kernel/ptrace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 2537cfb..5b88a1b 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -173,8 +173,8 @@ unsigned long kernel_stack_pointer(struct pt_regs *regs)
                return sp;
 
        prev_esp = (u32 *)(context);
-       if (prev_esp)
-               return (unsigned long)prev_esp;
+       if (*prev_esp)
+               return (unsigned long)*prev_esp;
 
        return (unsigned long)regs;
 }
-- 
2.7.4