1 files changed, 28 insertions, 0 deletions

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index b00b33a18390..2484e331a64d 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -166,6 +166,34 @@ static inline bool invalid_selector(u16 value)
 
 #define FLAG_MASK               FLAG_MASK_32
 
+/*
+ * X86_32 CPUs don't save ss and esp if the CPU is already in kernel mode
+ * when it traps.  The previous stack will be directly underneath the saved
+ * registers, and 'sp/ss' won't even have been saved. Thus the '&regs->sp'.
+ *
+ * Now, if the stack is empty, '&regs->sp' is out of range. In this
+ * case we try to take the previous stack. To always return a non-null
+ * stack pointer we fall back to regs as stack if no previous stack
+ * exists.
+ *
+ * This is valid only for kernel mode traps.
+ */
+unsigned long kernel_stack_pointer(struct pt_regs *regs)
+{
+        unsigned long context = (unsigned long)regs & ~(THREAD_SIZE - 1);
+        unsigned long sp = (unsigned long)&regs->sp;
+        struct thread_info *tinfo;
+
+        if (context == (sp & ~(THREAD_SIZE - 1)))
+                return sp;
+
+        tinfo = (struct thread_info *)context;
+        if (tinfo->previous_esp)
+                return tinfo->previous_esp;
+
+        return (unsigned long)regs;
+}
+
 static unsigned long *pt_regs_access(struct pt_regs *regs, unsigned long regno)
 {
         BUILD_BUG_ON(offsetof(struct pt_regs, bx) != 0);