x86_32: Return actual stack when requesting sp from regs

As x86_32 traps do not save sp when taken in kernel mode, we need to
accommodate the sp when requesting to get the register.

This affects kprobes.

Before:

 # echo 'p:ftrace sys_read+4 s=%sp' > /debug/tracing/kprobe_events
 # echo 1 > /debug/tracing/events/kprobes/enable
 # cat trace
            sshd-1345  [000] d...   489.117168: ftrace: (sys_read+0x4/0x70) s=b7e96768
            sshd-1345  [000] d...   489.117191: ftrace: (sys_read+0x4/0x70) s=b7e96768
             cat-1447  [000] d...   489.117392: ftrace: (sys_read+0x4/0x70) s=5a7
             cat-1447  [001] d...   489.118023: ftrace: (sys_read+0x4/0x70) s=b77ad05f
            less-1448  [000] d...   489.118079: ftrace: (sys_read+0x4/0x70) s=b7762e06
            less-1448  [000] d...   489.118117: ftrace: (sys_read+0x4/0x70) s=b7764970

After:
            sshd-1352  [000] d...   362.348016: ftrace: (sys_read+0x4/0x70) s=f3febfa8
            sshd-1352  [000] d...   362.348048: ftrace: (sys_read+0x4/0x70) s=f3febfa8
            bash-1355  [001] d...   362.348081: ftrace: (sys_read+0x4/0x70) s=f5075fa8
            sshd-1352  [000] d...   362.348082: ftrace: (sys_read+0x4/0x70) s=f3febfa8
            sshd-1352  [000] d...   362.690950: ftrace: (sys_read+0x4/0x70) s=f3febfa8
            bash-1355  [001] d...   362.691033: ftrace: (sys_read+0x4/0x70) s=f5075fa8

Link: http://lkml.kernel.org/r/1342208654.30075.22.camel@gandalf.stny.rr.com
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index dcfde52979c3e63be0d42627cc567cf76c733090..2233713d029737f7a111ebdb0ee8374ed8c39849 100644 (file)
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -246,6 +246,15 @@ static inline unsigned long regs_get_register(struct pt_regs *regs,
 {
        if (unlikely(offset > MAX_REG_OFFSET))
                return 0;
+#ifdef CONFIG_X86_32
+       /*
+        * Traps from the kernel do not save sp and ss.
+        * Use the helper function to retrieve sp.
+        */
+       if (offset == offsetof(struct pt_regs, sp) &&
+           regs->cs == __KERNEL_CS)
+               return kernel_stack_pointer(regs);
+#endif
        return *(unsigned long *)((unsigned long)regs + offset);
 }