[PATCH] fix is_at_popf() for compat tasks

When testing for the REX instruction prefix, first check
for 32-bit mode because in compat mode the REX prefix is an
increment instruction.

Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
Signed-off-by: Andi Kleen <ak@suse.de>

1 files changed, 4 insertions, 1 deletions

diff --git a/arch/x86_64/kernel/ptrace.c b/arch/x86_64/kernel/ptrace.c
index d35ec1bc696a..3a52c7bcfd12 100644
--- a/arch/x86_64/kernel/ptrace.c
+++ b/arch/x86_64/kernel/ptrace.c
@@ -141,8 +141,11 @@ static int is_at_popf(struct task_struct *child, struct pt_regs *regs)
                 case 0xf0: case 0xf2: case 0xf3:
                         continue;
 
-                /* REX prefixes */
                 case 0x40 ... 0x4f:
+                        if (regs->cs != __USER_CS)
+                                /* 32-bit mode: register increment */
+                                return 0;
+                        /* 64-bit mode: REX prefix */
                         continue;
 
                         /* CHECKME: f0, f2, f3 */