kprobes/x86: Fix removed int3 checking order

Fix kprobe/x86 to check removed int3 when failing to get kprobe
from hlist. Since we have a time window between checking int3
exists on probed address and getting kprobe on that address,
we can have following scenario:

 -------
 CPU1                     CPU2
 hit int3
 check int3 exists
                          remove int3
                          remove kprobe from hlist
 get kprobe from hlist
 no kprobe->OOPS!
 -------

This patch moves int3 checking if there is no kprobe on that
address for fixing this problem as follows:

 ------
 CPU1                     CPU2
 hit int3
                          remove int3
                          remove kprobe from hlist
 get kprobe from hlist
 no kprobe->check int3 exists
          ->rollback&retry
 ------

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Acked-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Cc: systemtap <systemtap@sources.redhat.com>
Cc: DLE <dle-develop@lists.sourceforge.net>
Cc: Dave Anderson <anderson@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20100427223348.2322.9112.stgit@localhost6.localdomain6>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 13 insertions, 14 deletions

diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
index b43bbaebe2c..1658efdfb4e 100644
--- a/arch/x86/kernel/kprobes.c
+++ b/arch/x86/kernel/kprobes.c
@@ -534,20 +534,6 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
         struct kprobe_ctlblk *kcb;
 
         addr = (kprobe_opcode_t *)(regs->ip - sizeof(kprobe_opcode_t));
-        if (*addr != BREAKPOINT_INSTRUCTION) {
-                /*
-                 * The breakpoint instruction was removed right
-                 * after we hit it.  Another cpu has removed
-                 * either a probepoint or a debugger breakpoint
-                 * at this address.  In either case, no further
-                 * handling of this interrupt is appropriate.
-                 * Back up over the (now missing) int3 and run
-                 * the original instruction.
-                 */
-                regs->ip = (unsigned long)addr;
-                return 1;
-        }
-
         /*
          * We don't want to be preempted for the entire
          * duration of kprobe processing. We conditionally
@@ -579,6 +565,19 @@ static int __kprobes kprobe_handler(struct pt_regs *regs)
                                 setup_singlestep(p, regs, kcb, 0);
                         return 1;
                 }
+        } else if (*addr != BREAKPOINT_INSTRUCTION) {
+                /*
+                 * The breakpoint instruction was removed right
+                 * after we hit it.  Another cpu has removed
+                 * either a probepoint or a debugger breakpoint
+                 * at this address.  In either case, no further
+                 * handling of this interrupt is appropriate.
+                 * Back up over the (now missing) int3 and run
+                 * the original instruction.
+                 */
+                regs->ip = (unsigned long)addr;
+                preempt_enable_no_resched();
+                return 1;
         } else if (kprobe_running()) {
                 p = __get_cpu_var(current_kprobe);
                 if (p->break_handler && p->break_handler(p, regs)) {