wait_task_stopped: simplify and fix races with SIGCONT/SIGKILL/untrace

wait_task_stopped() has multiple races with SIGCONT/SIGKILL.  tasklist_lock
does not pin the child in TASK_TRACED/TASK_STOPPED stated, almost all info
reported (including exit_code) may be wrong.

In fact, the code under write_lock_irq(tasklist_lock) is not safe.  The child
may be PTRACE_DETACH'ed at this time by another subthread, in that case it is
possible we are no longer its ->parent.

Change wait_task_stopped() to take ->siglock before inspecting the task.  This
guarantees that the child can't resume and (for example) clear its
->exit_code, so we don't need to use xchg(&p->exit_code) and re-check.  The
only exception is ptrace_stop() which changes ->state and ->exit_code without
->siglock held during abort.  But this can only happen if both the tracer and
the tracee are dying (coredump is in progress), we don't care.

With this patch wait_task_stopped() doesn't move the child to the end of
the ->parent list on success.  This optimization could be restored, but
in that case we have to take write_lock(tasklist) and do some nasty
checks.

Also change the do_wait() since we don't return EAGAIN any longer.

[akpm@linux-foundation.org: fix up after Willy renamed everything]
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 26 insertions, 62 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index b5ff2b121093..da293ac7e379 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1355,17 +1355,35 @@ static int wait_task_stopped(struct task_struct *p, int delayed_group_leader,
                              int noreap, struct siginfo __user *infop,
                              int __user *stat_addr, struct rusage __user *ru)
 {
-        int retval, exit_code;
+        int retval, exit_code, why;
+        uid_t uid = 0; /* unneeded, required by compiler */
         pid_t pid;
 
-        if (!p->exit_code)
-                return 0;
+        exit_code = 0;
+        spin_lock_irq(&p->sighand->siglock);
+
+        if (unlikely(!task_is_stopped_or_traced(p)))
+                goto unlock_sig;
+
         if (delayed_group_leader && !(p->ptrace & PT_PTRACED) &&
             p->signal->group_stop_count > 0)
                 /*
                  * A group stop is in progress and this is the group leader.
                  * We won't report until all threads have stopped.
                  */
+                goto unlock_sig;
+
+        exit_code = p->exit_code;
+        if (!exit_code)
+                goto unlock_sig;
+
+        if (!noreap)
+                p->exit_code = 0;
+
+        uid = p->uid;
+unlock_sig:
+        spin_unlock_irq(&p->sighand->siglock);
+        if (!exit_code)
                 return 0;
 
         /*
@@ -1375,65 +1393,15 @@ static int wait_task_stopped(struct task_struct *p, int delayed_group_leader,
          * keep holding onto the tasklist_lock while we call getrusage and
          * possibly take page faults for user memory.
          */
-        pid = task_pid_nr_ns(p, current->nsproxy->pid_ns);
         get_task_struct(p);
+        pid = task_pid_nr_ns(p, current->nsproxy->pid_ns);
+        why = (p->ptrace & PT_PTRACED) ? CLD_TRAPPED : CLD_STOPPED;
         read_unlock(&tasklist_lock);
 
-        if (unlikely(noreap)) {
-                uid_t uid = p->uid;
-                int why = (p->ptrace & PT_PTRACED) ? CLD_TRAPPED : CLD_STOPPED;
-
-                exit_code = p->exit_code;
-                if (unlikely(!exit_code) || unlikely(p->exit_state))
-                        goto bail_ref;
+        if (unlikely(noreap))
                 return wait_noreap_copyout(p, pid, uid,
                                            why, exit_code,
                                            infop, ru);
-        }
-
-        write_lock_irq(&tasklist_lock);
-
-        /*
-         * This uses xchg to be atomic with the thread resuming and setting
-         * it.  It must also be done with the write lock held to prevent a
-         * race with the EXIT_ZOMBIE case.
-         */
-        exit_code = xchg(&p->exit_code, 0);
-        if (unlikely(p->exit_state)) {
-                /*
-                 * The task resumed and then died.  Let the next iteration
-                 * catch it in EXIT_ZOMBIE.  Note that exit_code might
-                 * already be zero here if it resumed and did _exit(0).
-                 * The task itself is dead and won't touch exit_code again;
-                 * other processors in this function are locked out.
-                 */
-                p->exit_code = exit_code;
-                exit_code = 0;
-        }
-        if (unlikely(exit_code == 0)) {
-                /*
-                 * Another thread in this function got to it first, or it
-                 * resumed, or it resumed and then died.
-                 */
-                write_unlock_irq(&tasklist_lock);
-bail_ref:
-                put_task_struct(p);
-                /*
-                 * We are returning to the wait loop without having successfully
-                 * removed the process and having released the lock. We cannot
-                 * continue, since the "p" task pointer is potentially stale.
-                 *
-                 * Return -EAGAIN, and do_wait() will restart the loop from the
-                 * beginning. Do _not_ re-acquire the lock.
-                 */
-                return -EAGAIN;
-        }
-
-        /* move to end of parent's list to avoid starvation */
-        remove_parent(p);
-        add_parent(p);
-
-        write_unlock_irq(&tasklist_lock);
 
         retval = ru ? getrusage(p, RUSAGE_BOTH, ru) : 0;
         if (!retval && stat_addr)
@@ -1443,15 +1411,13 @@ bail_ref:
         if (!retval && infop)
                 retval = put_user(0, &infop->si_errno);
         if (!retval && infop)
-                retval = put_user((short)((p->ptrace & PT_PTRACED)
-                                          ? CLD_TRAPPED : CLD_STOPPED),
-                                  &infop->si_code);
+                retval = put_user(why, &infop->si_code);
         if (!retval && infop)
                 retval = put_user(exit_code, &infop->si_status);
         if (!retval && infop)
                 retval = put_user(pid, &infop->si_pid);
         if (!retval && infop)
-                retval = put_user(p->uid, &infop->si_uid);
+                retval = put_user(uid, &infop->si_uid);
         if (!retval)
                 retval = pid;
         put_task_struct(p);
@@ -1558,8 +1524,6 @@ repeat:
                                 retval = wait_task_stopped(p, ret == 2,
                                                 (options & WNOWAIT), infop,
                                                 stat_addr, ru);
-                                if (retval == -EAGAIN)
-                                        goto repeat;
                                 if (retval != 0) /* He released the lock.  */
                                         goto end;
                         } else if (p->exit_state == EXIT_ZOMBIE) {