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authorOleg Nesterov <oleg@tv-sign.ru>2008-02-08 04:19:01 -0800
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2008-02-08 09:22:26 -0800
commitee7c82da830ea860b1f9274f1f0cdf99f206e7c2 (patch)
tree449477479b25ec73a1c2242723b3fb1c09cd936d
parent6405f7f4675884b671bee66678e1c2859bdb0e56 (diff)
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>
-rw-r--r--kernel/exit.c88
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) {