signal: sigprocmask() should do retarget_shared_pending()

In short, almost every changing of current->blocked is wrong, or at least
can lead to the unexpected results.

For example. Two threads T1 and T2, T1 sleeps in sigtimedwait/pause/etc.
kill(tgid, SIG) can pick T2 for TIF_SIGPENDING. If T2 calls sigprocmask()
and blocks SIG before it notices the pending signal, nobody else can handle
this pending shared signal.

I am not sure this is bug, but at least this looks strange imho. T1 should
not sleep forever, there is a signal which should wake it up.

This patch moves the code which actually changes ->blocked into the new
helper, set_current_blocked() and changes this code to call
retarget_shared_pending() as exit_signals() does. We should only care about
the signals we just blocked, we use "newset & ~current->blocked" as a mask.

We do not check !sigisemptyset(newblocked), retarget_shared_pending() is
cheap unless mask & shared_pending.

Note: for this particular case we could simply change sigprocmask() to
return -EINTR if signal_pending(), but then we should change other callers
and, more importantly, if we need this fix then set_current_blocked() will
have more callers and some of them can't restart. See the next patch as a
random example.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Matt Fleming <matt.fleming@linux.intel.com>
Acked-by: Tejun Heo <tj@kernel.org>

1 files changed, 24 insertions, 5 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index e8308e3238c1..8aa3a2e226af 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2299,6 +2299,29 @@ long do_no_restart_syscall(struct restart_block *param)
         return -EINTR;
 }
 
+/**
+ * set_current_blocked - change current->blocked mask
+ * @newset: new mask
+ *
+ * It is wrong to change ->blocked directly, this helper should be used
+ * to ensure the process can't miss a shared signal we are going to block.
+ */
+void set_current_blocked(const sigset_t *newset)
+{
+        struct task_struct *tsk = current;
+
+        spin_lock_irq(&tsk->sighand->siglock);
+        if (signal_pending(tsk) && !thread_group_empty(tsk)) {
+                sigset_t newblocked;
+                /* A set of now blocked but previously unblocked signals. */
+                signandsets(&newblocked, newset, &current->blocked);
+                retarget_shared_pending(tsk, &newblocked);
+        }
+        tsk->blocked = *newset;
+        recalc_sigpending();
+        spin_unlock_irq(&tsk->sighand->siglock);
+}
+
 /*
  * This is also useful for kernel threads that want to temporarily
  * (or permanently) block certain signals.
@@ -2330,11 +2353,7 @@ int sigprocmask(int how, sigset_t *set, sigset_t *oldset)
                 return -EINVAL;
         }
 
-        spin_lock_irq(&tsk->sighand->siglock);
-        tsk->blocked = newset;
-        recalc_sigpending();
-        spin_unlock_irq(&tsk->sighand->siglock);
-
+        set_current_blocked(&newset);
         return 0;
 }