1ab7c2fc6
Denys Vlasenko
2016-11-03
Subject
ash: while (!got_sig) pause() is not reliable, use sigsuspend()
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shell/ash.c+6 -5
| @@ -3933,6 +3933,8 @@ wait_block_or_sig(int *status) | |
| int pid; | |
| do { | |
| + | sigset_t mask; |
| + | |
| /* Poll all children for changes in their state */ | |
| got_sigchld = 0; | |
| /* if job control is active, accept stopped processes too */ | |
| @@ -3941,14 +3943,13 @@ wait_block_or_sig(int *status) | |
| break; /* Error (e.g. EINTR, ECHILD) or pid */ | |
| /* Children exist, but none are ready. Sleep until interesting signal */ | |
| - | #if 0 /* dash does this */ |
| - | sigset_t mask; |
| + | #if 1 |
| sigfillset(&mask); | |
| sigprocmask(SIG_SETMASK, &mask, &mask); | |
| while (!got_sigchld && !pending_sig) | |
| sigsuspend(&mask); | |
| sigprocmask(SIG_SETMASK, &mask, NULL); | |
| - | #else |
| + | #else /* unsafe: a signal can set pending_sig after check, but before pause() */ |
| while (!got_sigchld && !pending_sig) | |
| pause(); | |
| #endif | |
| @@ -3987,9 +3988,9 @@ dowait(int block, struct job *job) | |
| * either enter a sleeping waitpid() (BUG), or need to busy-loop. | |
| * | |
| * Because of this, we run inside INT_OFF, but use a special routine | |
| - | * which combines waitpid() and pause(). |
| + | * which combines waitpid() and sigsuspend(). |
| * This is the reason why we need to have a handler for SIGCHLD: | |
| - | * SIG_DFL handler does not wake pause(). |
| + | * SIG_DFL handler does not wake sigsuspend(). |
| */ | |
| INT_OFF; | |
| if (block == DOWAIT_BLOCK_OR_SIG) { | |