[JOBS] Fix dowait signal race

This test program by Alexey Gladkov can cause dash to enter an
infinite loop in waitcmd.

#!/bin/dash
trap "echo TRAP" USR1
stub() {
echo ">>> STUB $1" >&2
sleep $1
echo "<<< STUB $1" >&2
kill -USR1 $$
}
stub 3 &
stub 2 &
until { echo "###"; wait; } do
echo "*** $?"
done

The problem is that if we get a signal after the wait3 system
call has returned but before we get to INTON in dowait, then
we can jump back up to the top and lose the exit status.  So
if we then wait for the job that has just exited, then it'll
stay there forever.

I made the original change that caused this bug to fix pretty
much the same bug but in the opposite direction.  That is, if
we get a signal after we enter wait3 but before we hit the kernel
then it too can cause the wait to go on forever (assuming the
child doesn't exit).

In fact this is pretty much exactly the scenario that you'll
find in glibc's documentation on pause().  The solution is given
there too, in the form of sigsuspend, which is the only way to
do the check and wait atomically.

So this patch fixes Alexey's race without reintroducing the old
bug by converting the blocking wait3 to a sigsuspend.

In order to do this we need to set a signal handler for SIGCHLD,
so the code has been modified to always do that.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

3 files changed, 42 insertions, 33 deletions

diff --git a/src/jobs.c b/src/jobs.c
index 2b6a752..69a84f7 100644
--- a/src/jobs.c
+++ b/src/jobs.c
@@ -75,6 +75,7 @@
 /* mode flags for dowait */
 #define DOWAIT_NORMAL 0
 #define DOWAIT_BLOCK 1
+#define DOWAIT_WAITCMD 2
 
 /* array of jobs */
 static struct job *jobtab;
@@ -589,8 +590,6 @@ waitcmd(int argc, char **argv)
 	int retval;
 	struct job *jp;
 
-	EXSIGON();
-
 	nextopt(nullstr);
 	retval = 0;
 
@@ -609,7 +608,8 @@ waitcmd(int argc, char **argv)
 				jp->waited = 1;
 				jp = jp->prev_job;
 			}
-			dowait(DOWAIT_BLOCK, 0);
+			if (dowait(DOWAIT_WAITCMD, 0) <= 0)
+				goto sigout;
 		}
 	}
 
@@ -631,7 +631,8 @@ start:
 			job = getjob(*argv, 0);
 		/* loop until process terminated or stopped */
 		while (job->state == JOBRUNNING)
-			dowait(DOWAIT_BLOCK, 0);
+			if (dowait(DOWAIT_WAITCMD, 0) <= 0)
+				goto sigout;
 		job->waited = 1;
 		retval = getstatus(job);
 repeat:
@@ -640,6 +641,10 @@ repeat:
 
 out:
 	return retval;
+
+sigout:
+	retval = 128 + pendingsigs;
+	goto out;
 }
 
 
@@ -998,16 +1003,16 @@ dowait(int block, struct job *job)
 	int pid;
 	int status;
 	struct job *jp;
-	struct job *thisjob;
+	struct job *thisjob = NULL;
 	int state;
 
+	INTOFF;
 	TRACE(("dowait(%d) called\n", block));
 	pid = waitproc(block, &status);
 	TRACE(("wait returns pid %d, status=%d\n", pid, status));
 	if (pid <= 0)
-		return pid;
-	INTOFF;
-	thisjob = NULL;
+		goto out;
+
 	for (jp = curjob; jp; jp = jp->prev_job) {
 		struct procstat *sp;
 		struct procstat *spend;
@@ -1115,34 +1120,33 @@ STATIC int onsigchild() {
 STATIC int
 waitproc(int block, int *status)
 {
-#ifdef BSD
-	int flags = 0;
+	sigset_t mask, oldmask;
+	int flags = block == DOWAIT_BLOCK ? 0 : WNOHANG;
+	int err;
+	int sig;
 
 #if JOBS
 	if (jobctl)
 		flags |= WUNTRACED;
 #endif
-	if (block == 0)
-		flags |= WNOHANG;
-	return wait3(status, flags, (struct rusage *)NULL);
-#else
-#ifdef SYSV
-	int (*save)();
 
-	if (block == 0) {
-		gotsigchild = 0;
-		save = signal(SIGCLD, onsigchild);
-		signal(SIGCLD, save);
-		if (gotsigchild == 0)
-			return 0;
-	}
-	return wait(status);
-#else
-	if (block == 0)
-		return 0;
-	return wait(status);
-#endif
-#endif
+	do {
+		err = wait3(status, flags, NULL);
+		if (err || !block)
+			break;
+
+		block = 0;
+
+		sigfillset(&mask);
+		sigprocmask(SIG_SETMASK, &mask, &oldmask);
+
+		while (!(sig = pendingsigs))
+			sigsuspend(&oldmask);
+
+		sigclearmask();
+	} while (sig == SIGCHLD);
+
+	return err;
 }
 
 /*
diff --git a/src/trap.c b/src/trap.c
index 58cd0cc..53663ae 100644
--- a/src/trap.c
+++ b/src/trap.c
@@ -71,7 +71,7 @@
 /* trap handler commands */
 char *trap[NSIG];
 /* current value of signal */
-static char sigmode[NSIG - 1];
+char sigmode[NSIG - 1];
 /* indicates specified signal received */
 char gotsig[NSIG - 1];
 /* last pending signal */
@@ -82,9 +82,10 @@ int exsig;
 extern char *signal_names[];
 
 #ifdef mkinit
-INCLUDE <signal.h>
+INCLUDE "trap.h"
 INIT {
-	signal(SIGCHLD, SIG_DFL);
+	sigmode[SIGCHLD - 1] = S_DFL;
+	setsignal(SIGCHLD);
 }
 #endif
 
@@ -207,6 +208,9 @@ setsignal(int signo)
 		}
 	}
 
+	if (signo == SIGCHLD)
+		action = S_CATCH;
+
 	t = &sigmode[signo - 1];
 	tsig = *t;
 	if (tsig == 0) {
diff --git a/src/trap.h b/src/trap.h
index e889136..50c1587 100644
--- a/src/trap.h
+++ b/src/trap.h
@@ -38,6 +38,7 @@
 
 extern char *trap[];
 extern char gotsig[];
+extern char sigmode[];
 extern volatile sig_atomic_t pendingsigs;
 
 int trapcmd(int, char **);