Replace suspicious popen_pid assignment with an obviously correct one

popen_pid = vfork() does work correctly, but for a subtle reason
that wrong assignment of 0 happens in the child _first_,
and _then_ correct value overwrites it in the parent.

(And in a hyphothetical system where vfork = fork,
popen_pid wouldn't be shared, so it will also be ok.)

However, it's not necessary to be difficult.
This change makes it so that assignment is done only in parent.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/strace.c b/strace.c
index 4c70b98..2741201 100644
--- a/strace.c
+++ b/strace.c
@@ -485,6 +485,7 @@ static FILE *
 strace_popen(const char *command)
 {
 	FILE *fp;
+	int pid;
 	int fds[2];
 
 	swap_uid();
@@ -493,11 +494,11 @@ strace_popen(const char *command)
 
 	set_cloexec_flag(fds[1]); /* never fails */
 
-	popen_pid = vfork();
-	if (popen_pid == -1)
+	pid = vfork();
+	if (pid < 0)
 		perror_msg_and_die("vfork");
 
-	if (popen_pid == 0) {
+	if (pid == 0) {
 		/* child */
 		close(fds[1]);
 		if (fds[0] != 0) {
@@ -510,6 +511,7 @@ strace_popen(const char *command)
 	}
 
 	/* parent */
+	popen_pid = pid;
 	close(fds[0]);
 	swap_uid();
 	fp = fdopen(fds[1], "w");