bring std(in|out|err) fd's in a sane state

If the kernel forks us as an usermodhelper, we don't have any of
the standard fd's and the first open() will start with fd=0.
This is inherited to all forked childs and confuses later forked
helpers where we want to read from a pipe connected to the helpers
stdout/stderr.

  # ls -l /proc/$(pidof udevd)/fd
  total 6
  dr-x------  2 root root  0 2005-08-18 12:44 .
  dr-xr-xr-x  4 root root  0 2005-08-18 12:44 ..
  lrwx------  1 root root 64 2005-08-18 12:44 0 -> /dev/null
  lrwx------  1 root root 64 2005-08-18 12:44 1 -> socket:[1274617]
  lr-x------  1 root root 64 2005-08-18 12:44 2 -> pipe:[1274618]
  l-wx------  1 root root 64 2005-08-18 12:44 3 -> pipe:[1274618]
  lrwx------  1 root root 64 2005-08-18 12:44 4 -> socket:[1274619]
  lrwx------  1 root root 64 2005-08-18 12:44 5 -> socket:[1274620]

Ouch! This will obviously not redirect sterr, it will kill the pipe
we established between the parent and the child:

  devnull = open("/dev/null", O_RDWR);
  dup2(devnull, STDERR_FILENO);

Signed-off-by: Kay Sievers <kay.sievers@suse.de>

1 files changed, 16 insertions, 0 deletions

diff --git a/udev.c b/udev.c
index beaa191c..926c4b52 100644
--- a/udev.c
+++ b/udev.c
@@ -72,6 +72,7 @@ int main(int argc, char *argv[], char *envp[])
 	const char *devpath;
 	const char *subsystem;
 	struct sigaction act;
+	int devnull;
 	int retval = -EINVAL;
 
 	if (argc == 2 && strcmp(argv[1], "-V") == 0) {
@@ -79,7 +80,22 @@ int main(int argc, char *argv[], char *envp[])
 		exit(0);
 	}
 
+	/* set std fd's to /dev/null, if the kernel forks us, we don't have them at all */
+	devnull = open("/dev/null", O_RDWR);
+	if (devnull >= 0)  {
+		if (devnull != STDIN_FILENO)
+			dup2(devnull, STDIN_FILENO);
+		if (devnull != STDOUT_FILENO)
+			dup2(devnull, STDOUT_FILENO);
+		if (devnull != STDERR_FILENO)
+			dup2(devnull, STDERR_FILENO);
+		if (devnull > STDERR_FILENO)
+			close(devnull);
+	}
+
 	logging_init("udev");
+	if (devnull < 0)
+		err("fatal, could not open /dev/null");
 	udev_init_config();
 	dbg("version %s", UDEV_VERSION);