mdev: do not follow symlinks in /sys (as was intended prior to rev 18811).

If this breaks things, please document why!
mdev,init: use shared code for fd sanitization

function                                             old     new   delta
bb_daemonize_or_rexec                                155     172     +17
mdev_main                                            500     505      +5
init_main                                            907     856     -51
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 22/-51)            Total: -29 bytes

init/init.c

@@ -221,20 +221,7 @@ static void console_init(void)
 	} else {
 		/* Make sure fd 0,1,2 are not closed
 		 * (so that they won't be used by future opens) */
-
+		bb_sanitize_stdio();
-		/* bb_sanitize_stdio(); - WRONG.
-		 * It fails if "/dev/null" doesnt exist, and for init
-		 * this is a real possibility! Open code it instead. */
-
-		int fd = open(bb_dev_null, O_RDWR);
-		if (fd < 0) {
-			/* Give me _ANY_ open descriptor! */
-			fd = xopen("/", O_RDONLY); /* we don't believe this can fail */
-		}
-		while ((unsigned)fd < 2)
-			fd = dup(fd);
-		if (fd > 2)
-			close(fd);
 	}
 
 	s = getenv("TERM");

libbb/vfork_daemon_rexec.c

@@ -265,7 +265,14 @@ void FAST_FUNC bb_daemonize_or_rexec(int flags, char **argv)
 		close(2);
 	}
 
-	fd = xopen(bb_dev_null, O_RDWR);
+	fd = open(bb_dev_null, O_RDWR);
+	if (fd < 0) {
+		/* NB: we can be called as bb_sanitize_stdio() from init
+		 * or mdev, and there /dev/null may legitimately not (yet) exist!
+		 * Do not use xopen above, but obtain _ANY_ open descriptor,
+		 * even bogus one as below. */
+		fd = xopen("/", O_RDONLY); /* don't believe this can fail */
+	}
 
 	while ((unsigned)fd < 2)
 		fd = dup(fd); /* have 0,1,2 open at least to /dev/null */

util-linux/mdev.c

@@ -19,7 +19,8 @@ struct globals {
 #define root_major (G.root_major)
 #define root_minor (G.root_minor)
 
-#define MAX_SYSFS_DEPTH 3 /* prevent infinite loops in /sys symlinks */
+/* Prevent infinite loops in /sys symlinks */
+#define MAX_SYSFS_DEPTH 3
 
 /* We use additional 64+ bytes in make_device() */
 #define SCRATCH_SIZE 80
@@ -392,11 +393,17 @@ int mdev_main(int argc, char **argv)
 	char *env_path;
 	RESERVE_CONFIG_BUFFER(temp, PATH_MAX + SCRATCH_SIZE);
 
-#ifdef YOU_WANT_TO_DEBUG_HOTPLUG_EVENTS
+	/* We can be called as hotplug helper */
 	/* Kernel cannot provide suitable stdio fds for us, do it ourself */
+#if 1
+	bb_sanitize_stdio();
+#else
+	/* Debug code */
 	/* Replace LOGFILE by other file or device name if you need */
 #define LOGFILE "/dev/console"
-	xmove_fd(xopen("/dev/null", O_RDONLY), STDIN_FILENO);
+	/* Just making sure fd 0 is not closed,
+	 * we don't really intend to read from it */
+	xmove_fd(xopen("/", O_RDONLY), STDIN_FILENO);
 	xmove_fd(xopen(LOGFILE, O_WRONLY|O_APPEND), STDOUT_FILENO);
 	xmove_fd(xopen(LOGFILE, O_WRONLY|O_APPEND), STDERR_FILENO);
 #endif
@@ -414,11 +421,11 @@ int mdev_main(int argc, char **argv)
 		root_minor = minor(st.st_dev);
 
 		recursive_action("/sys/block",
-			ACTION_RECURSE | ACTION_FOLLOWLINKS,
+			ACTION_RECURSE /* no ACTION_FOLLOWLINKS! */,
 			fileAction, dirAction, temp, 0);
 
 		recursive_action("/sys/class",
-			ACTION_RECURSE | ACTION_FOLLOWLINKS,
+			ACTION_RECURSE /* no ACTION_FOLLOWLINKS! */,
 			fileAction, dirAction, temp, 0);
 
 	} else {