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

3 files changed, 21 insertions, 20 deletions

diff --git a/init/init.c b/init/init.c
index 232896709..4b2bd9c77 100644
--- a/init/init.c
+++ b/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(); - 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);
+                bb_sanitize_stdio();
         }
 
         s = getenv("TERM");
diff --git a/libbb/vfork_daemon_rexec.c b/libbb/vfork_daemon_rexec.c
index 37d4c274e..da0dc03e5 100644
--- a/libbb/vfork_daemon_rexec.c
+++ b/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 */
diff --git a/util-linux/mdev.c b/util-linux/mdev.c
index c7109373d..9c4938a70 100644
--- a/util-linux/mdev.c
+++ b/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 {