atomically replace existing nodes and symlinks

Based on a patch from Scott James Remnant <scott@ubuntu.com>.

diff --git a/udev_node.c b/udev_node.c
index 338948fc54b90ec0b610c31cf632a5a994b5571e..8c0287405c537505bc80a32423c44090fabcffc4 100644 (file)
--- a/udev_node.c
+++ b/udev_node.c
@@ -33,9 +33,11 @@
 #include "udev_rules.h"
 #include "udev_selinux.h"
 
+#define TMP_FILE_EXT           ".udev-tmp"
 
 int udev_node_mknod(struct udevice *udev, const char *file, dev_t devt, mode_t mode, uid_t uid, gid_t gid)
 {
+       char file_tmp[PATH_SIZE + sizeof(TMP_FILE_EXT)];
        struct stat stats;
        int retval = 0;
 
@@ -44,28 +46,37 @@ int udev_node_mknod(struct udevice *udev, const char *file, dev_t devt, mode_t m
        else
                mode |= S_IFCHR;
 
-       if (lstat(file, &stats) != 0)
-               goto create;
-
        /* preserve node with already correct numbers, to prevent changing the inode number */
-       if ((stats.st_mode & S_IFMT) == (mode & S_IFMT) && (stats.st_rdev == devt)) {
-               info("preserve file '%s', because it has correct dev_t", file);
-               selinux_setfilecon(file, udev->dev->kernel, stats.st_mode);
-               goto perms;
+       if (lstat(file, &stats) == 0) {
+               if ((stats.st_mode & S_IFMT) == (mode & S_IFMT) && (stats.st_rdev == devt)) {
+                       info("preserve file '%s', because it has correct dev_t", file);
+                       selinux_setfilecon(file, udev->dev->kernel, stats.st_mode);
+                       goto perms;
+               }
        }
 
-       if (unlink(file) != 0)
-               err("unlink(%s) failed: %s", file, strerror(errno));
-       else
-               dbg("already present file '%s' unlinked", file);
-
-create:
        selinux_setfscreatecon(file, udev->dev->kernel, mode);
        retval = mknod(file, mode, devt);
        selinux_resetfscreatecon();
+       if (retval == 0)
+               goto perms;
+
+       info("atomically replace '%s'", file);
+       strlcpy(file_tmp, file, sizeof(file_tmp));
+       strlcat(file_tmp, TMP_FILE_EXT, sizeof(file_tmp));
+       selinux_setfscreatecon(file_tmp, udev->dev->kernel, mode);
+       retval = mknod(file_tmp, mode, devt);
+       selinux_resetfscreatecon();
        if (retval != 0) {
                err("mknod(%s, %#o, %u, %u) failed: %s",
-                   file, mode, major(devt), minor(devt), strerror(errno));
+                   file_tmp, mode, major(devt), minor(devt), strerror(errno));
+               goto exit;
+       }
+       retval = rename(file_tmp, file);
+       if (retval != 0) {
+               err("rename(%s, %s) failed: %s",
+                   file_tmp, file, strerror(errno));
+               unlink(file_tmp);
                goto exit;
        }
 
@@ -84,18 +95,19 @@ perms:
                        goto exit;
                }
        }
-
 exit:
        return retval;
 }
 
 static int node_symlink(const char *node, const char *slink)
 {
+       struct stat stats;
        char target[PATH_SIZE] = "";
-       char buf[PATH_SIZE];
+       char slink_tmp[PATH_SIZE + sizeof(TMP_FILE_EXT)];
        int i = 0;
        int tail = 0;
        int len;
+       int retval = 0;
 
        /* use relative link */
        while (node[i] && (node[i] == slink[i])) {
@@ -110,28 +122,62 @@ static int node_symlink(const char *node, const char *slink)
        }
        strlcat(target, &node[tail], sizeof(target));
 
-       /* look if symlink already exists */
-       len = readlink(slink, buf, sizeof(buf));
-       if (len > 0) {
-               buf[len] = '\0';
-               if (strcmp(target, buf) == 0) {
-                       info("preserve already existing symlink '%s' to '%s'", slink, target);
-                       selinux_setfilecon(slink, NULL, S_IFLNK);
-                       goto exit;
+       /* preserve link with correct target, do not replace node of other device */
+       if (lstat(slink, &stats) == 0) {
+               if (S_ISBLK(stats.st_mode) || S_ISCHR(stats.st_mode)) {
+                       struct stat stats2;
+
+                       info("found existing node instead of symlink '%s'", slink);
+                       if (lstat(node, &stats2) == 0) {
+                               if ((stats.st_mode & S_IFMT) == (stats2.st_mode & S_IFMT) &&
+                                   stats.st_rdev == stats2.st_rdev) {
+                                       info("replace device node '%s' with symlink to our node '%s'", slink, node);
+                               } else {
+                                       err("device node '%s' already exists, link '%s' will not overwrite it", node, slink);
+                                       goto exit;
+                               }
+                       }
+               } else if (S_ISLNK(stats.st_mode)) {
+                       char buf[PATH_SIZE];
+
+                       info("found existing symlink '%s'", slink);
+                       len = readlink(slink, buf, sizeof(buf));
+                       if (len > 0) {
+                               buf[len] = '\0';
+                               if (strcmp(target, buf) == 0) {
+                                       info("preserve already existing symlink '%s' to '%s'", slink, target);
+                                       selinux_setfilecon(slink, NULL, S_IFLNK);
+                                       goto exit;
+                               }
+                       }
                }
-               info("link '%s' points to different target '%s', delete it", slink, buf);
-               unlink(slink);
        }
 
-       /* create link */
        info("creating symlink '%s' to '%s'", slink, target);
        selinux_setfscreatecon(slink, NULL, S_IFLNK);
-       if (symlink(target, slink) != 0)
-               err("symlink(%s, %s) failed: %s", target, slink, strerror(errno));
+       retval = symlink(target, slink);
        selinux_resetfscreatecon();
+       if (retval == 0)
+               goto exit;
 
+       info("atomically replace '%s'", slink);
+       strlcpy(slink_tmp, slink, sizeof(slink_tmp));
+       strlcat(slink_tmp, TMP_FILE_EXT, sizeof(slink_tmp));
+       selinux_setfscreatecon(slink_tmp, NULL, S_IFLNK);
+       retval = symlink(target, slink_tmp);
+       selinux_resetfscreatecon();
+       if (retval != 0) {
+               err("symlink(%s, %s) failed: %s", target, slink_tmp, strerror(errno));
+               goto exit;
+       }
+       retval = rename(slink_tmp, slink);
+       if (retval != 0) {
+               err("rename(%s, %s) failed: %s", slink_tmp, slink, strerror(errno));
+               unlink(slink_tmp);
+               goto exit;
+       }
 exit:
-       return 0;
+       return retval;
 }
 
 static int update_link(struct udevice *udev, const char *name)
@@ -303,7 +349,7 @@ int udev_node_add(struct udevice *udev)
                        gid = lookup_group(udev->group);
        }
 
-       info("creating device node '%s', major = '%d', minor = '%d', " "mode = '%#o', uid = '%d', gid = '%d'",
+       info("creating device node '%s', major=%d, minor=%d, mode=%#o, uid=%d, gid=%d",
             filename, major(udev->devt), minor(udev->devt), udev->mode, uid, gid);
 
        if (!udev->test_run)