umount: use UMOUNT_NOFOLLOW for non-root users

The UMOUNT_NOFOLLOW umount2() flag is supported since kernel 2.6.34.

Signed-off-by: Karel Zak <kzak@redhat.com>

diff --git a/mount/umount.c b/mount/umount.c
index 42671f465eefd42dd1391e373c9d46582de2d632..347945c82eb6c58a49b92de103af914396fdac5f 100644 (file)
--- a/mount/umount.c
+++ b/mount/umount.c
@@ -45,15 +45,22 @@ umount2(const char *path, int flags) {
 }
 #endif /* __NR_umount2 */
 
-#if !defined(MNT_FORCE)
-/* dare not try to include <linux/mount.h> -- lots of errors */
-#define MNT_FORCE 1
+#ifndef MNT_FORCE
+# define MNT_FORCE        0x00000001   /* Attempt to forcibily umount */
 #endif
 
 #endif /* MNT_FORCE */
 
-#if !defined(MNT_DETACH)
-#define MNT_DETACH 2
+#ifndef MNT_DETACH
+# define MNT_DETACH       0x00000002   /* Just detach from the tree */
+#endif
+
+#ifndef UMOUNT_NOFOLLOW
+# define UMOUNT_NOFOLLOW  0x00000008   /* Don't follow symlink on umount */
+#endif
+
+#ifndef UMOUNT_UNUSED
+# define UMOUNT_UNUSED    0x80000000   /* Flag guaranteed to be unused */
 #endif
 
 
@@ -197,6 +204,61 @@ static void complain(int err, const char *dev) {
   }
 }
 
+/* Check whether the kernel supports UMOUNT_NOFOLLOW flag */
+static int umount_nofollow_support(void)
+{
+       int res = umount2("", UMOUNT_UNUSED);
+       if (res != -1 || errno != EINVAL)
+               return 0;
+
+       res = umount2("", UMOUNT_NOFOLLOW);
+       if (res != -1 || errno != ENOENT)
+               return 0;
+
+       return 1;
+}
+
+static const char *chdir_to_parent(const char *node, char **resbuf)
+{
+       char *tmp, *res;
+       const char *parent;
+       char buf[65536];
+
+       *resbuf = xstrdup(node);
+
+       tmp = strrchr(*resbuf, '/');
+       if (!tmp)
+               die (2, _("umount: internal error: invalid abs path: %s"), node);
+
+       if (tmp != *resbuf) {
+               *tmp = '\0';
+               res = tmp + 1;
+               parent = *resbuf;
+       } else if (tmp[1] != '\0') {
+               res = tmp + 1;
+               parent = "/";
+       } else {
+               res = ".";
+               parent = "/";
+       }
+
+       if (chdir(parent) == -1)
+               die (2, _("umount: failed to chdir to %s: %s"),
+                                       parent, strerror(errno));
+
+       if (!getcwd(buf, sizeof(buf)))
+               die (2, _("umount: failed to obtain current directory: %s"),
+                       strerror(errno));
+
+       if (strcmp(buf, parent) != 0)
+               die (2, _("umount: mountpoint moved (%s -> %s)"), parent, buf);
+
+       if (verbose)
+               printf(_("current directory moved to %s\n"), res);
+
+       return res;
+}
+
 /* Umount a single device.  Return a status code, so don't exit
    on a non-fatal error.  We lock/unlock around each umount.  */
 static int
@@ -206,7 +268,9 @@ umount_one (const char *spec, const char *node, const char *type,
        int isroot;
        int res = 0;
        int status;
-       const char *loopdev;
+       int extra_flags = 0;
+       const char *loopdev, *target = node;
+       char *targetbuf = NULL;
        int myloop = 0;
 
        /* Special case for root.  As of 0.99pl10 we can (almost) unmount root;
@@ -237,15 +301,23 @@ umount_one (const char *spec, const char *node, const char *type,
        if (delloop && is_loop_device(spec))
                myloop = 1;
 
+       if (restricted) {
+               if (umount_nofollow_support())
+                       extra_flags |= UMOUNT_NOFOLLOW;
+
+               /* call umount(2) with relative path to avoid races */
+               target = chdir_to_parent(node, &targetbuf);
+       }
+
        if (lazy) {
-               res = umount2 (node, MNT_DETACH);
+               res = umount2 (target, MNT_DETACH | extra_flags);
                if (res < 0)
                        umnt_err = errno;
                goto writemtab;
        }
 
        if (force) {            /* only supported for NFS */
-               res = umount2 (node, MNT_FORCE);
+               res = umount2 (target, MNT_FORCE | extra_flags);
                if (res == -1) {
                        int errsv = errno;
                        perror("umount2");
@@ -253,11 +325,15 @@ umount_one (const char *spec, const char *node, const char *type,
                        if (errno == ENOSYS) {
                                if (verbose)
                                        printf(_("no umount2, trying umount...\n"));
-                               res = umount (node);
+                               res = umount (target);
                        }
                }
-       } else
-               res = umount (node);
+       } else if (extra_flags)
+               res = umount2 (target, extra_flags);
+       else
+               res = umount (target);
+
+       free(targetbuf);
 
        if (res < 0)
                umnt_err = errno;