mount: rewrite is_readonly()

This new implementation is more optimistic and always calls access(2).
The fallback solution (modify atime by futimens(2)) should be used
very rarely.

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

diff --git a/mount/mount.c b/mount/mount.c
index 84986e36b48639ef2e78deecc4de91ff197b2a40..22828b4e2a77145045775b664bd515b98f1fd70b 100644 (file)
--- a/mount/mount.c
+++ b/mount/mount.c
@@ -1308,38 +1308,46 @@ cdrom_setspeed(const char *spec) {
 }
 
 /*
- * Check if @node is read-only filesystem by access() or futimens().
- *
- * Note that access(2) uses real-UID (= useless for suid programs)
- * and euidaccess(2) does not check for read-only FS.
+ * Check if @path is on read-only filesystem independently on file permissions.
  */
 static int
-is_readonly(const char *node)
+is_readonly(const char *path)
 {
-       int res = 0;
+       int fd;
+
+       if (access(path, W_OK) == 0)
+               return 0;
+       if (errno == EROFS)
+               return 1;
+       if (errno != EACCES)
+               return 0;
 
-       if (getuid() == geteuid()) {
-               if (access(node, W_OK) == -1 && errno == EROFS)
-                       res = 1;
-       }
 #ifdef HAVE_FUTIMENS
-       else {
+       /*
+        * access(2) returns EACCES on read-only FS:
+        *
+        * - for set-uid application if one component of the path is not
+        *   accessible for the current rUID. (Note that euidaccess(2) does not
+        *   check for EROFS at all).
+        *
+        * - for read-write filesystem with read-only VFS node (aka -o remount,ro,bind)
+        */
+       fd = open(path, O_RDONLY);
+       if (fd >= 0) {
                struct timespec times[2];
-               int fd = open(node, O_RDONLY);
-
-               if (fd < 0)
-                       goto done;
+               int errsv = 0;
 
                times[0].tv_nsec = UTIME_NOW;   /* atime */
                times[1].tv_nsec = UTIME_OMIT;  /* mtime */
 
-               if (futimens(fd, times) == -1 && errno == EROFS)
-                       res = 1;
+               if (futimens(fd, times) == -1)
+                       errsv = errno;
                close(fd);
+
+               return errsv == EROFS;
        }
-done:
 #endif
-       return res;
+       return 0;
 }
 
 /*