Btrfs-progs: fix resolving of loop devices

The LOOP_GET_STATUS ioctl truncates filenames to 64 characters. We should get
the backing file for a given loop device from /sys/. This is how losetup does it
as well.

Signed-off-by: Nirbheek Chauhan <nirbheek.chauhan@collabora.co.uk>
Signed-off-by: Gene Czarcinski <gene@czarc.net>
Tested-By: Hector Oron <hector.oron@collabora.co.uk>

diff --git a/utils.c b/utils.c
index 8e1e66c..ba017fd 100644 (file)
--- a/utils.c
+++ b/utils.c
@@ -20,6 +20,7 @@
 #define __USE_XOPEN2K
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #ifndef __CHECKER__
 #include <sys/ioctl.h>
 #include <sys/mount.h>
@@ -652,21 +653,22 @@ int is_loop_device (const char* device) {
  * the associated file (e.g. /images/my_btrfs.img) */
 int resolve_loop_device(const char* loop_dev, char* loop_file, int max_len)
 {
-       int loop_fd;
-       int ret_ioctl;
-       struct loop_info loopinfo;
+       int ret;
+       FILE *f;
+       char fmt[20];
+       char p[PATH_MAX];
+       char real_loop_dev[PATH_MAX];
 
-       if ((loop_fd = open(loop_dev, O_RDONLY)) < 0)
+       if (!realpath(loop_dev, real_loop_dev))
+               return -errno;
+       snprintf(p, PATH_MAX, "/sys/block/%s/loop/backing_file", strrchr(real_loop_dev, '/'));
+       if (!(f = fopen(p, "r")))
                return -errno;
 
-       ret_ioctl = ioctl(loop_fd, LOOP_GET_STATUS, &loopinfo);
-       close(loop_fd);
-
-       if (ret_ioctl == 0) {
-               strncpy(loop_file, loopinfo.lo_name, max_len);
-               if (max_len > 0)
-                       loop_file[max_len-1] = 0;
-       } else
+       snprintf(fmt, 20, "%%%i[^\n]", max_len-1);
+       ret = fscanf(f, fmt, loop_file);
+       fclose(f);
+       if (ret == EOF)
                return -errno;
 
        return 0;