[CRIU] [PATCH] UBUNTU: SAUCE: overlayfs: use shiftfs hacks only with shiftfs as underlay

[Andrei Vagin]

The hack was introduced in ("UBUNTU: SAUCE: overlayfs: allow with
shiftfs as underlay") and it broke checkpoint/restore of docker
contains:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1857257

The following script can be used to trigger the issue:
  #!/bin/bash

  cat > test.py << EOF
  import sys

  f = open("/proc/self/maps")

  for l in f.readlines():
    if "python" not in l:
      continue
    print(l)
    s = l.split()
    start, end = s[0].split("-")
    fname = s[-1]
    print(start, end, fname)
    break
  else:
    sys.exit(1)

  test_file1 = open(fname)
  test_file2 = open("/proc/self/map_files/%s-%s" % (start, end))

  fdinfo1 = open("/proc/self/fdinfo/%d" % test_file1.fileno()).read()
  fdinfo2 = open("/proc/self/fdinfo/%d" % test_file2.fileno()).read()

  if fdinfo1 != fdinfo2:
    print("FAIL")
    print(test_file1)
    print(fdinfo1)
    print(test_file2)
    print(fdinfo2)
    sys.exit(1)
  print("PASS")
  EOF
  sudo docker run -it --privileged --rm -v `pwd`:/mnt python python /mnt/test.py

Cc: Christian Brauner <christian.brauner at ubuntu.com>
Cc: Stefan Bader <stefan.bader at canonical.com>
Cc: Connor Kuehl <connor.kuehl at canonical.com>
Cc: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
Fixes: 58009298c6bd ("UBUNTU: SAUCE: overlayfs: allow with shiftfs as underlay")
Signed-off-by: Andrei Vagin <avagin at gmail.com>
---
 fs/overlayfs/file.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 7b86d6080c22..c57cfe50f6cc 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -12,6 +12,7 @@
 #include <linux/xattr.h>
 #include <linux/uio.h>
 #include <linux/uaccess.h>
+#include <linux/magic.h>
 #include "overlayfs.h"
 
 static char ovl_whatisit(struct inode *inode, struct inode *realinode)
@@ -35,8 +36,12 @@ static struct file *ovl_open_realfile(const struct file *file,
 
 	old_cred = ovl_override_creds(inode->i_sb);
 	ovl_path_real(file->f_path.dentry, &realpath);
-	realfile = open_with_fake_path(&realpath, flags, realinode,
-				       current_cred());
+	if (realpath.dentry->d_sb->s_magic == SHIFTFS_MAGIC)
+		realfile = open_with_fake_path(&realpath, flags, realinode,
+					       current_cred());
+	else
+		realfile = open_with_fake_path(&file->f_path, flags, realinode,
+					       current_cred());
 	revert_creds(old_cred);
 
 	pr_debug("open(%p[%pD2/%c], 0%o) -> (%p, 0%o)\n",
-- 
2.17.1