[PATCH] UBUNTU: SAUCE: overlayfs: fix incorrect mnt_id of files opened from map_files

[alexander at mihalicyn.com]

From: Alexander Mikhalitsyn <alexander at mihalicyn.com>

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

Thanks to Andrei Vagin for the reproducer and investigation of this problem.

Cc: Andrei Vagin <avagin at gmail.com>
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: Alexander Mikhalitsyn <alexander at mihalicyn.com>
---
 fs/overlayfs/file.c      | 17 +++++++++++++++++
 fs/overlayfs/overlayfs.h | 12 ++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 0d3ea0cf3e98..7e63eb52bb0d 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -351,6 +351,23 @@ static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
 		vma->vm_file = file;
 		fput(realfile);
 	} else {
+		/*
+		 * In map_files_get_link() (fs/proc/base.c)
+		 * we need to determine correct path from overlayfs.
+		 * But real_mount(realfile->f_path.mnt) may be not
+		 * equal to real_mount(file->f_path.mnt). In such case
+		 * fdinfo of the same file which was opened from
+		 * /proc/<pid>/map_files/... and "usual" path
+		 * will show different mnt_id.
+		 *
+		 * We solve issue like in aufs by using additional
+		 * field on struct vm_area_struct called "vm_prfile"
+		 * which is used only for fdinfo/"printing" needs.
+		 *
+		 * See also mm/prfile.c
+		 */
+		ovl_vm_prfile_set(vma, file);
+
 		/* Drop reference count from previous vm_file value */
 		fput(file);
 	}
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 5f0188807666..a4e6023b4fdb 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -442,3 +442,15 @@ int ovl_set_origin(struct dentry *dentry, struct dentry *lower,
 
 /* export.c */
 extern const struct export_operations ovl_export_operations;
+
+/* handle vma->vm_prfile */
+static inline void ovl_vm_prfile_set(struct vm_area_struct *vma,
+				    struct file *file)
+{
+	get_file(file);
+	vma->vm_prfile = file;
+#ifndef CONFIG_MMU
+	get_file(file);
+	vma->vm_region->vm_prfile = file;
+#endif
+}
-- 
2.30.2