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Make sure auto-mounted subpath mount source is already mounted

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For example, we have two filesystems, one is embedded into another:

/a/test		# first filesystem with a directory "/a/test/b2"
/a/test/b2	# not auto mounted yet second filesystem, notice "/a/test/b2" is
	        # a new directory on this filesystem after this filesystem is mounted

For subpath mount "/a/test/b2",  `openat("/a/test", "b2")` gets directory "b2" on the first
filesystem, then "mount -c" will use this wrong directory as source directory.

`fstatat("/a/test", "b2/")` forces triggering auto mount of second filesystem, so
`openat("/a/test", "b2")` gets correct source directory for "mount -c".

This fixes issue https://github.com/kubernetes/kubernetes/issues/110818#issuecomment-1175736550

References:

1. https://man7.org/linux/man-pages/man2/openat.2.html

   If pathname refers to an automount point that has not yet
   been triggered, so no other filesystem is mounted on it,
   then the call returns a file descriptor referring to the
   automount directory without triggering a mount.

2. https://man7.org/linux/man-pages/man2/open_by_handle_at.2.html

   name_to_handle_at() does not trigger a mount when the final
   component of the pathname is an automount point.  When a
   filesystem supports both file handles and automount points, a
   name_to_handle_at() call on an automount point will return with
   error EOVERFLOW without having increased handle_bytes.  This can
   happen since Linux 4.13 with NFS when accessing a directory which
   is on a separate filesystem on the server.  In this case, the
   automount can be triggered by adding a "/" to the end of the
   pathname.
This commit is contained in:
Yubao Liu 2022-07-10 19:48:16 +08:00
parent 29705ce027
commit 834ce75f88
1 changed files with 6 additions and 1 deletions
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7
pkg/volume/util/subpath/subpath_linux.go
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@ -566,18 +566,23 @@ func doSafeOpen(pathname string, base string) (int, error) {
// Follow the segments one by one using openat() to make
// sure the user cannot change already existing directories into symlinks.
for _, seg := range segments {
var deviceStat unix.Stat_t
currentPath = filepath.Join(currentPath, seg)
if !mount.PathWithinBase(currentPath, base) {
return -1, fmt.Errorf("path %s is outside of allowed base %s", currentPath, base)
}
// Trigger auto mount if it's an auto-mounted directory, ignore error if not a directory.
// Notice the trailing slash is mandatory, see "automount" in openat(2) and open_by_handle_at(2).
unix.Fstatat(parentFD, seg+"/", &deviceStat, unix.AT_SYMLINK_NOFOLLOW)
klog.V(5).Infof("Opening path %s", currentPath)
childFD, err = syscall.Openat(parentFD, seg, openFDFlags|unix.O_CLOEXEC, 0)
if err != nil {
return -1, fmt.Errorf("cannot open %s: %s", currentPath, err)
}
var deviceStat unix.Stat_t
err := unix.Fstat(childFD, &deviceStat)
if err != nil {
return -1, fmt.Errorf("error running fstat on %s with %v", currentPath, err)