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overlay: use private merged directory
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use a private "merged" directory when mounting from an additional
store.

Operations like "Diff()" and "Changes()" cause an implicit mount when
the naive differ is used.

The issue was not observed earlier because native overlay can achieve
these operations without using a mount.

Since these mounts are performed read-only, and overlay supports
multiple mounts using the same lowerdirs, use a private location for
the "merged" directory.  The location is owned by the current
writeable store, that is locked for writing.

Closes: #2033

Signed-off-by: Giuseppe Scrivano <[email protected]>
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giuseppe committed Jul 17, 2024
1 parent 5aa9cde commit 120642a
Showing 1 changed file with 30 additions and 8 deletions.
38 changes: 30 additions & 8 deletions drivers/overlay/overlay.go
Original file line number Diff line number Diff line change
Expand Up @@ -848,14 +848,14 @@ func (d *Driver) Status() [][2]string {
// Metadata returns meta data about the overlay driver such as
// LowerDir, UpperDir, WorkDir and MergeDir used to store data.
func (d *Driver) Metadata(id string) (map[string]string, error) {
dir := d.dir(id)
dir, _, inAdditionalStore := d.dir2(id, false)
if err := fileutils.Exists(dir); err != nil {
return nil, err
}

metadata := map[string]string{
"WorkDir": path.Join(dir, "work"),
"MergedDir": path.Join(dir, "merged"),
"MergedDir": d.getMergedDir(id, dir, inAdditionalStore),
"UpperDir": path.Join(dir, "diff"),
}

Expand Down Expand Up @@ -1703,10 +1703,10 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
}
}

mergedDir := path.Join(dir, "merged")
mergedDir := d.getMergedDir(id, dir, inAdditionalStore)
// Attempt to create the merged dir only if it doesn't exist.
if err := fileutils.Exists(mergedDir); err != nil && os.IsNotExist(err) {
if err := idtools.MkdirAs(mergedDir, 0o700, rootUID, rootGID); err != nil && !os.IsExist(err) {
if err := idtools.MkdirAllAs(mergedDir, 0o700, rootUID, rootGID); err != nil && !os.IsExist(err) {
return "", err
}
}
Expand Down Expand Up @@ -1856,7 +1856,9 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
mountFunc = func(source string, target string, mType string, flags uintptr, label string) error {
return mountOverlayFrom(d.home, source, target, mType, flags, label)
}
mountTarget = path.Join(id, "merged")
if !inAdditionalStore {
mountTarget = path.Join(id, "merged")
}
}

// overlay has a check in place to prevent mounting the same file system twice
Expand All @@ -1875,13 +1877,25 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
return mergedDir, nil
}

// getMergedDir returns the directory path that should be used as the mount point for the overlayfs.
func (d *Driver) getMergedDir(id, dir string, inAdditionalStore bool) string {
// If the layer is in an additional store, the lock we might hold only a reading lock. To prevent
// races with other processes, use a private directory under the main store rundir, where we hold
// a read/write lock.
if inAdditionalStore {
return path.Join(d.runhome, id, "merged")
}
return path.Join(dir, "merged")
}

// Put unmounts the mount path created for the give id.
func (d *Driver) Put(id string) error {
dir, _, inAdditionalStore := d.dir2(id, false)
if err := fileutils.Exists(dir); err != nil {
return err
}
mountpoint := path.Join(dir, "merged")
mountpoint := d.getMergedDir(id, dir, inAdditionalStore)

if count := d.ctr.Decrement(mountpoint); count > 0 {
return nil
}
Expand Down Expand Up @@ -1938,7 +1952,15 @@ func (d *Driver) Put(id string) error {
}
}

if !inAdditionalStore {
if inAdditionalStore {
// check the base name for extra safety
if strings.HasPrefix(mountpoint, d.runhome) && filepath.Base(mountpoint) == "merged" {
err := os.RemoveAll(filepath.Dir(mountpoint))
if err != nil {
logrus.Warningf("Failed to remove mountpoint %s overlay: %s: %v", id, mountpoint, err)
}
}
} else {
uid, gid := int(0), int(0)
fi, err := os.Stat(mountpoint)
if err != nil {
Expand All @@ -1955,7 +1977,7 @@ func (d *Driver) Put(id string) error {
// rename(2) can be used on an empty directory, as it is the mountpoint after umount, and it retains
// its atomic semantic. In this way the "merged" directory is never removed.
if err := unix.Rename(tmpMountpoint, mountpoint); err != nil {
logrus.Debugf("Failed to replace mountpoint %s overlay: %s - %v", id, mountpoint, err)
logrus.Debugf("Failed to replace mountpoint %s overlay: %s: %v", id, mountpoint, err)
return fmt.Errorf("replacing mount point %q: %w", mountpoint, err)
}
}
Expand Down

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