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[release/1.6] fix: check for tmpfs when evaluating if userxattr should be used #7788

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merged 1 commit into from Dec 9, 2022
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[release/1.6] fix: check for tmpfs when evaluating if userxattr should be used #7788

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26 changes: 26 additions & 0 deletions snapshots/overlay/overlayutils/check.go
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Expand Up @@ -23,6 +23,7 @@ import (
"fmt"
"os"
"path/filepath"
"syscall"

kernel "github.com/containerd/containerd/contrib/seccomp/kernelversion"
"github.com/containerd/containerd/log"
Expand All @@ -31,6 +32,11 @@ import (
"github.com/containerd/continuity/fs"
)

const (
// see https://man7.org/linux/man-pages/man2/statfs.2.html
tmpfsMagic = 0x01021994
)

// SupportsMultipleLowerDir checks if the system supports multiple lowerdirs,
// which is required for the overlay snapshotter. On 4.x kernels, multiple lowerdirs
// are always available (so this check isn't needed), and backported to RHEL and
Expand Down Expand Up @@ -88,6 +94,21 @@ func Supported(root string) error {
return SupportsMultipleLowerDir(root)
}

// IsPathOnTmpfs returns whether the path is on a tmpfs or not.
//
// It uses statfs to check if the fs type is TMPFS_MAGIC (0x01021994)
// see https://man7.org/linux/man-pages/man2/statfs.2.html
func IsPathOnTmpfs(d string) bool {
stat := syscall.Statfs_t{}
err := syscall.Statfs(d, &stat)
if err != nil {
log.L.WithError(err).Warnf("Could not retrieve statfs for %v", d)
return false
}

return stat.Type == tmpfsMagic
}

// NeedsUserXAttr returns whether overlayfs should be mounted with the "userxattr" mount option.
//
// The "userxattr" option is needed for mounting overlayfs inside a user namespace with kernel >= 5.11.
Expand All @@ -114,6 +135,11 @@ func NeedsUserXAttr(d string) (bool, error) {
return false, nil
}

// userxattr not permitted on tmpfs https://man7.org/linux/man-pages/man5/tmpfs.5.html
if IsPathOnTmpfs(d) {
return false, nil
}

// Fast path on kernels >= 5.11
//
// Keep in mind that distro vendors might be going to backport the patch to older kernels
Expand Down