[vfs] Implement xattr for overlayfs.

PiperOrigin-RevId: 329825497

3 files changed, 192 insertions, 16 deletions

diff --git a/pkg/sentry/fsimpl/overlay/copy_up.go b/pkg/sentry/fsimpl/overlay/copy_up.go
index 13735eb05..ba7b8495a 100644
--- a/pkg/sentry/fsimpl/overlay/copy_up.go
+++ b/pkg/sentry/fsimpl/overlay/copy_up.go
@@ -91,6 +91,10 @@ func (d *dentry) copyUpLocked(ctx context.Context) error {
 		if err != nil {
 			ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to delete upper layer file after copy-up error: %v", err)
 		}
+		if d.upperVD.Ok() {
+			d.upperVD.DecRef(ctx)
+			d.upperVD = vfs.VirtualDentry{}
+		}
 	}
 	switch ftype {
 	case linux.S_IFREG:
@@ -234,7 +238,10 @@ func (d *dentry) copyUpLocked(ctx context.Context) error {
 		panic(fmt.Sprintf("unexpected file type %o", ftype))
 	}
 
-	// TODO(gvisor.dev/issue/1199): copy up xattrs
+	if err := d.copyXattrsLocked(ctx); err != nil {
+		cleanupUndoCopyUp()
+		return err
+	}
 
 	// Update the dentry's device and inode numbers (except for directories,
 	// for which these remain overlay-assigned).
@@ -246,14 +253,10 @@ func (d *dentry) copyUpLocked(ctx context.Context) error {
 			Mask: linux.STATX_INO,
 		})
 		if err != nil {
-			d.upperVD.DecRef(ctx)
-			d.upperVD = vfs.VirtualDentry{}
 			cleanupUndoCopyUp()
 			return err
 		}
 		if upperStat.Mask&linux.STATX_INO == 0 {
-			d.upperVD.DecRef(ctx)
-			d.upperVD = vfs.VirtualDentry{}
 			cleanupUndoCopyUp()
 			return syserror.EREMOTE
 		}
@@ -265,3 +268,42 @@ func (d *dentry) copyUpLocked(ctx context.Context) error {
 	atomic.StoreUint32(&d.copiedUp, 1)
 	return nil
 }
+
+// copyXattrsLocked copies a subset of lower's extended attributes to upper.
+// Attributes that configure an overlay in the lower are not copied up.
+//
+// Preconditions: d.copyMu must be locked for writing.
+func (d *dentry) copyXattrsLocked(ctx context.Context) error {
+	vfsObj := d.fs.vfsfs.VirtualFilesystem()
+	lowerPop := &vfs.PathOperation{Root: d.lowerVDs[0], Start: d.lowerVDs[0]}
+	upperPop := &vfs.PathOperation{Root: d.upperVD, Start: d.upperVD}
+
+	lowerXattrs, err := vfsObj.ListxattrAt(ctx, d.fs.creds, lowerPop, 0)
+	if err != nil {
+		if err == syserror.EOPNOTSUPP {
+			// There are no guarantees as to the contents of lowerXattrs.
+			return nil
+		}
+		ctx.Warningf("failed to copy up xattrs because ListxattrAt failed: %v", err)
+		return err
+	}
+
+	for _, name := range lowerXattrs {
+		// Do not copy up overlay attributes.
+		if isOverlayXattr(name) {
+			continue
+		}
+
+		value, err := vfsObj.GetxattrAt(ctx, d.fs.creds, lowerPop, &vfs.GetxattrOptions{Name: name, Size: 0})
+		if err != nil {
+			ctx.Warningf("failed to copy up xattrs because GetxattrAt failed: %v", err)
+			return err
+		}
+
+		if err := vfsObj.SetxattrAt(ctx, d.fs.creds, upperPop, &vfs.SetxattrOptions{Name: name, Value: value}); err != nil {
+			ctx.Warningf("failed to copy up xattrs because SetxattrAt failed: %v", err)
+			return err
+		}
+	}
+	return nil
+}
diff --git a/pkg/sentry/fsimpl/overlay/filesystem.go b/pkg/sentry/fsimpl/overlay/filesystem.go
index 63df86481..46528c99c 100644
--- a/pkg/sentry/fsimpl/overlay/filesystem.go
+++ b/pkg/sentry/fsimpl/overlay/filesystem.go
@@ -15,6 +15,7 @@
 package overlay
 
 import (
+	"strings"
 	"sync/atomic"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
@@ -27,10 +28,15 @@ import (
 	"gvisor.dev/gvisor/pkg/syserror"
 )
 
+// _OVL_XATTR_PREFIX is an extended attribute key prefix to identify overlayfs
+// attributes.
+// Linux: fs/overlayfs/overlayfs.h:OVL_XATTR_PREFIX
+const _OVL_XATTR_PREFIX = linux.XATTR_TRUSTED_PREFIX + "overlay."
+
 // _OVL_XATTR_OPAQUE is an extended attribute key whose value is set to "y" for
 // opaque directories.
 // Linux: fs/overlayfs/overlayfs.h:OVL_XATTR_OPAQUE
-const _OVL_XATTR_OPAQUE = linux.XATTR_TRUSTED_PREFIX + "overlay.opaque"
+const _OVL_XATTR_OPAQUE = _OVL_XATTR_PREFIX + "opaque"
 
 func isWhiteout(stat *linux.Statx) bool {
 	return stat.Mode&linux.S_IFMT == linux.S_IFCHR && stat.RdevMajor == 0 && stat.RdevMinor == 0
@@ -1347,18 +1353,42 @@ func (fs *filesystem) UnlinkAt(ctx context.Context, rp *vfs.ResolvingPath) error
 	return nil
 }
 
+// isOverlayXattr returns whether the given extended attribute configures the
+// overlay.
+func isOverlayXattr(name string) bool {
+	return strings.HasPrefix(name, _OVL_XATTR_PREFIX)
+}
+
 // ListxattrAt implements vfs.FilesystemImpl.ListxattrAt.
 func (fs *filesystem) ListxattrAt(ctx context.Context, rp *vfs.ResolvingPath, size uint64) ([]string, error) {
 	var ds *[]*dentry
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckDrop(ctx, &ds)
-	_, err := fs.resolveLocked(ctx, rp, &ds)
+	d, err := fs.resolveLocked(ctx, rp, &ds)
 	if err != nil {
 		return nil, err
 	}
-	// TODO(gvisor.dev/issue/1199): Linux overlayfs actually allows listxattr,
-	// but not any other xattr syscalls. For now we just reject all of them.
-	return nil, syserror.ENOTSUP
+
+	return fs.listXattr(ctx, d, size)
+}
+
+func (fs *filesystem) listXattr(ctx context.Context, d *dentry, size uint64) ([]string, error) {
+	vfsObj := d.fs.vfsfs.VirtualFilesystem()
+	top := d.topLayer()
+	names, err := vfsObj.ListxattrAt(ctx, fs.creds, &vfs.PathOperation{Root: top, Start: top}, size)
+	if err != nil {
+		return nil, err
+	}
+
+	// Filter out all overlay attributes.
+	n := 0
+	for _, name := range names {
+		if !isOverlayXattr(name) {
+			names[n] = name
+			n++
+		}
+	}
+	return names[:n], err
 }
 
 // GetxattrAt implements vfs.FilesystemImpl.GetxattrAt.
@@ -1366,11 +1396,29 @@ func (fs *filesystem) GetxattrAt(ctx context.Context, rp *vfs.ResolvingPath, opt
 	var ds *[]*dentry
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckDrop(ctx, &ds)
-	_, err := fs.resolveLocked(ctx, rp, &ds)
+	d, err := fs.resolveLocked(ctx, rp, &ds)
 	if err != nil {
 		return "", err
 	}
-	return "", syserror.ENOTSUP
+
+	return fs.getXattr(ctx, d, rp.Credentials(), &opts)
+}
+
+func (fs *filesystem) getXattr(ctx context.Context, d *dentry, creds *auth.Credentials, opts *vfs.GetxattrOptions) (string, error) {
+	if err := d.checkXattrPermissions(creds, opts.Name, vfs.MayRead); err != nil {
+		return "", err
+	}
+
+	// Return EOPNOTSUPP when fetching an overlay attribute.
+	// See fs/overlayfs/super.c:ovl_own_xattr_get().
+	if isOverlayXattr(opts.Name) {
+		return "", syserror.EOPNOTSUPP
+	}
+
+	// Analogous to fs/overlayfs/super.c:ovl_other_xattr_get().
+	vfsObj := d.fs.vfsfs.VirtualFilesystem()
+	top := d.topLayer()
+	return vfsObj.GetxattrAt(ctx, fs.creds, &vfs.PathOperation{Root: top, Start: top}, opts)
 }
 
 // SetxattrAt implements vfs.FilesystemImpl.SetxattrAt.
@@ -1378,11 +1426,36 @@ func (fs *filesystem) SetxattrAt(ctx context.Context, rp *vfs.ResolvingPath, opt
 	var ds *[]*dentry
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckDrop(ctx, &ds)
-	_, err := fs.resolveLocked(ctx, rp, &ds)
+	d, err := fs.resolveLocked(ctx, rp, &ds)
 	if err != nil {
 		return err
 	}
-	return syserror.ENOTSUP
+
+	return fs.setXattrLocked(ctx, d, rp.Mount(), rp.Credentials(), &opts)
+}
+
+// Precondition: fs.renameMu must be locked.
+func (fs *filesystem) setXattrLocked(ctx context.Context, d *dentry, mnt *vfs.Mount, creds *auth.Credentials, opts *vfs.SetxattrOptions) error {
+	if err := d.checkXattrPermissions(creds, opts.Name, vfs.MayWrite); err != nil {
+		return err
+	}
+
+	// Return EOPNOTSUPP when setting an overlay attribute.
+	// See fs/overlayfs/super.c:ovl_own_xattr_set().
+	if isOverlayXattr(opts.Name) {
+		return syserror.EOPNOTSUPP
+	}
+
+	// Analogous to fs/overlayfs/super.c:ovl_other_xattr_set().
+	if err := mnt.CheckBeginWrite(); err != nil {
+		return err
+	}
+	defer mnt.EndWrite()
+	if err := d.copyUpLocked(ctx); err != nil {
+		return err
+	}
+	vfsObj := d.fs.vfsfs.VirtualFilesystem()
+	return vfsObj.SetxattrAt(ctx, fs.creds, &vfs.PathOperation{Root: d.upperVD, Start: d.upperVD}, opts)
 }
 
 // RemovexattrAt implements vfs.FilesystemImpl.RemovexattrAt.
@@ -1390,11 +1463,36 @@ func (fs *filesystem) RemovexattrAt(ctx context.Context, rp *vfs.ResolvingPath,
 	var ds *[]*dentry
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckDrop(ctx, &ds)
-	_, err := fs.resolveLocked(ctx, rp, &ds)
+	d, err := fs.resolveLocked(ctx, rp, &ds)
 	if err != nil {
 		return err
 	}
-	return syserror.ENOTSUP
+
+	return fs.removeXattrLocked(ctx, d, rp.Mount(), rp.Credentials(), name)
+}
+
+// Precondition: fs.renameMu must be locked.
+func (fs *filesystem) removeXattrLocked(ctx context.Context, d *dentry, mnt *vfs.Mount, creds *auth.Credentials, name string) error {
+	if err := d.checkXattrPermissions(creds, name, vfs.MayWrite); err != nil {
+		return err
+	}
+
+	// Like SetxattrAt, return EOPNOTSUPP when removing an overlay attribute.
+	// Linux passes the remove request to xattr_handler->set.
+	// See fs/xattr.c:vfs_removexattr().
+	if isOverlayXattr(name) {
+		return syserror.EOPNOTSUPP
+	}
+
+	if err := mnt.CheckBeginWrite(); err != nil {
+		return err
+	}
+	defer mnt.EndWrite()
+	if err := d.copyUpLocked(ctx); err != nil {
+		return err
+	}
+	vfsObj := d.fs.vfsfs.VirtualFilesystem()
+	return vfsObj.RemovexattrAt(ctx, fs.creds, &vfs.PathOperation{Root: d.upperVD, Start: d.upperVD}, name)
 }
 
 // PrependPath implements vfs.FilesystemImpl.PrependPath.
diff --git a/pkg/sentry/fsimpl/overlay/overlay.go b/pkg/sentry/fsimpl/overlay/overlay.go
index 00562667f..e706f9d4e 100644
--- a/pkg/sentry/fsimpl/overlay/overlay.go
+++ b/pkg/sentry/fsimpl/overlay/overlay.go
@@ -570,6 +570,16 @@ func (d *dentry) checkPermissions(creds *auth.Credentials, ats vfs.AccessTypes)
 	return vfs.GenericCheckPermissions(creds, ats, linux.FileMode(atomic.LoadUint32(&d.mode)), auth.KUID(atomic.LoadUint32(&d.uid)), auth.KGID(atomic.LoadUint32(&d.gid)))
 }
 
+func (d *dentry) checkXattrPermissions(creds *auth.Credentials, name string, ats vfs.AccessTypes) error {
+	mode := linux.FileMode(atomic.LoadUint32(&d.mode))
+	kuid := auth.KUID(atomic.LoadUint32(&d.uid))
+	kgid := auth.KGID(atomic.LoadUint32(&d.gid))
+	if err := vfs.GenericCheckPermissions(creds, ats, mode, kuid, kgid); err != nil {
+		return err
+	}
+	return vfs.CheckXattrPermissions(creds, ats, mode, kuid, name)
+}
+
 // statInternalMask is the set of stat fields that is set by
 // dentry.statInternalTo().
 const statInternalMask = linux.STATX_TYPE | linux.STATX_MODE | linux.STATX_UID | linux.STATX_GID | linux.STATX_INO
@@ -622,6 +632,32 @@ func (fd *fileDescription) dentry() *dentry {
 	return fd.vfsfd.Dentry().Impl().(*dentry)
 }
 
+// Listxattr implements vfs.FileDescriptionImpl.Listxattr.
+func (fd *fileDescription) Listxattr(ctx context.Context, size uint64) ([]string, error) {
+	return fd.filesystem().listXattr(ctx, fd.dentry(), size)
+}
+
+// Getxattr implements vfs.FileDescriptionImpl.Getxattr.
+func (fd *fileDescription) Getxattr(ctx context.Context, opts vfs.GetxattrOptions) (string, error) {
+	return fd.filesystem().getXattr(ctx, fd.dentry(), auth.CredentialsFromContext(ctx), &opts)
+}
+
+// Setxattr implements vfs.FileDescriptionImpl.Setxattr.
+func (fd *fileDescription) Setxattr(ctx context.Context, opts vfs.SetxattrOptions) error {
+	fs := fd.filesystem()
+	fs.renameMu.RLock()
+	defer fs.renameMu.RUnlock()
+	return fs.setXattrLocked(ctx, fd.dentry(), fd.vfsfd.Mount(), auth.CredentialsFromContext(ctx), &opts)
+}
+
+// Removexattr implements vfs.FileDescriptionImpl.Removexattr.
+func (fd *fileDescription) Removexattr(ctx context.Context, name string) error {
+	fs := fd.filesystem()
+	fs.renameMu.RLock()
+	defer fs.renameMu.RUnlock()
+	return fs.removeXattrLocked(ctx, fd.dentry(), fd.vfsfd.Mount(), auth.CredentialsFromContext(ctx), name)
+}
+
 // LockPOSIX implements vfs.FileDescriptionImpl.LockPOSIX.
 func (fd *fileDescription) LockPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, start, length uint64, whence int16, block fslock.Blocker) error {
 	return fd.Locks().LockPOSIX(ctx, &fd.vfsfd, uid, t, start, length, whence, block)