Add ENAMETOOLONG checks in overlayfs.

Linux overlayfs uses the max of all layers' filename length limit. Do the same
in gVisor.

This is needed to get PHP runtime test ext/standard/tests/strings/007.phpt to
pass with overlayfs.

PiperOrigin-RevId: 493942515

pkg/sentry/fsimpl/overlay/filesystem.go

@@ -161,6 +161,9 @@ afterSymlink:
 		rp.Advance()
 		return d.parent, d.parent.topLookupLayer(), nil
 	}
+	if uint64(len(name)) > fs.maxFilenameLen {
+		return nil, lookupLayerNone, linuxerr.ENAMETOOLONG
+	}
 	child, topLookupLayer, err := fs.getChildLocked(ctx, d, name, ds)
 	if err != nil {
 		return nil, topLookupLayer, err
@@ -504,6 +507,9 @@ func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, ct
 	if name == "." || name == ".." {
 		return linuxerr.EEXIST
 	}
+	if uint64(len(name)) > fs.maxFilenameLen {
+		return linuxerr.ENAMETOOLONG
+	}
 	if parent.vfsd.IsDead() {
 		return linuxerr.ENOENT
 	}
@@ -1090,6 +1096,9 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 		}
 		return linuxerr.EBUSY
 	}
+	if uint64(len(newName)) > fs.maxFilenameLen {
+		return linuxerr.ENAMETOOLONG
+	}
 	// Do not check for newName length, since different filesystem
 	// implementations impose different name limits. upperfs.RenameAt() will fail
 	// appropriately if it has to.

pkg/sentry/fsimpl/overlay/overlay.go

@@ -125,6 +125,9 @@ type filesystem struct {
 	// lastDirIno is the last inode number assigned to a directory. lastDirIno
 	// is protected by dirInoCacheMu.
 	lastDirIno uint64
+
+	// MaxFilenameLen is the maximum filename length allowed by the overlayfs.
+	maxFilenameLen uint64
 }
 
 // +stateify savable
@@ -264,9 +267,23 @@ func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 		dirDevMinor:    dirDevMinor,
 		lowerDevMinors: make(map[layerDevNumber]uint32),
 		dirInoCache:    make(map[layerDevNoAndIno]uint64),
+		maxFilenameLen: linux.NAME_MAX,
 	}
 	fs.vfsfs.Init(vfsObj, &fstype, fs)
 
+	// Configure max filename length. Similar to what Linux does in
+	// fs/overlayfs/super.c:ovl_fill_super() -> ... -> ovl_check_namelen().
+	if fsopts.UpperRoot.Ok() {
+		if err := fs.updateMaxNameLen(ctx, creds, vfsObj, fs.opts.UpperRoot); err != nil {
+			ctx.Debugf("overlay.FilesystemType.GetFilesystem: failed to StatFSAt on upper layer root: %v", err)
+		}
+	}
+	for _, lowerRoot := range fsopts.LowerRoots {
+		if err := fs.updateMaxNameLen(ctx, creds, vfsObj, lowerRoot); err != nil {
+			ctx.Debugf("overlay.FilesystemType.GetFilesystem: failed to StatFSAt on lower layer root: %v", err)
+		}
+	}
+
 	// Construct the root dentry.
 	root := fs.newDentry()
 	root.refs = atomicbitops.FromInt64(1)
@@ -368,6 +385,21 @@ func (fs *filesystem) Release(ctx context.Context) {
 	}
 }
 
+// updateMaxNameLen is analogous to fs/overlayfs/super.c:ovl_check_namelen().
+func (fs *filesystem) updateMaxNameLen(ctx context.Context, creds *auth.Credentials, vfsObj *vfs.VirtualFilesystem, vd vfs.VirtualDentry) error {
+	statfs, err := vfsObj.StatFSAt(ctx, creds, &vfs.PathOperation{
+		Root:  vd,
+		Start: vd,
+	})
+	if err != nil {
+		return err
+	}
+	if statfs.NameLength > fs.maxFilenameLen {
+		fs.maxFilenameLen = statfs.NameLength
+	}
+	return nil
+}
+
 func (fs *filesystem) statFS(ctx context.Context) (linux.Statfs, error) {
 	// Always statfs the root of the topmost layer. Compare Linux's
 	// fs/overlayfs/super.c:ovl_statfs().