Don't enforce NAME_MAX in fs.Dirent.walk().

Maximum filename length is filesystem-dependent, and obtained via
statfs::f_namelen. This limit is usually 255 bytes (NAME_MAX), but not
always. For example, VFAT supports filenames of up to 255... UCS-2
characters, which Linux conservatively takes to mean UTF-8-encoded
bytes: fs/fat/inode.c:fat_statfs(), FAT_LFN_LEN * NLS_MAX_CHARSET_SIZE.
As a result, Linux's VFS does not enforce NAME_MAX:

$ rg --maxdepth=1 '\WNAME_MAX\W' fs/ include/linux/
fs/libfs.c
38:     buf->f_namelen = NAME_MAX;
64:     if (dentry->d_name.len > NAME_MAX)

include/linux/relay.h
74:     char base_filename[NAME_MAX];   /* saved base filename */

include/linux/fscrypt.h
149: * filenames up to NAME_MAX bytes, since base64 encoding expands the length.

include/linux/exportfs.h
176: *    understanding that it is already pointing to a a %NAME_MAX+1 sized

Remove this check from core VFS, and add it to ramfs (and by extension
tmpfs), where it is actually applicable:
mm/shmem.c:shmem_dir_inode_operations.lookup == simple_lookup *does*
enforce NAME_MAX.

PiperOrigin-RevId: 245324748
Change-Id: I17567c4324bfd60e31746a5270096e75db963fac

pkg/sentry/fs/dirent.go

@@ -459,11 +459,6 @@ func (d *Dirent) walk(ctx context.Context, root *Dirent, name string, walkMayUnl
 		return nil, syscall.ENOTDIR
 	}
 
-	// The component must be less than NAME_MAX.
-	if len(name) > linux.NAME_MAX {
-		return nil, syscall.ENAMETOOLONG
-	}
-
 	if name == "" || name == "." {
 		d.IncRef()
 		return d, nil

pkg/sentry/fs/gofer/path.go

@@ -27,9 +27,17 @@ import (
 	"gvisor.googlesource.com/gvisor/pkg/syserror"
 )
 
+// maxFilenameLen is the maximum length of a filename. This is dictated by 9P's
+// encoding of strings, which uses 2 bytes for the length prefix.
+const maxFilenameLen = (1 << 16) - 1
+
 // Lookup loads an Inode at name into a Dirent based on the session's cache
 // policy.
 func (i *inodeOperations) Lookup(ctx context.Context, dir *fs.Inode, name string) (*fs.Dirent, error) {
+	if len(name) > maxFilenameLen {
+		return nil, syserror.ENAMETOOLONG
+	}
+
 	cp := i.session().cachePolicy
 	if cp.cacheReaddir() {
 		// Check to see if we have readdirCache that indicates the
@@ -72,6 +80,10 @@ func (i *inodeOperations) Lookup(ctx context.Context, dir *fs.Inode, name string
 //
 // Ownership is currently ignored.
 func (i *inodeOperations) Create(ctx context.Context, dir *fs.Inode, name string, flags fs.FileFlags, perm fs.FilePermissions) (*fs.File, error) {
+	if len(name) > maxFilenameLen {
+		return nil, syserror.ENAMETOOLONG
+	}
+
 	// Create replaces the directory fid with the newly created/opened
 	// file, so clone this directory so it doesn't change out from under
 	// this node.
@@ -139,6 +151,10 @@ func (i *inodeOperations) Create(ctx context.Context, dir *fs.Inode, name string
 
 // CreateLink uses Create to create a symlink between oldname and newname.
 func (i *inodeOperations) CreateLink(ctx context.Context, dir *fs.Inode, oldname string, newname string) error {
+	if len(newname) > maxFilenameLen {
+		return syserror.ENAMETOOLONG
+	}
+
 	owner := fs.FileOwnerFromContext(ctx)
 	if _, err := i.fileState.file.symlink(ctx, oldname, newname, p9.UID(owner.UID), p9.GID(owner.GID)); err != nil {
 		return err
@@ -149,6 +165,10 @@ func (i *inodeOperations) CreateLink(ctx context.Context, dir *fs.Inode, oldname
 
 // CreateHardLink implements InodeOperations.CreateHardLink.
 func (i *inodeOperations) CreateHardLink(ctx context.Context, inode *fs.Inode, target *fs.Inode, newName string) error {
+	if len(newName) > maxFilenameLen {
+		return syserror.ENAMETOOLONG
+	}
+
 	targetOpts, ok := target.InodeOperations.(*inodeOperations)
 	if !ok {
 		return syscall.EXDEV
@@ -167,6 +187,10 @@ func (i *inodeOperations) CreateHardLink(ctx context.Context, inode *fs.Inode, t
 
 // CreateDirectory uses Create to create a directory named s under inodeOperations.
 func (i *inodeOperations) CreateDirectory(ctx context.Context, dir *fs.Inode, s string, perm fs.FilePermissions) error {
+	if len(s) > maxFilenameLen {
+		return syserror.ENAMETOOLONG
+	}
+
 	owner := fs.FileOwnerFromContext(ctx)
 	if _, err := i.fileState.file.mkdir(ctx, s, p9.FileMode(perm.LinuxMode()), p9.UID(owner.UID), p9.GID(owner.GID)); err != nil {
 		return err
@@ -184,6 +208,10 @@ func (i *inodeOperations) CreateDirectory(ctx context.Context, dir *fs.Inode, s
 
 // Bind implements InodeOperations.Bind.
 func (i *inodeOperations) Bind(ctx context.Context, dir *fs.Inode, name string, ep transport.BoundEndpoint, perm fs.FilePermissions) (*fs.Dirent, error) {
+	if len(name) > maxFilenameLen {
+		return nil, syserror.ENAMETOOLONG
+	}
+
 	if i.session().endpoints == nil {
 		return nil, syscall.EOPNOTSUPP
 	}
@@ -252,6 +280,10 @@ func (*inodeOperations) CreateFifo(context.Context, *fs.Inode, string, fs.FilePe
 
 // Remove implements InodeOperations.Remove.
 func (i *inodeOperations) Remove(ctx context.Context, dir *fs.Inode, name string) error {
+	if len(name) > maxFilenameLen {
+		return syserror.ENAMETOOLONG
+	}
+
 	var key device.MultiDeviceKey
 	removeSocket := false
 	if i.session().endpoints != nil {
@@ -284,6 +316,10 @@ func (i *inodeOperations) Remove(ctx context.Context, dir *fs.Inode, name string
 
 // Remove implements InodeOperations.RemoveDirectory.
 func (i *inodeOperations) RemoveDirectory(ctx context.Context, dir *fs.Inode, name string) error {
+	if len(name) > maxFilenameLen {
+		return syserror.ENAMETOOLONG
+	}
+
 	// 0x200 = AT_REMOVEDIR.
 	if err := i.fileState.file.unlinkAt(ctx, name, 0x200); err != nil {
 		return err
@@ -301,6 +337,10 @@ func (i *inodeOperations) RemoveDirectory(ctx context.Context, dir *fs.Inode, na
 
 // Rename renames this node.
 func (i *inodeOperations) Rename(ctx context.Context, oldParent *fs.Inode, oldName string, newParent *fs.Inode, newName string, replacement bool) error {
+	if len(newName) > maxFilenameLen {
+		return syserror.ENAMETOOLONG
+	}
+
 	// Unwrap the new parent to a *inodeOperations.
 	newParentInodeOperations, ok := newParent.InodeOperations.(*inodeOperations)
 	if !ok {

pkg/sentry/fs/ramfs/dir.go

@@ -192,6 +192,10 @@ func (d *Dir) removeChildLocked(ctx context.Context, name string) (*fs.Inode, er
 
 // Remove removes the named non-directory.
 func (d *Dir) Remove(ctx context.Context, _ *fs.Inode, name string) error {
+	if len(name) > linux.NAME_MAX {
+		return syserror.ENAMETOOLONG
+	}
+
 	d.mu.Lock()
 	defer d.mu.Unlock()
 	inode, err := d.removeChildLocked(ctx, name)
@@ -206,6 +210,10 @@ func (d *Dir) Remove(ctx context.Context, _ *fs.Inode, name string) error {
 
 // RemoveDirectory removes the named directory.
 func (d *Dir) RemoveDirectory(ctx context.Context, _ *fs.Inode, name string) error {
+	if len(name) > linux.NAME_MAX {
+		return syserror.ENAMETOOLONG
+	}
+
 	d.mu.Lock()
 	defer d.mu.Unlock()
 
@@ -234,6 +242,10 @@ func (d *Dir) RemoveDirectory(ctx context.Context, _ *fs.Inode, name string) err
 
 // Lookup loads an inode at p into a Dirent.
 func (d *Dir) Lookup(ctx context.Context, _ *fs.Inode, p string) (*fs.Dirent, error) {
+	if len(p) > linux.NAME_MAX {
+		return nil, syserror.ENAMETOOLONG
+	}
+
 	d.mu.Lock()
 	defer d.mu.Unlock()
 
@@ -265,6 +277,10 @@ func (d *Dir) walkLocked(ctx context.Context, p string) (*fs.Inode, error) {
 // createInodeOperationsCommon creates a new child node at this dir by calling
 // makeInodeOperations. It is the common logic for creating a new child.
 func (d *Dir) createInodeOperationsCommon(ctx context.Context, name string, makeInodeOperations func() (*fs.Inode, error)) (*fs.Inode, error) {
+	if len(name) > linux.NAME_MAX {
+		return nil, syserror.ENAMETOOLONG
+	}
+
 	d.mu.Lock()
 	defer d.mu.Unlock()
 
@@ -314,6 +330,10 @@ func (d *Dir) CreateLink(ctx context.Context, dir *fs.Inode, oldname, newname st
 
 // CreateHardLink creates a new hard link.
 func (d *Dir) CreateHardLink(ctx context.Context, dir *fs.Inode, target *fs.Inode, name string) error {
+	if len(name) > linux.NAME_MAX {
+		return syserror.ENAMETOOLONG
+	}
+
 	d.mu.Lock()
 	defer d.mu.Unlock()
 
@@ -465,6 +485,9 @@ func Rename(ctx context.Context, oldParent fs.InodeOperations, oldName string, n
 	if !ok {
 		return syserror.EXDEV
 	}
+	if len(newName) > linux.NAME_MAX {
+		return syserror.ENAMETOOLONG
+	}
 
 	np.mu.Lock()
 	defer np.mu.Unlock()

test/syscalls/linux/chdir.cc

@@ -54,11 +54,6 @@ TEST(ChdirTest, NotDir) {
   EXPECT_THAT(chdir(temp_file.path().c_str()), SyscallFailsWithErrno(ENOTDIR));
 }
 
-TEST(ChdirTest, NameTooLong) {
-  std::string name(NAME_MAX + 1, 'a');
-  ASSERT_THAT(chdir(name.c_str()), SyscallFailsWithErrno(ENAMETOOLONG));
-}
-
 TEST(ChdirTest, NotExist) {
   EXPECT_THAT(chdir("/foo/bar"), SyscallFailsWithErrno(ENOENT));
 }

test/syscalls/linux/statfs.cc

@@ -49,6 +49,7 @@ TEST(StatfsTest, NameLen) {
   struct statfs st;
   EXPECT_THAT(statfs("/dev/shm", &st), SyscallSucceeds());
 
+  // This assumes that /dev/shm is tmpfs.
   EXPECT_EQ(st.f_namelen, NAME_MAX);
 }