6 files changed, 101 insertions, 6 deletions
diff --git a/pkg/sentry/fsimpl/tmpfs/BUILD b/pkg/sentry/fsimpl/tmpfs/BUILD
index 9a076ad71..007be1572 100644
--- a/pkg/sentry/fsimpl/tmpfs/BUILD
+++ b/pkg/sentry/fsimpl/tmpfs/BUILD
@@ -52,7 +52,6 @@ go_library(
         "//pkg/sentry/fs",
         "//pkg/sentry/fs/fsutil",
         "//pkg/sentry/fs/lock",
-        "//pkg/sentry/kernel",
         "//pkg/sentry/kernel/auth",
         "//pkg/sentry/kernel/pipe",
         "//pkg/sentry/kernel/time",
@@ -106,7 +105,6 @@ go_test(
         "//pkg/sentry/contexttest",
         "//pkg/sentry/fs/lock",
         "//pkg/sentry/kernel/auth",
-        "//pkg/sentry/kernel/contexttest",
         "//pkg/sentry/vfs",
         "//pkg/syserror",
         "//pkg/usermem",
diff --git a/pkg/sentry/fsimpl/tmpfs/filesystem.go b/pkg/sentry/fsimpl/tmpfs/filesystem.go
index e0ad82769..80fa7b29d 100644
--- a/pkg/sentry/fsimpl/tmpfs/filesystem.go
+++ b/pkg/sentry/fsimpl/tmpfs/filesystem.go
@@ -772,5 +772,24 @@ func (fs *filesystem) RemovexattrAt(ctx context.Context, rp *vfs.ResolvingPath,
 func (fs *filesystem) PrependPath(ctx context.Context, vfsroot, vd vfs.VirtualDentry, b *fspath.Builder) error {
 	fs.mu.RLock()
 	defer fs.mu.RUnlock()
-	return genericPrependPath(vfsroot, vd.Mount(), vd.Dentry().Impl().(*dentry), b)
+	mnt := vd.Mount()
+	d := vd.Dentry().Impl().(*dentry)
+	for {
+		if mnt == vfsroot.Mount() && &d.vfsd == vfsroot.Dentry() {
+			return vfs.PrependPathAtVFSRootError{}
+		}
+		if &d.vfsd == mnt.Root() {
+			return nil
+		}
+		if d.parent == nil {
+			if d.name != "" {
+				// This must be an anonymous memfd file.
+				b.PrependComponent("/" + d.name)
+				return vfs.PrependPathSyntheticError{}
+			}
+			return vfs.PrependPathAtNonMountRootError{}
+		}
+		b.PrependComponent(d.name)
+		d = d.parent
+	}
 }
diff --git a/pkg/sentry/fsimpl/tmpfs/regular_file.go b/pkg/sentry/fsimpl/tmpfs/regular_file.go
index 57e5e28ec..3f433d666 100644
--- a/pkg/sentry/fsimpl/tmpfs/regular_file.go
+++ b/pkg/sentry/fsimpl/tmpfs/regular_file.go
@@ -88,6 +88,7 @@ type regularFile struct {
 func (fs *filesystem) newRegularFile(creds *auth.Credentials, mode linux.FileMode) *inode {
 	file := &regularFile{
 		memFile: fs.memFile,
+		seals:   linux.F_SEAL_SEAL,
 	}
 	file.inode.init(file, fs, creds, linux.S_IFREG|mode)
 	file.inode.nlink = 1 // from parent directory
@@ -577,3 +578,44 @@ exitLoop:
 
 	return done, retErr
 }
+
+// GetSeals returns the current set of seals on a memfd inode.
+func GetSeals(fd *vfs.FileDescription) (uint32, error) {
+	f, ok := fd.Impl().(*regularFileFD)
+	if !ok {
+		return 0, syserror.EINVAL
+	}
+	rf := f.inode().impl.(*regularFile)
+	rf.dataMu.RLock()
+	defer rf.dataMu.RUnlock()
+	return rf.seals, nil
+}
+
+// AddSeals adds new file seals to a memfd inode.
+func AddSeals(fd *vfs.FileDescription, val uint32) error {
+	f, ok := fd.Impl().(*regularFileFD)
+	if !ok {
+		return syserror.EINVAL
+	}
+	rf := f.inode().impl.(*regularFile)
+	rf.mapsMu.Lock()
+	defer rf.mapsMu.Unlock()
+	rf.dataMu.RLock()
+	defer rf.dataMu.RUnlock()
+
+	if rf.seals&linux.F_SEAL_SEAL != 0 {
+		// Seal applied which prevents addition of any new seals.
+		return syserror.EPERM
+	}
+
+	// F_SEAL_WRITE can only be added if there are no active writable maps.
+	if rf.seals&linux.F_SEAL_WRITE == 0 && val&linux.F_SEAL_WRITE != 0 {
+		if rf.writableMappingPages > 0 {
+			return syserror.EBUSY
+		}
+	}
+
+	// Seals can only be added, never removed.
+	rf.seals |= val
+	return nil
+}
diff --git a/pkg/sentry/fsimpl/tmpfs/regular_file_test.go b/pkg/sentry/fsimpl/tmpfs/regular_file_test.go
index f2bc96d51..64e1c40ad 100644
--- a/pkg/sentry/fsimpl/tmpfs/regular_file_test.go
+++ b/pkg/sentry/fsimpl/tmpfs/regular_file_test.go
@@ -21,8 +21,8 @@ import (
 	"testing"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/contexttest"
 	"gvisor.dev/gvisor/pkg/sentry/fs/lock"
-	"gvisor.dev/gvisor/pkg/sentry/kernel/contexttest"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/syserror"
 	"gvisor.dev/gvisor/pkg/usermem"
diff --git a/pkg/sentry/fsimpl/tmpfs/stat_test.go b/pkg/sentry/fsimpl/tmpfs/stat_test.go
index f52755092..f7ee4aab2 100644
--- a/pkg/sentry/fsimpl/tmpfs/stat_test.go
+++ b/pkg/sentry/fsimpl/tmpfs/stat_test.go
@@ -19,8 +19,8 @@ import (
 	"testing"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/sentry/contexttest"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
-	"gvisor.dev/gvisor/pkg/sentry/kernel/contexttest"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 )
 
diff --git a/pkg/sentry/fsimpl/tmpfs/tmpfs.go b/pkg/sentry/fsimpl/tmpfs/tmpfs.go
index 405928bd0..1e781aecd 100644
--- a/pkg/sentry/fsimpl/tmpfs/tmpfs.go
+++ b/pkg/sentry/fsimpl/tmpfs/tmpfs.go
@@ -94,7 +94,7 @@ type FilesystemOpts struct {
 }
 
 // GetFilesystem implements vfs.FilesystemType.GetFilesystem.
-func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, source string, opts vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
+func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, _ string, opts vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
 	memFileProvider := pgalloc.MemoryFileProviderFromContext(ctx)
 	if memFileProvider == nil {
 		panic("MemoryFileProviderFromContext returned nil")
@@ -139,6 +139,11 @@ func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 	return &fs.vfsfs, &root.vfsd, nil
 }
 
+// NewFilesystem returns a new tmpfs filesystem.
+func NewFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials) (*vfs.Filesystem, *vfs.Dentry, error) {
+	return FilesystemType{}.GetFilesystem(ctx, vfsObj, creds, "", vfs.GetFilesystemOptions{})
+}
+
 // Release implements vfs.FilesystemImpl.Release.
 func (fs *filesystem) Release() {
 	fs.vfsfs.VirtualFilesystem().PutAnonBlockDevMinor(fs.devMinor)
@@ -658,3 +663,34 @@ func (fd *fileDescription) Setxattr(ctx context.Context, opts vfs.SetxattrOption
 func (fd *fileDescription) Removexattr(ctx context.Context, name string) error {
 	return fd.inode().removexattr(auth.CredentialsFromContext(ctx), name)
 }
+
+// NewMemfd creates a new tmpfs regular file and file description that can back
+// an anonymous fd created by memfd_create.
+func NewMemfd(mount *vfs.Mount, creds *auth.Credentials, allowSeals bool, name string) (*vfs.FileDescription, error) {
+	fs, ok := mount.Filesystem().Impl().(*filesystem)
+	if !ok {
+		panic("NewMemfd() called with non-tmpfs mount")
+	}
+
+	// Per Linux, mm/shmem.c:__shmem_file_setup(), memfd inodes are set up with
+	// S_IRWXUGO.
+	mode := linux.FileMode(0777)
+	inode := fs.newRegularFile(creds, mode)
+	rf := inode.impl.(*regularFile)
+	if allowSeals {
+		rf.seals = 0
+	}
+
+	d := fs.newDentry(inode)
+	defer d.DecRef()
+	d.name = name
+
+	// Per Linux, mm/shmem.c:__shmem_file_setup(), memfd files are set up with
+	// FMODE_READ | FMODE_WRITE.
+	var fd regularFileFD
+	flags := uint32(linux.O_RDWR)
+	if err := fd.vfsfd.Init(&fd, flags, mount, &d.vfsd, &vfs.FileDescriptionOptions{}); err != nil {
+		return nil, err
+	}
+	return &fd.vfsfd, nil
+}