Implement FUSE_READLINK

Fixes #3316

9 files changed, 55 insertions, 14 deletions

diff --git a/pkg/abi/linux/fuse.go b/pkg/abi/linux/fuse.go
index ea5a7fd43..5de1433d7 100644
--- a/pkg/abi/linux/fuse.go
+++ b/pkg/abi/linux/fuse.go
@@ -483,3 +483,14 @@ func (r *FUSESymLinkIn) MarshalUnsafe(buf []byte) {
 func (r *FUSESymLinkIn) SizeBytes() int {
 	return len(r.Name) + len(r.Target) + 2
 }
+
+// FUSEEmptyIn is used by operations without request body.
+type FUSEEmptyIn struct{ marshal.StubMarshallable }
+
+// MarshalUnsafe do nothing for marshal.
+func (r *FUSEEmptyIn) MarshalUnsafe(buf []byte) {}
+
+// SizeBytes is 0 for empty request.
+func (r *FUSEEmptyIn) SizeBytes() int {
+	return 0
+}
diff --git a/pkg/sentry/fsimpl/fuse/fusefs.go b/pkg/sentry/fsimpl/fuse/fusefs.go
index 0021e2933..8db337a2e 100644
--- a/pkg/sentry/fsimpl/fuse/fusefs.go
+++ b/pkg/sentry/fsimpl/fuse/fusefs.go
@@ -225,6 +225,9 @@ type inode struct {
 
 	// version of the inode.
 	version uint64
+
+	// link is result of following a symbolic link.
+	link string
 }
 
 func (fs *filesystem) newRootInode(creds *auth.Credentials, mode linux.FileMode) *kernfs.Dentry {
@@ -406,6 +409,33 @@ func (i *inode) newEntry(ctx context.Context, name string, fileType linux.FileMo
 	return child.VFSDentry(), nil
 }
 
+// Readlink implements kernfs.Inode.Readlink.
+func (i *inode) Readlink(ctx context.Context, mnt *vfs.Mount) (string, error) {
+	if i.Mode().FileType()&linux.S_IFLNK == 0 {
+		return "", syserror.EINVAL
+	}
+	if i.link == "" {
+		kernelTask := kernel.TaskFromContext(ctx)
+		if kernelTask == nil {
+			log.Warningf("fusefs.Inode.Readlink: couldn't get kernel task from context")
+			return "", syserror.EINVAL
+		}
+		req, err := i.fs.conn.NewRequest(auth.CredentialsFromContext(ctx), uint32(kernelTask.ThreadID()), i.NodeID, linux.FUSE_READLINK, &linux.FUSEEmptyIn{})
+		if err != nil {
+			return "", err
+		}
+		res, err := i.fs.conn.Call(kernelTask, req)
+		if err != nil {
+			return "", err
+		}
+		i.link = string(res.data[res.hdr.SizeBytes():])
+		if !mnt.Options().ReadOnly {
+			i.attributeTime = 0
+		}
+	}
+	return i.link, nil
+}
+
 // statFromFUSEAttr makes attributes from linux.FUSEAttr to linux.Statx. The
 // opts.Sync attribute is ignored since the synchronization is handled by the
 // FUSE server.
diff --git a/pkg/sentry/fsimpl/kernfs/filesystem.go b/pkg/sentry/fsimpl/kernfs/filesystem.go
index 904203070..7aaf1146d 100644
--- a/pkg/sentry/fsimpl/kernfs/filesystem.go
+++ b/pkg/sentry/fsimpl/kernfs/filesystem.go
@@ -548,7 +548,7 @@ func (fs *Filesystem) ReadlinkAt(ctx context.Context, rp *vfs.ResolvingPath) (st
 	if !d.Impl().(*Dentry).isSymlink() {
 		return "", syserror.EINVAL
 	}
-	return inode.Readlink(ctx)
+	return inode.Readlink(ctx, rp.Mount())
 }
 
 // RenameAt implements vfs.FilesystemImpl.RenameAt.
diff --git a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
index c0b863ba4..ef63a1947 100644
--- a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
+++ b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
@@ -172,7 +172,7 @@ func (InodeNoDynamicLookup) Valid(ctx context.Context) bool {
 type InodeNotSymlink struct{}
 
 // Readlink implements Inode.Readlink.
-func (InodeNotSymlink) Readlink(context.Context) (string, error) {
+func (InodeNotSymlink) Readlink(context.Context, *vfs.Mount) (string, error) {
 	return "", syserror.EINVAL
 }
 
diff --git a/pkg/sentry/fsimpl/kernfs/kernfs.go b/pkg/sentry/fsimpl/kernfs/kernfs.go
index 67a0347fe..f656e2a8b 100644
--- a/pkg/sentry/fsimpl/kernfs/kernfs.go
+++ b/pkg/sentry/fsimpl/kernfs/kernfs.go
@@ -437,7 +437,7 @@ type inodeDynamicLookup interface {
 type inodeSymlink interface {
 	// Readlink returns the target of a symbolic link. If an inode is not a
 	// symlink, the implementation should return EINVAL.
-	Readlink(ctx context.Context) (string, error)
+	Readlink(ctx context.Context, mnt *vfs.Mount) (string, error)
 
 	// Getlink returns the target of a symbolic link, as used by path
 	// resolution:
diff --git a/pkg/sentry/fsimpl/kernfs/symlink.go b/pkg/sentry/fsimpl/kernfs/symlink.go
index 64731a3e4..a9812fcef 100644
--- a/pkg/sentry/fsimpl/kernfs/symlink.go
+++ b/pkg/sentry/fsimpl/kernfs/symlink.go
@@ -52,7 +52,7 @@ func (s *StaticSymlink) Init(creds *auth.Credentials, devMajor uint32, devMinor
 }
 
 // Readlink implements Inode.
-func (s *StaticSymlink) Readlink(_ context.Context) (string, error) {
+func (s *StaticSymlink) Readlink(_ context.Context, _ *vfs.Mount) (string, error) {
 	return s.target, nil
 }
 
diff --git a/pkg/sentry/fsimpl/proc/task_fds.go b/pkg/sentry/fsimpl/proc/task_fds.go
index 3f0d78461..5374538c9 100644
--- a/pkg/sentry/fsimpl/proc/task_fds.go
+++ b/pkg/sentry/fsimpl/proc/task_fds.go
@@ -209,7 +209,7 @@ func (fs *filesystem) newFDSymlink(task *kernel.Task, fd int32, ino uint64) *ker
 	return d
 }
 
-func (s *fdSymlink) Readlink(ctx context.Context) (string, error) {
+func (s *fdSymlink) Readlink(ctx context.Context, _ *vfs.Mount) (string, error) {
 	file, _ := getTaskFD(s.task, s.fd)
 	if file == nil {
 		return "", syserror.ENOENT
diff --git a/pkg/sentry/fsimpl/proc/task_files.go b/pkg/sentry/fsimpl/proc/task_files.go
index 356036b9b..4f7f9cb00 100644
--- a/pkg/sentry/fsimpl/proc/task_files.go
+++ b/pkg/sentry/fsimpl/proc/task_files.go
@@ -668,7 +668,7 @@ func (fs *filesystem) newExeSymlink(task *kernel.Task, ino uint64) *kernfs.Dentr
 }
 
 // Readlink implements kernfs.Inode.
-func (s *exeSymlink) Readlink(ctx context.Context) (string, error) {
+func (s *exeSymlink) Readlink(ctx context.Context, _ *vfs.Mount) (string, error) {
 	if !kernel.ContextCanTrace(ctx, s.task, false) {
 		return "", syserror.EACCES
 	}
@@ -808,11 +808,11 @@ func (fs *filesystem) newNamespaceSymlink(task *kernel.Task, ino uint64, ns stri
 }
 
 // Readlink implements Inode.
-func (s *namespaceSymlink) Readlink(ctx context.Context) (string, error) {
+func (s *namespaceSymlink) Readlink(ctx context.Context, mnt *vfs.Mount) (string, error) {
 	if err := checkTaskState(s.task); err != nil {
 		return "", err
 	}
-	return s.StaticSymlink.Readlink(ctx)
+	return s.StaticSymlink.Readlink(ctx, mnt)
 }
 
 // Getlink implements Inode.Getlink.
diff --git a/pkg/sentry/fsimpl/proc/tasks_files.go b/pkg/sentry/fsimpl/proc/tasks_files.go
index 8c41729e4..68c541046 100644
--- a/pkg/sentry/fsimpl/proc/tasks_files.go
+++ b/pkg/sentry/fsimpl/proc/tasks_files.go
@@ -51,7 +51,7 @@ func (fs *filesystem) newSelfSymlink(creds *auth.Credentials, ino uint64, pidns
 	return d
 }
 
-func (s *selfSymlink) Readlink(ctx context.Context) (string, error) {
+func (s *selfSymlink) Readlink(ctx context.Context, _ *vfs.Mount) (string, error) {
 	t := kernel.TaskFromContext(ctx)
 	if t == nil {
 		// Who is reading this link?
@@ -64,8 +64,8 @@ func (s *selfSymlink) Readlink(ctx context.Context) (string, error) {
 	return strconv.FormatUint(uint64(tgid), 10), nil
 }
 
-func (s *selfSymlink) Getlink(ctx context.Context, _ *vfs.Mount) (vfs.VirtualDentry, string, error) {
-	target, err := s.Readlink(ctx)
+func (s *selfSymlink) Getlink(ctx context.Context, mnt *vfs.Mount) (vfs.VirtualDentry, string, error) {
+	target, err := s.Readlink(ctx, mnt)
 	return vfs.VirtualDentry{}, target, err
 }
 
@@ -94,7 +94,7 @@ func (fs *filesystem) newThreadSelfSymlink(creds *auth.Credentials, ino uint64,
 	return d
 }
 
-func (s *threadSelfSymlink) Readlink(ctx context.Context) (string, error) {
+func (s *threadSelfSymlink) Readlink(ctx context.Context, _ *vfs.Mount) (string, error) {
 	t := kernel.TaskFromContext(ctx)
 	if t == nil {
 		// Who is reading this link?
@@ -108,8 +108,8 @@ func (s *threadSelfSymlink) Readlink(ctx context.Context) (string, error) {
 	return fmt.Sprintf("%d/task/%d", tgid, tid), nil
 }
 
-func (s *threadSelfSymlink) Getlink(ctx context.Context, _ *vfs.Mount) (vfs.VirtualDentry, string, error) {
-	target, err := s.Readlink(ctx)
+func (s *threadSelfSymlink) Getlink(ctx context.Context, mnt *vfs.Mount) (vfs.VirtualDentry, string, error) {
+	target, err := s.Readlink(ctx, mnt)
 	return vfs.VirtualDentry{}, target, err
 }