Implement FUSE_READLINK

Fixes #3316

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
+}

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.

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.

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
 }
 

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:

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
 }
 

pkg/sentry/fsimpl/proc/task_fds.go

@@ -214,7 +214,7 @@ 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

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.

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
 }
 

test/fuse/BUILD

@@ -26,3 +26,8 @@ syscall_test(
     fuse = "True",
     test = "//test/fuse/linux:symlink_test",
 )
+
+syscall_test(
+    fuse = "True",
+    test = "//test/fuse/linux:readlink_test",
+)

test/fuse/linux/BUILD

@@ -71,6 +71,19 @@ cc_binary(
     ],
 )
 
+cc_binary(
+    name = "readlink_test",
+    testonly = 1,
+    srcs = ["readlink_test.cc"],
+    deps = [
+        gtest,
+        ":fuse_base",
+        "//test/util:fuse_util",
+        "//test/util:test_main",
+        "//test/util:test_util",
+    ],
+)
+
 cc_library(
     name = "fuse_base",
     testonly = 1,

test/fuse/linux/readlink_test.cc

@@ -0,0 +1,85 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/fuse.h>
+#include <sys/stat.h>
+#include <sys/statfs.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <string>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "test/fuse/linux/fuse_base.h"
+#include "test/util/fuse_util.h"
+#include "test/util/test_util.h"
+
+namespace gvisor {
+namespace testing {
+
+namespace {
+
+class ReadlinkTest : public FuseTest {
+ protected:
+  const std::string test_file_ = "test_file_";
+  const mode_t perms_ = S_IRWXU | S_IRWXG | S_IRWXO;
+};
+
+TEST_F(ReadlinkTest, ReadSymLink) {
+  const std::string symlink_path =
+      JoinPath(mount_point_.path().c_str(), test_file_);
+  SetServerInodeLookup(test_file_, S_IFLNK | perms_);
+
+  struct fuse_out_header out_header = {
+      .len = static_cast<uint32_t>(sizeof(struct fuse_out_header)) +
+             static_cast<uint32_t>(test_file_.length()) + 1,
+  };
+  std::string link = test_file_;
+  auto iov_out = FuseGenerateIovecs(out_header, link);
+  SetServerResponse(FUSE_READLINK, iov_out);
+  const std::string actual_link =
+      ASSERT_NO_ERRNO_AND_VALUE(ReadLink(symlink_path));
+
+  struct fuse_in_header in_header;
+  auto iov_in = FuseGenerateIovecs(in_header);
+  GetServerActualRequest(iov_in);
+
+  EXPECT_EQ(in_header.len, sizeof(in_header));
+  EXPECT_EQ(in_header.opcode, FUSE_READLINK);
+  EXPECT_EQ(0, memcmp(actual_link.c_str(), link.data(), link.size()));
+
+  // next readlink should have link cached, so shouldn't have new request to
+  // server.
+  uint32_t recieved_before = GetServerTotalReceivedBytes();
+  ASSERT_NO_ERRNO(ReadLink(symlink_path));
+  EXPECT_EQ(GetServerTotalReceivedBytes(), recieved_before);
+}
+
+TEST_F(ReadlinkTest, NotSymlink) {
+  const std::string test_file_path =
+      JoinPath(mount_point_.path().c_str(), test_file_);
+  SetServerInodeLookup(test_file_, S_IFREG | perms_);
+
+  std::vector<char> buf(PATH_MAX + 1);
+  ASSERT_THAT(readlink(test_file_path.c_str(), buf.data(), PATH_MAX),
+              SyscallFailsWithErrno(EINVAL));
+}
+
+}  // namespace
+
+}  // namespace testing
+}  // namespace gvisor