Add LinkAt support to gofer

Updates #1198

PiperOrigin-RevId: 325350818

6 files changed, 143 insertions, 5 deletions

diff --git a/images/basic/linktest/Dockerfile b/images/basic/linktest/Dockerfile
new file mode 100644
index 000000000..baebc9b76
--- /dev/null
+++ b/images/basic/linktest/Dockerfile
@@ -0,0 +1,7 @@
+FROM ubuntu:bionic
+
+WORKDIR /root
+COPY . .
+
+RUN apt-get update && apt-get install -y gcc
+RUN gcc -O2 -o link_test link_test.c
diff --git a/images/basic/linktest/link_test.c b/images/basic/linktest/link_test.c
new file mode 100644
index 000000000..45ab00abe
--- /dev/null
+++ b/images/basic/linktest/link_test.c
@@ -0,0 +1,93 @@
+// 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 <err.h>
+#include <fcntl.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+// Basic test for linkat(2). Syscall tests requires CAP_DAC_READ_SEARCH and it
+// cannot use tricks like userns as root. For this reason, run a basic link test
+// to ensure some coverage.
+int main(int argc, char** argv) {
+  const char kOldPath[] = "old.txt";
+  int fd = open(kOldPath, O_RDWR | O_CREAT | O_TRUNC, 0600);
+  if (fd < 0) {
+    errx(1, "open(%s) failed", kOldPath);
+  }
+  const char kData[] = "some random content";
+  if (write(fd, kData, sizeof(kData)) < 0) {
+    err(1, "write failed");
+  }
+  close(fd);
+
+  struct stat old_stat;
+  if (stat(kOldPath, &old_stat)) {
+    errx(1, "stat(%s) failed", kOldPath);
+  }
+
+  const char kNewPath[] = "new.txt";
+  if (link(kOldPath, kNewPath)) {
+    errx(1, "link(%s, %s) failed", kOldPath, kNewPath);
+  }
+
+  struct stat new_stat;
+  if (stat(kNewPath, &new_stat)) {
+    errx(1, "stat(%s) failed", kNewPath);
+  }
+
+  // Check that files are the same.
+  if (old_stat.st_dev != new_stat.st_dev) {
+    errx(1, "files st_dev is different, want: %lu, got: %lu", old_stat.st_dev,
+         new_stat.st_dev);
+  }
+  if (old_stat.st_ino != new_stat.st_ino) {
+    errx(1, "files st_ino is different, want: %lu, got: %lu", old_stat.st_ino,
+         new_stat.st_ino);
+  }
+
+  // Check that link count is correct.
+  if (new_stat.st_nlink != old_stat.st_nlink + 1) {
+    errx(1, "wrong nlink, want: %lu, got: %lu", old_stat.st_nlink + 1,
+         new_stat.st_nlink);
+  }
+
+  // Check taht contents are the same.
+  fd = open(kNewPath, O_RDONLY);
+  if (fd < 0) {
+    errx(1, "open(%s) failed", kNewPath);
+  }
+  char buf[sizeof(kData)] = {};
+  if (read(fd, buf, sizeof(buf)) < 0) {
+    err(1, "read failed");
+  }
+  close(fd);
+
+  if (strcmp(buf, kData) != 0) {
+    errx(1, "file content mismatch: %s", buf);
+  }
+
+  // Cleanup.
+  if (unlink(kNewPath)) {
+    errx(1, "unlink(%s) failed", kNewPath);
+  }
+  if (unlink(kOldPath)) {
+    errx(1, "unlink(%s) failed", kOldPath);
+  }
+
+  // Success!
+  return 0;
+}
diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index e6af37d0d..582b744bb 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -15,6 +15,7 @@
 package gofer
 
 import (
+	"math"
 	"sync"
 	"sync/atomic"
 
@@ -724,8 +725,29 @@ func (fs *filesystem) LinkAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.
 		if rp.Mount() != vd.Mount() {
 			return syserror.EXDEV
 		}
-		// 9P2000.L supports hard links, but we don't.
-		return syserror.EPERM
+		d := vd.Dentry().Impl().(*dentry)
+		if d.isDir() {
+			return syserror.EPERM
+		}
+		gid := auth.KGID(atomic.LoadUint32(&d.gid))
+		uid := auth.KUID(atomic.LoadUint32(&d.uid))
+		mode := linux.FileMode(atomic.LoadUint32(&d.mode))
+		if err := vfs.MayLink(rp.Credentials(), mode, uid, gid); err != nil {
+			return err
+		}
+		if d.nlink == 0 {
+			return syserror.ENOENT
+		}
+		if d.nlink == math.MaxUint32 {
+			return syserror.EMLINK
+		}
+		if err := parent.file.link(ctx, d.file, childName); err != nil {
+			return err
+		}
+
+		// Success!
+		atomic.AddUint32(&d.nlink, 1)
+		return nil
 	}, nil)
 }
 
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 2e5575d8d..6ae796c6d 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -558,8 +558,6 @@ type dentry struct {
 	// filesystem.renameMu.
 	name string
 
-	// We don't support hard links, so each dentry maps 1:1 to an inode.
-
 	// file is the unopened p9.File that backs this dentry. file is immutable.
 	//
 	// If file.isNil(), this dentry represents a synthetic file, i.e. a file
diff --git a/scripts/docker_tests.sh b/scripts/docker_tests.sh
index 07e9f3109..be0b0a3ec 100755
--- a/scripts/docker_tests.sh
+++ b/scripts/docker_tests.sh
@@ -23,5 +23,5 @@ test_runsc //test/image:image_test //test/e2e:integration_test
 
 install_runsc_for_test docker --vfs2
 IMAGE_FILTER="Hello|Httpd|Ruby|Stdio"
-INTEGRATION_FILTER="LifeCycle|Pause|Connect|JobControl|Overlay|Exec|DirCreation/root"
+INTEGRATION_FILTER="LifeCycle|Pause|Connect|JobControl|Overlay|Exec|DirCreation/root|Link"
 test_runsc //test/e2e:integration_test //test/image:image_test --test_filter="${IMAGE_FILTER}|${INTEGRATION_FILTER}"
diff --git a/test/e2e/integration_test.go b/test/e2e/integration_test.go
index 6fe6d304f..71ec4791e 100644
--- a/test/e2e/integration_test.go
+++ b/test/e2e/integration_test.go
@@ -467,6 +467,24 @@ func TestHostOverlayfsRewindDir(t *testing.T) {
 	}
 }
 
+// Basic test for linkat(2). Syscall tests requires CAP_DAC_READ_SEARCH and it
+// cannot use tricks like userns as root. For this reason, run a basic link test
+// to ensure some coverage.
+func TestLink(t *testing.T) {
+	ctx := context.Background()
+	d := dockerutil.MakeContainer(ctx, t)
+	defer d.CleanUp(ctx)
+
+	if got, err := d.Run(ctx, dockerutil.RunOpts{
+		Image:   "basic/linktest",
+		WorkDir: "/root",
+	}, "./link_test"); err != nil {
+		t.Fatalf("docker run failed: %v", err)
+	} else if got != "" {
+		t.Errorf("test failed:\n%s", got)
+	}
+}
+
 func TestMain(m *testing.M) {
 	dockerutil.EnsureSupportedDockerVersion()
 	flag.Parse()