Add LinkAt support to gofer

Updates #1198

PiperOrigin-RevId: 325350818

2 files changed, 100 insertions, 0 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;
+}