[vfs] Disable nlink tests for overlayfs.

Overlayfs intentionally does not compute nlink for directories (because it can
be really expensive). Linux returns 1, VFS2 returns 2 and VFS1 actually
calculates the correct value.

PiperOrigin-RevId: 330967139

test/syscalls/linux/stat.cc

@@ -97,6 +97,11 @@ TEST_F(StatTest, FstatatSymlink) {
 }
 
 TEST_F(StatTest, Nlinks) {
+  // Skip this test if we are testing overlayfs because overlayfs does not
+  // (intentionally) return the correct nlink value for directories.
+  // See fs/overlayfs/inode.c:ovl_getattr().
+  SKIP_IF(ASSERT_NO_ERRNO_AND_VALUE(IsOverlayfs(GetAbsoluteTestTmpdir())));
+
   TempPath basedir = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
 
   // Directory is initially empty, it should contain 2 links (one from itself,
@@ -432,6 +437,11 @@ TEST_F(StatTest, ZeroLinksOpenFdRegularFileChild_NoRandomSave) {
 
 // Test link counts with a directory as the child.
 TEST_F(StatTest, LinkCountsWithDirChild) {
+  // Skip this test if we are testing overlayfs because overlayfs does not
+  // (intentionally) return the correct nlink value for directories.
+  // See fs/overlayfs/inode.c:ovl_getattr().
+  SKIP_IF(ASSERT_NO_ERRNO_AND_VALUE(IsOverlayfs(GetAbsoluteTestTmpdir())));
+
   const TempPath dir = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
 
   // Before a child is added the two links are "." and the link from the parent.

test/syscalls/linux/statfs.cc

@@ -27,10 +27,6 @@ namespace testing {
 
 namespace {
 
-// From linux/magic.h. For some reason, not defined in the headers for some
-// build environments.
-#define OVERLAYFS_SUPER_MAGIC 0x794c7630
-
 TEST(StatfsTest, CannotStatBadPath) {
   auto temp_file = NewTempAbsPathInDir("/tmp");
 

test/util/fs_util.cc

@@ -649,5 +649,19 @@ PosixErrorOr<bool> IsTmpfs(const std::string& path) {
 }
 #endif  // __linux__
 
+PosixErrorOr<bool> IsOverlayfs(const std::string& path) {
+  struct statfs stat;
+  if (statfs(path.c_str(), &stat)) {
+    if (errno == ENOENT) {
+      // Nothing at path, don't raise this as an error. Instead, just report no
+      // overlayfs at path.
+      return false;
+    }
+    return PosixError(errno,
+                      absl::StrFormat("statfs(\"%s\", %#p)", path, &stat));
+  }
+  return stat.f_type == OVERLAYFS_SUPER_MAGIC;
+}
+
 }  // namespace testing
 }  // namespace gvisor

test/util/fs_util.h

@@ -38,6 +38,10 @@ constexpr int kOLargeFile = 00400000;
 #error "Unknown architecture"
 #endif
 
+// From linux/magic.h. For some reason, not defined in the headers for some
+// build environments.
+#define OVERLAYFS_SUPER_MAGIC 0x794c7630
+
 // Returns a status or the current working directory.
 PosixErrorOr<std::string> GetCWD();
 
@@ -184,6 +188,9 @@ PosixErrorOr<std::string> ProcessExePath(int pid);
 PosixErrorOr<bool> IsTmpfs(const std::string& path);
 #endif  // __linux__
 
+// IsOverlayfs returns true if the file at path is backed by overlayfs.
+PosixErrorOr<bool> IsOverlayfs(const std::string& path);
+
 namespace internal {
 // Not part of the public API.
 std::string JoinPathImpl(std::initializer_list<absl::string_view> paths);