Check remaining traversal limit when creating a file through a symlink.

This fixes the case when an app tries to create a file that already exists, and
is a symlink to itself. A test was added.

PiperOrigin-RevId: 256044811

pkg/sentry/syscalls/linux/sys_file.go

@@ -354,6 +354,12 @@ func createAt(t *kernel.Task, dirFD kdefs.FD, addr usermem.Addr, flags uint, mod
 				break
 			}
 
+			// Are we able to resolve further?
+			if remainingTraversals == 0 {
+				found.DecRef()
+				return syscall.ELOOP
+			}
+
 			// Resolve the symlink to a path via Readlink.
 			path, err := found.Inode.Readlink(t)
 			if err != nil {

test/syscalls/linux/symlink.cc

@@ -312,6 +312,19 @@ TEST_P(ParamSymlinkTest, OpenLinkCreatesTarget) {
   ASSERT_THAT(unlink(target.c_str()), SyscallSucceeds());
 }
 
+// Test that opening a self-symlink with O_CREAT will fail with ELOOP.
+TEST_P(ParamSymlinkTest, CreateExistingSelfLink) {
+  ASSERT_THAT(chdir(GetAbsoluteTestTmpdir().c_str()), SyscallSucceeds());
+
+  const std::string linkpath = GetParam();
+  ASSERT_THAT(symlink(linkpath.c_str(), linkpath.c_str()), SyscallSucceeds());
+
+  EXPECT_THAT(open(linkpath.c_str(), O_CREAT, 0666),
+              SyscallFailsWithErrno(ELOOP));
+
+  ASSERT_THAT(unlink(linkpath.c_str()), SyscallSucceeds());
+}
+
 // Test that opening an existing symlink with O_CREAT|O_EXCL will fail with
 // EEXIST.
 TEST_P(ParamSymlinkTest, OpenLinkExclFails) {