Don't mask errors in createAt loop.

The error set in the loop in createAt was being masked
by other errors declared with ":=". This allowed an
ErrResolveViaReadlink error to escape, which can cause
a sentry panic.

Added test case which repros without the fix.

PiperOrigin-RevId: 257061767

pkg/sentry/syscalls/linux/sys_file.go

@@ -353,7 +353,8 @@ func createAt(t *kernel.Task, dirFD int32, addr usermem.Addr, flags uint, mode l
 				// No more resolution necessary.
 				defer resolved.DecRef()
 				break
-			} else if err != fs.ErrResolveViaReadlink {
+			}
+			if err != fs.ErrResolveViaReadlink {
 				return err
 			}
 
@@ -363,15 +364,17 @@ func createAt(t *kernel.Task, dirFD int32, addr usermem.Addr, flags uint, mode l
 			}
 
 			// Resolve the symlink to a path via Readlink.
-			path, err := found.Inode.Readlink(t)
+			var path string
+			path, err = found.Inode.Readlink(t)
 			if err != nil {
 				break
 			}
 			remainingTraversals--
 
 			// Get the new parent from the target path.
+			var newParent *fs.Dirent
 			newParentPath, newName := fs.SplitLast(path)
-			newParent, err := t.MountNamespace().FindInode(t, root, parent, newParentPath, &remainingTraversals)
+			newParent, err = t.MountNamespace().FindInode(t, root, parent, newParentPath, &remainingTraversals)
 			if err != nil {
 				break
 			}

test/syscalls/linux/symlink.cc

@@ -325,6 +325,21 @@ TEST_P(ParamSymlinkTest, CreateExistingSelfLink) {
   ASSERT_THAT(unlink(linkpath.c_str()), SyscallSucceeds());
 }
 
+// Test that opening a file that is a symlink to its parent directory fails
+// with ELOOP.
+TEST_P(ParamSymlinkTest, CreateExistingParentLink) {
+  ASSERT_THAT(chdir(GetAbsoluteTestTmpdir().c_str()), SyscallSucceeds());
+
+  const std::string linkpath = GetParam();
+  const std::string target = JoinPath(linkpath, "child");
+  ASSERT_THAT(symlink(target.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) {