tools/nolibc: Add a simple test for writing to a FILE and reading it back

Add a test that exercises create->write->seek->read to check that using the
stream functions (fwrite() etc) is not totally broken.

The only edge cases this is testing for are:
- Reading the file after writing but without rewinding reads nothing.
- Trying to read more items than the file contains returns the count of
  fully read items.

Signed-off-by: Daniel Palmer <daniel@thingy.jp>
Link: https://patch.msgid.link/20260105023629.1502801-4-daniel@thingy.jp
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

Author: fifteenhex

tools/testing/selftests/nolibc/nolibc-test.c

@@ -878,6 +878,58 @@ int test_file_stream(void)
 	return 0;
 }
 
+int test_file_stream_wsr(void)
+{
+	const char dataout[] = "foo";
+	const size_t datasz = sizeof(dataout);
+	char datain[datasz];
+	int fd, r;
+	FILE *f;
+
+	fd = open("/tmp", O_TMPFILE | O_RDWR, 0644);
+	if (fd == -1)
+		return -1;
+
+	f = fdopen(fd, "w+");
+	if (!f)
+		return -1;
+
+	errno = 0;
+	r = fwrite(dataout, 1, datasz, f);
+	if (r != datasz)
+		return -1;
+
+	/* Attempt to read from the file without rewinding,
+	 * we should read 0 items.
+	 */
+	r = fread(datain, 1, datasz, f);
+	if (r)
+		return -1;
+
+	/* Rewind the file to the start */
+	r = fseek(f, 0, SEEK_SET);
+	if (r)
+		return -1;
+
+	/* Attempt to read back more than was written to
+	 * make sure we handle short reads properly.
+	 * fread() should return the number of complete items.
+	 */
+	r = fread(datain, 1, datasz + 1, f);
+	if (r != datasz)
+		return -1;
+
+	/* Data we read should match the data we just wrote */
+	if (memcmp(datain, dataout, datasz) != 0)
+		return -1;
+
+	r = fclose(f);
+	if (r)
+		return -1;
+
+	return 0;
+}
+
 enum fork_type {
 	FORK_STANDARD,
 	FORK_VFORK,
@@ -1352,6 +1404,7 @@ int run_syscall(int min, int max)
 		CASE_TEST(fchdir_stdin);      EXPECT_SYSER(1, fchdir(STDIN_FILENO), -1, ENOTDIR); break;
 		CASE_TEST(fchdir_badfd);      EXPECT_SYSER(1, fchdir(-1), -1, EBADF); break;
 		CASE_TEST(file_stream);       EXPECT_SYSZR(1, test_file_stream()); break;
+		CASE_TEST(file_stream_wsr);   EXPECT_SYSZR(1, test_file_stream_wsr()); break;
 		CASE_TEST(fork);              EXPECT_SYSZR(1, test_fork(FORK_STANDARD)); break;
 		CASE_TEST(getdents64_root);   EXPECT_SYSNE(1, test_getdents64("/"), -1); break;
 		CASE_TEST(getdents64_null);   EXPECT_SYSER(1, test_getdents64("/dev/null"), -1, ENOTDIR); break;