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>

diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index e83c1e7e2beb680eb26db3075c4c5ae6b16e56db..1b9d3b2e2491c93d266c51237616747a8249a446 100644 (file)
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/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;