[PATCH v3 2/2] selftests/nolibc: Add a very basic test for fallocate()

Daniel Palmer
2026-04-30T16:41:41Z

1: Create a tmp file, fallocate() to make it a bit bigger, check the
size is what was expected.

2: Try to fallocate() (1 << 20), this should work.

3: Try to fallocate() (1 << 52), this should cause ENOSPC or EFBIG.

2 and 3 are basically to make sure if the offset or size are split
into a pair of registers for the syscall that we are passing them
the correct way around.

sparc32 seems to return EOPNOTSUPP for fallocate() so skip the
test there.

Signed-off-by: Daniel Palmer <daniel@thingy.jp>
---
tools/testing/selftests/nolibc/nolibc-test.c | 69 ++++++++++++++++++++
1 file changed, 69 insertions(+)

diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index 08610cacf030..a586e9d9ede2 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -89,6 +89,14 @@ static const int is_glibc =
#endif
;

+static const int is_sparc32 =
+#if defined(__sparc__) && !defined(__arch64__)
+ 1
+#else
+ 0
+#endif
+;
+
#if !defined(NOLIBC)
/* Some disabled tests may not compile. */

@@ -898,6 +906,66 @@ int test_getpagesize(void)
return !c;
}

+int test_fallocate(void)
+{
+ struct stat st;
+ int fd, r;
+
+ /* Create a new tmp file */
+ fd = open("/tmp", O_TMPFILE | O_RDWR, 0644);
+ if (fd == -1)
+ return -1;
+
+ /* Expand it to 42 bytes */
+ r = fallocate(fd, 0, 0, 42);
+ if (r)
+ goto close_tmpfile;
+
+ /* Get the new stat */
+ r = fstat(fd, &st);
+ if (r)
+ goto close_tmpfile;
+
+ /* It should be 42 bytes long */
+ if (st.st_size != 42) {
+ r = -1;
+ goto close_tmpfile;
+ }
+
+ /* Now try to allocate 1MiB. This puts a single bit
+ * into one of the registers if the size is split into
+ * two registers. This shouldn't fail if the bit is in
+ * the correct register.
+ */
+ r = fallocate(fd, 0, 0, (1ll << 20));
+ if (r)
+ goto close_tmpfile;
+
+ /* Check a massive size that puts a single bit into
+ * the other register if splitting.
+ * This should return an error and errno = ENOSPC or
+ * EFBIG indicating the value was passed correctly but it
+ * was rejected.
+ */
+ r = fallocate(fd, 0, 0, (1ll << (20 + 32)));
+ if (r != -1) {
+ r = -1;
+ goto close_tmpfile;
+ }
+ if (errno != ENOSPC && errno != EFBIG) {
+ r = -1;
+ goto close_tmpfile;
+ }
+
+ /* Test passed */
+ r = 0;
+
+close_tmpfile:
+ close(fd);
+
+ return r;
+}
+
int test_file_stream(void)
{
FILE *f;
@@ -1507,6 +1575,7 @@ int run_syscall(int min, int max)
CASE_TEST(dup3_0); tmp = dup3(0, 100, 0); EXPECT_SYSNE(1, tmp, -1); close(tmp); break;
CASE_TEST(dup3_m1); tmp = dup3(-1, 100, 0); EXPECT_SYSER(1, tmp, -1, EBADF); if (tmp != -1) close(tmp); break;
CASE_TEST(execve_root); EXPECT_SYSER(1, execve("/", (char*[]){ [0] = "/", [1] = NULL }, NULL), -1, EACCES); break;
+ CASE_TEST(fallocate); EXPECT_SYSZR(!is_sparc32, test_fallocate()); break;
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;
--
2.53.0