Add three more checks for buflen and numwritten. The buflen should be at
least two, that means at least one char and the null-end. The error case
check is added by checking numwriten < 0 instead of numwritten < 1. And the
truncate case is checked. The test will exit if any of these conditions
aren't met.

Additionally, add more print information when a write failure occurs or
a truncated write happens, providing clearer diagnostics.

Signed-off-by: Chunyu Hu <chuhu@redhat.com>
---
Changes in v7:
  - add an empty line before the 'buflen < 1' line.  And change to
   'buflen < 2 to limit the buflen at least to be 2, so at least one char
    is allowed.
  - handle the numwriten < 0 case, which is the error path.
  - use ksft_exit_fail_perror in error exit path.
  - use %.*s to print the buf when error happened
Changes in v6:
  - save/restore errno before and after the close(), so the log could show
    the correct error info on failure. Suggested by AI.
Chagnes in v5:
  - new patch for making improve on write_file. Add more safety checks and
    diagnostics info in log
---
 tools/testing/selftests/mm/vm_util.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index ad96d19d1b85..00aaf33387f5 100644
--- a/tools/testing/selftests/mm/vm_util.c
+++ b/tools/testing/selftests/mm/vm_util.c
@@ -767,15 +767,24 @@ int unpoison_memory(unsigned long pfn)
 
 void write_file(const char *path, const char *buf, size_t buflen)
 {
-	int fd;
+	int fd, saved_errno;
 	ssize_t numwritten;
 
+	if (buflen < 2)
+		ksft_exit_fail_msg("Incorrect buffer len: %zu\n", buflen);
+
 	fd = open(path, O_WRONLY);
 	if (fd == -1)
-		ksft_exit_fail_msg("%s open failed: %s\n", path, strerror(errno));
+		ksft_exit_fail_perror("%s open failed", path);
 
 	numwritten = write(fd, buf, buflen - 1);
+	saved_errno = errno;
 	close(fd);
-	if (numwritten < 1)
-		ksft_exit_fail_msg("Write failed\n");
+	errno = saved_errno;
+	if (numwritten < 0)
+		ksft_exit_fail_perror("%s write(%.*s) failed", path, (int)(buflen - 1),
+				buf);
+	if (numwritten != buflen - 1)
+		ksft_exit_fail_msg("%s write(%.*s) is truncated, expected %zu bytes, got %zd bytes\n",
+				path, (int)(buflen - 1), buf, buflen - 1, numwritten);
 }
-- 
2.53.0