[PATCH v6 4/6] selftests/mm/vm_util: robust write_file()

[Chunyu Hu <chuhu@redhat.com>]

Add two more checks for buflen and numwritten. The buflen should be at
least one, otherwise the 'buflen - 1' could underflow and cause trouble.
The numwritten should be equal to 'buflen - 1'. 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 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 | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index ad96d19d1b85..572ccb99de8e 100644
--- a/tools/testing/selftests/mm/vm_util.c
+++ b/tools/testing/selftests/mm/vm_util.c
@@ -767,15 +767,23 @@ 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 < 1)
+		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));
 
 	numwritten = write(fd, buf, buflen - 1);
+	saved_errno = errno;
 	close(fd);
+	errno = saved_errno;
 	if (numwritten < 1)
-		ksft_exit_fail_msg("Write failed\n");
+		ksft_exit_fail_msg("%s write(%s) failed: %s\n", path, buf,
+				strerror(errno));
+	if (numwritten != buflen - 1)
+		ksft_exit_fail_msg("%s write(%s) is truncated, expected %zu bytes, got %zd bytes\n",
+				path, buf, buflen - 1, numwritten);
 }
-- 
2.53.0