[PATCH v8 4/6] selftests/mm/vm_util: robust write_file()

Chunyu Hu
2026-04-02T01:46:42Z

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 v8:
- go back to ksft_exit_fail_msg in the two error exit path as no clean
ksft_exit_fail_perror that supports va_list
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 | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index ad96d19d1b85..db94564f4431 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));

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_msg("%s write(%.*s) failed: %s\n", path, (int)(buflen - 1),
+ buf, strerror(errno));
+ 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