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

[Chunyu Hu <chuhu@redhat.com>]

On Fri, Mar 27, 2026 at 07:46:03AM +0000, Lorenzo Stoakes (Oracle) wrote:
> On Tue, Mar 24, 2026 at 09:33:14AM +0800, Chunyu Hu wrote:
> > 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)
> 
> As per Sashiko (presumably didn't look :P), let's make this < 2 since we
> write buflen - 1 bytes below which we don't ever want to be 0.

That makes sense. Let me change to 2 in next version. Thanks!

> 
> > +		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
> >
>