Re: [PATCH v2 03/11] selftests/mm: fix "warning: expression which evaluates to zero..." in mlock2-tests.c

[John Hubbard]

On 6/5/23 08:43, Peter Xu wrote:
> On Fri, Jun 02, 2023 at 07:15:50PM -0700, John Hubbard wrote:
> > The stop variable is a char*, and the code was assigning a char value to
> > it. This was generating a warning when compiling with clang.
> >
> > However, as both David and Peter pointed out, stop is not even used
> > after the problematic assignment to a char type. So just delete that
> > line entirely.
> >
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Cc: Peter Xu <peterx@xxxxxxxxxx>
> > Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> > ---
> >   tools/testing/selftests/mm/mlock2-tests.c | 1 -
> >   1 file changed, 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/mm/mlock2-tests.c b/tools/testing/selftests/mm/mlock2-tests.c
> > index 11b2301f3aa3..80cddc0de206 100644
> > --- a/tools/testing/selftests/mm/mlock2-tests.c
> > +++ b/tools/testing/selftests/mm/mlock2-tests.c
> > @@ -50,7 +50,6 @@ static int get_vm_area(unsigned long addr, struct vm_boundaries *area)
> >   			printf("cannot parse /proc/self/maps\n");
> >   			goto out;
> >   		}
> > -		stop = '\0';
> >   
> >   		sscanf(line, "%lx", &start);
> >   		sscanf(end_addr, "%lx", &end);
>
> I'd rather simply make it "*stop = '\0'", or as David suggested dropping
> stop completely when we're it (assumes that scanf() will always work with
> number ending with space ' ').

Actually it does not assume that. Rather, it follows the documented behavior
of strchr(3), which is:

    The strchr() and strrchr() functions return a pointer to the matched
    character or NULL if the character is not found. The terminating
    null byte is considered part of the string, so that if c is
    specified as '\0', these functions return a pointer to the
    terminator.

And we have this code now:

	stop = strchr(end_addr, ' ');
	if (!stop) {
		printf("cannot parse /proc/self/maps\n");
		goto out;
	}

So, either stop has a valid char* in it, or we goto out. There are no
fragile assumptions in there, as far as I can see anyway.

> No strong opinion here, though.

OK, I think it's kind of a flip of the coin whether to write this:

	stop = strchr(end_addr, ' ');
	if (!stop) {

or this:

	if (!strchr(end_addr, ' ')) {

So I'll just leave it as the first one, which (depending on the
day of the week) might read slightly clearer. :)


thanks,
--
John Hubbard
NVIDIA