linux-kernel - Re: [PATCH 04/12] selftests/mm: fix a char* assignment in mlock2-tests.c

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Date:   Mon, 5 Jun 2023 11:38:58 -0400
From:   Peter Xu <peterx@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/12] selftests/mm: fix a char* assignment in
 mlock2-tests.c

On Fri, Jun 02, 2023 at 11:52:42AM -0700, John Hubbard wrote:
> On 6/2/23 08:24, Peter Xu wrote:
> > On Fri, Jun 02, 2023 at 12:04:57PM +0200, David Hildenbrand wrote:
> > > On 02.06.23 03:33, John Hubbard wrote:
> > > > The stop variable is a char*, so use "\0" when assigning to it, rather
> > > > than attempting to assign a character type. This was generating a
> > > > warning when compiling with clang.
> > > > 
> > > > Signed-off-by: John Hubbard <jhubbard@...dia.com>
> > > > ---
> > > >    tools/testing/selftests/mm/mlock2-tests.c | 2 +-
> > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/tools/testing/selftests/mm/mlock2-tests.c b/tools/testing/selftests/mm/mlock2-tests.c
> > > > index 11b2301f3aa3..8ee95077dc25 100644
> > > > --- a/tools/testing/selftests/mm/mlock2-tests.c
> > > > +++ b/tools/testing/selftests/mm/mlock2-tests.c
> > > > @@ -50,7 +50,7 @@ static int get_vm_area(unsigned long addr, struct vm_boundaries *area)
> > > >    			printf("cannot parse /proc/self/maps\n");
> > > >    			goto out;
> > > >    		}
> > > > -		stop = '\0';
> > > > +		stop = "\0";
> > > >    		sscanf(line, "%lx", &start);
> > > >    		sscanf(end_addr, "%lx", &end);
> > > 
> > > 
> > > I'm probably missing something, but what is the stop variable supposed to do
> > > here? It's completely unused, no?
> > > 
> > > if (!strchr(end_addr, ' ')) {
> > > 	printf("cannot parse /proc/self/maps\n");
> > > 	goto out;
> > > }
> 
> Yes it is! I certainly had tunnel vision on that one. I've changed the
> patch to simply delete that line, for v2, thanks.
> 
> > 
> > I guess it wanted to do "*stop = '\0'" but it just didn't matter a lot
> > since the sscanf() just worked..
> > 
> 
> Maybe, yes. Hard to tell the original intent at this point...it might
> have been used in an early draft version of the loop that didn't get
> posted, perhaps.

I'm pretty sure of it.. see the pattern:

		end_addr = strchr(line, '-');
		if (!end_addr) {
			printf("cannot parse /proc/self/maps\n");
			goto out;
		}
		*end_addr = '\0';

And...

		stop = strchr(end_addr, ' ');
		if (!stop) {
			printf("cannot parse /proc/self/maps\n");
			goto out;
		}
		stop = '\0';    <------------------- only diff here

-- 
Peter Xu