linux-kernel - Re: [PATCH v2 09/10] selftests/nolibc: test return value of read() in test

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Date:   Tue, 1 Aug 2023 09:48:59 +0200
From:   Thomas Weißschuh <linux@...ssschuh.net>
To:     Willy Tarreau <w@....eu>
Cc:     Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, Yuan Tan <tanyuan@...ylab.org>,
        Zhangjin Wu <falcon@...ylab.org>
Subject: Re: [PATCH v2 09/10] selftests/nolibc: test return value of read()
 in test_vfprintf

On 2023-08-01 08:59:17+0200, Willy Tarreau wrote:
> On Tue, Aug 01, 2023 at 07:30:16AM +0200, Thomas Weißschuh wrote:
> > If read() fails and returns -1 buf would be accessed out of bounds.
> > 
> > Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> > ---
> >  tools/testing/selftests/nolibc/nolibc-test.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
> > index 82714051c72f..a334f8450a34 100644
> > --- a/tools/testing/selftests/nolibc/nolibc-test.c
> > +++ b/tools/testing/selftests/nolibc/nolibc-test.c
> > @@ -1031,6 +1031,12 @@ static int expect_vfprintf(int llen, int c, const char *expected, const char *fm
> >  	lseek(fd, 0, SEEK_SET);
> >  
> >  	r = read(fd, buf, sizeof(buf) - 1);
> > +	if (r == -1) {
> > +		llen += printf(" read() = %s", errorname(errno));
> > +		result(llen, FAIL);
> > +		return 1;
> > +	}
> > +
> >  	buf[r] = '\0';
> 
> In fact given the nature of this file (test if we properly implemented
> our syscalls), I think that a more conservative approach is deserved
> because if we messed up on read() we can have anything on return and we
> don't want to trust that. As such I would suggest that we declare r as
> ssize_t and verify that it's neither negative nor larger than
> sizeof(buf)-1, which becomes:
> 
>         if ((size_t)r >= sizeof(buf)) {
>             ... fail ...
>         }

As r == w is validated just below anyways we could move the assignment
buf[r] = '\0' after that check and then we don't need a new block.

> You'll also have to turn w to ssize_t then due to the test later BTW.

Will do in any case.