Re: [PATCH v1 bpf-next] selftest: bpf: Correct mssind comparison in test_tcp_custom_syncookie.c.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Martin KaFai Lau <martin.lau@linux.dev>
Date: Tue, 20 Aug 2024 17:11:48 -0700
> On 8/19/24 12:42 PM, Kuniyuki Iwashima wrote:
> > Smatch reported a possible off-by-one in tcp_validate_cookie().
> > 
> > However, it's false positive because the possible range of mssind is
> > limited from 0 to 3 by the preceding calculation.
> > 
> >    mssind = (cookie & (3 << 6)) >> 6;
> > 
> > There's no real issue, but let's make Smatch happy to suppress the same
> > reports.
> > 
> > Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
> > Closes: https://lore.kernel.org/bpf/6ae12487-d3f1-488b-9514-af0dac96608f@stanley.mountain/
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> > ---
> >   tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c b/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c
> > index 44ee0d037f95..36b842133033 100644
> > --- a/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c
> > +++ b/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c
> > @@ -487,12 +487,12 @@ static int tcp_validate_cookie(struct tcp_syncookie *ctx)
> >   
> >   	mssind = (cookie & (3 << 6)) >> 6;
> 
> This should have bound the mssind.
> 
> >   	if (ctx->ipv4) {
> > -		if (mssind > ARRAY_SIZE(msstab4))
> > +		if (mssind >= ARRAY_SIZE(msstab4))
> 
> Does the verifier complain without this if check?

It didn't :)

I'll remove the checks in v2.

Thanks!