Re: [PATCH] f_flower: Remove always zero checks

[Simon Horman <horms@kernel.org>]

On Wed, Jul 17, 2024 at 07:21:30PM +0200, Guillaume Nault wrote:
> On Wed, Jul 10, 2024 at 04:11:39PM -0700, Stephen Hemminger wrote:
> > On Sun,  7 Jul 2024 20:27:41 +0300
> > Maks Mishin <maks.mishinfz@gmail.com> wrote:
> > 
> > > Expression 'ttl & ~(255 >> 0)' is always zero, because right operand
> > > has 8 trailing zero bits, which is greater or equal than the size
> > > of the left operand == 8 bits.
> > > 
> > > Found by RASU JSC.
> > > 
> > > Signed-off-by: Maks Mishin <maks.mishinFZ@gmail.com>
> > > ---
> > >  tc/f_flower.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/tc/f_flower.c b/tc/f_flower.c
> > > index 08c1001a..244f0f7e 100644
> > > --- a/tc/f_flower.c
> > > +++ b/tc/f_flower.c
> > > @@ -1523,7 +1523,7 @@ static int flower_parse_mpls_lse(int *argc_p, char ***argv_p,
> > >  
> > >  			NEXT_ARG();
> > >  			ret = get_u8(&ttl, *argv, 10);
> > > -			if (ret < 0 || ttl & ~(MPLS_LS_TTL_MASK >> MPLS_LS_TTL_SHIFT)) {
> > > +			if (ret < 0) {
> > >  				fprintf(stderr, "Illegal \"ttl\"\n");
> > >  				return -1;
> > >  			}
> > > @@ -1936,7 +1936,7 @@ static int flower_parse_opt(const struct filter_util *qu, char *handle,
> > >  			}
> > >  			mpls_format_old = true;
> > >  			ret = get_u8(&ttl, *argv, 10);
> > > -			if (ret < 0 || ttl & ~(MPLS_LS_TTL_MASK >> MPLS_LS_TTL_SHIFT)) {
> > > +			if (ret < 0) {
> > >  				fprintf(stderr, "Illegal \"mpls_ttl\"\n");
> > >  				return -1;
> > >  			}
> > 
> > That is correct mathematically, but perhaps the original author had different idea.
> > Could we have review from someone familiar with MPLS support please.
> 
> The MPLS TTL field is an 8 bits value. Therefore any successful return
> of get_u8() should be valid and this test is indeed not needed.
> 
> I guess the original intent was to keep consistency with how the other
> MPLS parameters are validated. But TTL is indeed a special case as it's
> the only parameter with a "common" length.
> 
> Reviewed-by: Guillaume Nault <gnault@redhat.com>

Thanks Guillaume,

Yes, I agree that was very likely the intent.

Reviewed-by: Simon Horman <horms@kernel.org>