netdev - Re: [PATCH] f_flower: Remove always zero checks

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Message-ID: <20240710161139.578d20dc@hermes.local>
Date: Wed, 10 Jul 2024 16:11:39 -0700
From: Stephen Hemminger <stephen@...workplumber.org>
To: Maks Mishin <maks.mishinfz@...il.com>, Guillaume Nault
 <gnault@...hat.com>
Cc: netdev@...r.kernel.org
Subject: Re: [PATCH] f_flower: Remove always zero checks

On Sun,  7 Jul 2024 20:27:41 +0300
Maks Mishin <maks.mishinfz@...il.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@...il.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.