netdev - Re: [PATCH net V2 1/2] net: stmmac: Premature loop termination check was ignored on rx

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Date:   Fri, 17 Mar 2023 22:21:17 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jochen Henneberg <jh@...neberg-systemdesign.com>
Cc:     netdev@...r.kernel.org,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Ong Boon Leong <boon.leong.ong@...el.com>,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net V2 1/2] net: stmmac: Premature loop termination
 check was ignored on rx

On Thu, 16 Mar 2023 08:59:39 +0100 Jochen Henneberg wrote:
> The premature loop termination check makes sense only in case of the
> jump to read_again where the count may have been updated. But
> read_again did not include the check.
> 
> Fixes: ec222003bd94 ("net: stmmac: Prepare to add Split Header support")
> Signed-off-by: Jochen Henneberg <jh@...neberg-systemdesign.com>
> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index e4902a7bb61e..ea51c7c93101 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -5221,10 +5221,10 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
>  			len = 0;
>  		}
>  
> +read_again:
>  		if (count >= limit)
>  			break;

Are you sure? Can you provide more detailed analysis?
Do you observe a problem / error in real life or is this theoretical?

As far as I can tell only path which jumps to read_again after doing
count++ is via the drain_data jump, but I can't tell how it's
discarding subsequent segments in that case..

> -read_again:
>  		buf1_len = 0;
>  		buf2_len = 0;
>  		entry = next_entry;