netdev - Re: [PATCH net-next 1/3] soc: fsl_qbman: use be16_to_cpu() in qm_sg_entry_get

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Message-ID: <34c972b7-ba53-46ba-ada9-df741bc21e47@csgroup.eu>
Date: Mon, 4 Nov 2024 15:33:03 +0100
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Breno Leitao <leitao@...ian.org>,
 Madalin Bucur <madalin.bucur@....com>, Ioana Ciornei
 <ioana.ciornei@....com>, Radu Bulie <radu-andrei.bulie@....com>,
 Sean Anderson <sean.anderson@...ux.dev>, linux-kernel@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH net-next 1/3] soc: fsl_qbman: use be16_to_cpu() in
 qm_sg_entry_get_off()



Le 29/10/2024 à 17:43, Vladimir Oltean a écrit :
> struct qm_sg_entry :: offset is a 13-bit field, declared as __be16.
> 
> When using be32_to_cpu(), a wrong value will be calculated on little
> endian systems (Arm), because type promotion from 16-bit to 32-bit,
> which is done before the byte swap and always in the CPU native
> endianness, changes the value of the scatter/gather list entry offset in
> big-endian interpretation (adds two zero bytes in the LSB interpretation).
> The result of the byte swap is ANDed with GENMASK(12, 0), so the result
> is always zero, because only those bytes added by type promotion remain
> after the application of the bit mask.
> 
> The impact of the bug is that scatter/gather frames with a non-zero
> offset into the buffer are treated by the driver as if they had a zero
> offset. This is all in theory, because in practice, qm_sg_entry_get_off()
> has a single caller, where the bug is inconsequential, because at that
> call site the buffer offset will always be zero, as will be explained in
> the subsequent change.
> 
> Flagged by sparse:
> 
> warning: cast to restricted __be32
> warning: cast from restricted __be16
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Acked-by: Christophe Leroy <christophe.leroy@...roup.eu>

> ---
>   include/soc/fsl/qman.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/soc/fsl/qman.h b/include/soc/fsl/qman.h
> index 0d3d6beb7fdb..7f7a4932d7f1 100644
> --- a/include/soc/fsl/qman.h
> +++ b/include/soc/fsl/qman.h
> @@ -242,7 +242,7 @@ static inline void qm_sg_entry_set_f(struct qm_sg_entry *sg, int len)
>   
>   static inline int qm_sg_entry_get_off(const struct qm_sg_entry *sg)
>   {
> -	return be32_to_cpu(sg->offset) & QM_SG_OFF_MASK;
> +	return be16_to_cpu(sg->offset) & QM_SG_OFF_MASK;
>   }
>   
>   /* "Frame Dequeue Response" */