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

[Breno Leitao]

On Tue, Oct 29, 2024 at 06:43:15PM +0200, Vladimir Oltean wrote:
> 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 at nxp.com>

Reviewed-by: Breno Leitao <leitao at debian.org>