Re: [PATCH net-next 3/3] net: devmem: decode DMA addresses for TX

[Pavel Begunkov]

On 9/22/26 21:43, Stanislav Fomichev wrote:
> On 32-bit architectures where dma_addr_t is wider than unsigned long,
> page_pool_set_dma_addr_netmem() stores page-aligned DMA addresses shifted
> by PAGE_SHIFT. The net_iov branch of __skb_frag_dma_map() adds byte offsets
> to the encoded value, so the NIC is programmed with an invalid DMA address.
> This can trigger an IOMMU fault or DMA from unintended memory.
>
> Consolidate DMA address encoding, decoding, and representability checks in
> netmem helpers. Use the common decoder from the page pool and net_iov TX
> paths so both interpret stored addresses consistently.
>
> Fixes: bd61848900bf ("net: devmem: Implement TX path")
> Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxxx>
> ---
>   include/linux/skbuff.h          |  7 +++++--
>   include/net/netmem.h            | 27 +++++++++++++++++++++++++++
>   include/net/page_pool/helpers.h | 10 +---------
>   net/core/page_pool_priv.h       | 14 ++------------
>   4 files changed, 35 insertions(+), 23 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 421f6fc45451..3740d4b15b70 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3833,9 +3833,12 @@ static inline dma_addr_t __skb_frag_dma_map(struct device *dev,
>   					    size_t offset, size_t size,
>   					    enum dma_data_direction dir)
>   {
> +	dma_addr_t addr;
> +
>   	if (skb_frag_is_net_iov(frag)) {
> -		return netmem_to_net_iov(frag->netmem)->desc.dma_addr +
> -		       offset + frag->offset;
> +		addr = netmem_dma_addr_decode(
> +			netmem_get_dma_addr(frag->netmem));
> +		return addr + offset + frag->offset;
Looks like just page_pool_get_dma_addr_netmem(). Just a note, but it'd
be nice to rename netmem_get_dma_addr() into something less confusing
as well.

--
Pavel Begunkov