Re: [PATCH net v3 8/8] xsk: don't support AF_XDP on 32-bit architectures

[David Laight <david.laight.linux@gmail.com>]

On Wed, 22 Apr 2026 11:36:50 +0800
Jason Xing <kerneljasonxing@gmail.com> wrote:

> From: Jason Xing <kernelxing@tencent.com>
> 
> In copy mode TX, xsk_skb_destructor_set_addr() stores the 64-bit
> descriptor address into skb_shinfo(skb)->destructor_arg (void *) via a
> uintptr_t cast:
> 
>     skb_shinfo(skb)->destructor_arg = (void *)((uintptr_t)addr | 0x1UL);
> 
> On 32-bit architectures uintptr_t is 32 bits, so the upper 32 bits of
> the descriptor address are silently dropped. In XDP_ZEROCOPY unaligned
> mode the chunk offset is encoded in bits 48-63 of the descriptor
> address (XSK_UNALIGNED_BUF_OFFSET_SHIFT = 48), meaning the offset is
> lost entirely. The completion queue then returns a truncated address to
> userspace, making buffer recycling impossible.

I had a look at how this is used.

I suspect that XSK_UNALIGNED_BUF_OFFSET_SHIFT can just be made smaller.
The 'addr' isn't really a normal address of any kind, it is a really
and offset into an array made up of pages of memory.
The actual address (kernel virtual of dma) is generated by:
	ptr->array[addr >> PAGE_SHIFT] + addr & ~PAGE_MASK
(after removing the unaligned chunk offset from 'addr').

It is actually quite likely that there are enough free high bits
even in 32bit mode.

	David

> 
> Since we hear no one is using AF_XDP on 32-bit arch, we decided to
> strictly stop supporting it at compile time.
> 
> Closes: https://lore.kernel.org/all/20260419045824.D9E5EC2BCAF@smtp.kernel.org/
> Fixes: 0ebc27a4c67d ("xsk: avoid data corruption on cq descriptor number")
> Suggested-by: Stanislav Fomichev <sdf@fomichev.me>
> Signed-off-by: Jason Xing <kernelxing@tencent.com>
> ---
>  net/xdp/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/xdp/Kconfig b/net/xdp/Kconfig
> index 71af2febe72a..819aa5795f50 100644
> --- a/net/xdp/Kconfig
> +++ b/net/xdp/Kconfig
> @@ -1,7 +1,7 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  config XDP_SOCKETS
>  	bool "XDP sockets"
> -	depends on BPF_SYSCALL
> +	depends on BPF_SYSCALL && 64BIT
>  	default n
>  	help
>  	  XDP sockets allows a channel between XDP programs and