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

[Alexander Lobakin <aleksander.lobakin@intel.com>]

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

> 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.

What if we relax the restriction a bit? For example, refuse to configure
an XSk socket in unaligned mode if on a 32-bit arch? Or add a check
under CONFIG_32_BIT like it was done in Page Pool:

skb_shinfo(skb)->destructor_arg = (void *)((uintptr_t)addr | 0x1UL);

#ifdef CONFIG_32BIT
if (((uintptr_t)skb_shinfo(skb)->destructor_arg) & ~0x1UL) != addr)
	// WARN_ONCE or whatever + error path
#endif

I never used XSk on a 32-bit arch, but back when I was working on 32-bit
MIPS 1G routers, I wanted to add native XSk support to the Eth driver.
Sure, just for fun, now that we have cheap AArch64 and other 64-bit
embedded chips, 32-bit embedded networking SoCs are almost dead, but
OTOH, as you can see, other subsystems like PP still try to support 32 bit.
Especially given that this issue applies to only to the skb XSk path,
not native in-driver implementations.

> 
> 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

Thanks,
Olek