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

[Jason Xing <kerneljasonxing@gmail.com>]

On Thu, Apr 23, 2026 at 12:10 AM Alexander Lobakin
<aleksander.lobakin@intel.com> wrote:
>
> 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

As to the bug itself, yes, It only affects the unaligned mode.

I wonder if we can support this after someone requires us to support
32-bit arch and use it in the real world, then we can use the previous
patch to complete the full support (which doesn't harm the path on
64-bit arch).

The code looks like this based on your suggestion. Just for the record.
diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
index 58da2f4f4397..03417b04592f 100644
--- a/net/xdp/xdp_umem.c
+++ b/net/xdp/xdp_umem.c
@@ -177,6 +177,9 @@ static int xdp_umem_reg(struct xdp_umem *umem,
struct xdp_umem_reg *mr)
if (mr->flags & ~XDP_UMEM_FLAGS_VALID)
return -EINVAL;
+ if (!IS_ENABLED(CONFIG_64BIT) && unaligned_chunks)
+ return -EOPNOTSUPP;
+
if (!unaligned_chunks && !is_power_of_2(chunk_size))
return -EINVAL;

Actually I'm fine with either of them. Right now I'm not so sure which
direction this patch should take :)

Thanks,
Jason

> 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