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

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Thu, Apr 23, 2026 at 12:37:07AM +0800, Jason Xing wrote:
> 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 :)

Abstracting from discussion, that is not a patch we would want against
-net. We should not forbid 32bit on stable kernels, maybe there is someone
in the 'basement' using 32bit xsk on stable kernel. Plus the fixes tag vs
patch's content looks weird ;)

> 
> 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