Re: [PATCH net v2 1/3] xsk: align TX metadata layout across ABIs

["Arnd Bergmann" <arnd@arndb.de>]

On Thu, Aug 13, 2026, at 21:07, Stanislav Fomichev wrote:
> Add explicit padding before launch_time so xsk_tx_metadata has the same
> layout on 32-bit and 64-bit systems.
>
> On i386 and m68k, the old native 32-bit layout put launch_time at offset 12
> and had a natural size of 20 bytes. Using sizeof(struct xsk_tx_metadata) as
> tx_metadata_len was already rejected because the length must be a multiple
> of eight, so the straightforward use of the interface was broken on those
> ABIs. Userspace could still register a padded length of 24 bytes, though;
> mixing the old and new layouts then silently reads launch_time from the
> wrong offset and misprograms packet launch times. This intentionally
> replaces that incompatible layout while the interface is still new.
>
> Fixes: ca4419f15abd ("xsk: Add launch time hardware offload support to 
> XDP Tx metadata")
> Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>

This is probably the right approach, given lack of alternatives.
In the changelog text, it may be worth noting a few more points:

- A few additional architectures have the same issue: csky,
  nios2, openrisc and sh.
- the commit that introduced the mistake was part of linux-6.15,
  so the 6.18-lts release also needs an ABI change. I don't
  think the "while the interface is still new" wording
  makes sense here.
- what actually saves us here is that none of the affected
  architectures are likely to have notable use cases for
  xdp that would care about the ABI break.
  The one that is most likely to have affected users
  is x86-compat, and that is also the only one that is
  broken right now.

      Arnd