Re: [PATCH net-next v2 0/2] net: a sockopt_t quirk for the options that write past optlen

[Stanislav Fomichev <sdf.kernel@gmail.com>]

On 09/14, Breno Leitao wrote:
> This series continues the migration of our protocols to sockopt_t, as
> described in [1].
> 
> There are some protocols that use optlen as the header size, and the real
> buffer size comes from a field inside the header. This means a bug, given
> that optlen should be the full buffer size, but there are indications [2]
> that we have programs that use the bad behaviour above, and we want to
> avoid breaking them (or, honestly, avoid being cursed by Linus).
> 
> That said, create a quirk helper that preserves the same behaviour, even
> using sockopt_t. The way to do it is simple:
> 
> 1) Only do it for userspace callers (ubuf), otherwise a bug here will
>    corrupt the kernel instead of a simple SIGSEGV.
> 2) Expand optval mid-air based on the header field.
> 
> IP_MSFILTER is the first user, and the smallest one: a single caller, no
> compat variant, and a reply written front to back. MCAST_MSFILTER on ipv4
> and ipv6 comes next, and TCP_AO_GET_KEYS has the same shape. Do this
> quirk on IP_MSFILTER to make sure the dynamic is ok, so, we can expand
> it later.
> 
> None of this is meant to change what userspace sees.
> 
> Link: https://lore.kernel.org/all/20260401-getsockopt-v2-0-611df6771aff@debian.org/ [1]
> Link: https://lore.kernel.org/all/20260806-mcast_fix-v1-0-bed0a5518e57@debian.org/ [2]
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
> Changes in v2:
> - sockopt_expand_out() measures the request against opt->optlen instead
>   of the iterator's remaining count, and asserts that nothing has been
>   written through iter_out yet. Comparing against the remaining count
>   made the verdict depend on how far a callback had already got, and a
>   late call would rewind the write cursor to the head of optval.
> - Cap the requested size at INT_MAX, since optlen and the getsockopt ABI
>   are int.
> - do_ip_getsockopt() writes optlen back only when ip_mc_msfget()
>   succeeded. Without the guard, a read-only optlen turned -EINVAL,
>   -ENODEV and -EADDRNOTAVAIL into -EFAULT.
> - Name IP_MSFILTER in patch 1, document the WARN_ON_ONCE() and that no
>   in-tree path reaches it, and mention sockptr_to_sockopt() losing its
>   static.
> - Link to v1: https://patch.msgid.link/20260910-getsockopt_phase6-v1-0-e681e102d5b8@debian.org

Acked-by: Stanislav Fomichev <sdf@fomichev.me>