Re: [PATCH net-next] net: uapi: Provide an UAPI definition of 'struct sockaddr'

[Florian Weimer <fweimer@redhat.com>]

* Arnd Bergmann:

> On Wed, Jan 7, 2026, at 00:13, Jakub Kicinski wrote:
>> On Tue, 6 Jan 2026 11:32:52 +0100 Thomas Weißschuh wrote:
>>> As for the failure in netdev CI however I am not so sure.
>>> Looking at net-next-2026-01-05--12-00, the only failures triggered by my
>>> change are also the ones from the bpf-ci. Are these the ones you meant,
>>> or am I missing some others?
>>
>> Multiple things broke at once so slightly hard to fish the relevant
>> stuff out from here:
>>
>> https://netdev.bots.linux.dev/contest.html?branch=net-next-2026-01-05--15-00&pass=0&pw-n=0
>>
>> Here's one:
>>
>> make[1]: Entering directory 
>> '/home/virtme/testing/wt-3/tools/testing/selftests/net'
>>   CC       busy_poller
>> In file included from [01m[K/usr/include/sys/socket.h:33[m[K,
>>                  from [01m[K/usr/include/netinet/in.h:23[m[K,
>>                  from [01m[K/usr/include/arpa/inet.h:22[m[K,
>>                  from [01m[Kbusy_poller.c:14[m[K:
>> [01m[K/usr/include/bits/socket.h:182:8:[m[K [01;31m[Kerror: 
>> [m[Kredefinition of '[01m[Kstruct sockaddr[m[K'
>
>>                  from [01m[Kbusy_poller.c:12[m[K:
>> [01m[K/home/virtme/testing/wt-3/usr/include/linux/socket.h:37:8:[m[K 
>> [01;36m[Knote: [m[Koriginally defined here
>
> Maybe we can change all the instances of 'struct sockaddr' in
> include/uapi/ to reference a new 'struct __kernel_sockaddr',
> and then redirect that one if the libc header got included
> first?
>
> struct __kernel_sockaddr {
>        __kernel_sa_family_t    sa_family;      /* address family, AF_xxx       */
>        char sa_data_min[14];           /* Minimum 14 bytes of protocol address */
> };
> #ifdef _SYS_SOCKET_H
> #define __kernel_sockaddr sockaddr
> #endif
>
> This will still fail when a user application includes linux/if.h
> before sys/socket.h and then expects the structures in linux/if.h
> to contain the libc version of sockaddr, but hopefully that is
> much rarer. A survey of codesearch.debian.net shows almost all
> users of linux/if.h first including sys/socket.h, and most of
> them not caring about struct sockaddr either.

If you call the data member sa_data just like glibc, it will only fail
in C++, not C.  GCC considers the two definitions sufficiently
equivalent (even though glibc adds a may_alias attribute to meet POSIX
requirements), and duplicate definitions are permitted in C.

C++ with modules will probably support duplicate definitions, too, but I
haven't checked if it's possible to get this work with GCC 16.

Thanks,
Florian