Re: [PATCH net-next v3 0/4] net: move .getsockopt away from __user buffers

[Stefan Metzmacher]

Am 08.04.26 um 15:56 schrieb Stefan Metzmacher:
> Am 08.04.26 um 13:26 schrieb David Laight:
> > On Wed, 08 Apr 2026 03:30:28 -0700
> > Breno Leitao <leitao@xxxxxxxxxx> wrote:
> >
> > > Currently, the .getsockopt callback requires __user pointers:
> > >
> > >    int (*getsockopt)(struct socket *sock, int level,
> > >                      int optname, char __user *optval, int __user *optlen);
> > >
> > > This prevents kernel callers (io_uring, BPF) from using getsockopt on
> > > levels other than SOL_SOCKET, since they pass kernel pointers.
> > >
> > > Following Linus' suggestion [0], this series introduces sockopt_t, a
> > > type-safe wrapper around iov_iter, and a getsockopt_iter callback that
> > > works with both user and kernel buffers. AF_PACKET and CAN raw are
> > > converted as initial users, with selftests covering the trickiest
> > > conversion patterns.
> >
> > What are you doing about the cases where 'optlen' is a complete lie?
> > IIRC there is one related to some form of async io where it is just
> > the length of the header, the actual buffer length depends on
> > data in the header.
> > This doesn't matter with the existing code for applications, when they
> > get it wrong they just crash.
> > But kernel users will need to pass the actual buffer length separately
> > from optlen.
> > It also affects any code that tries to cache the actual data and copy
> > it back to userspace in the syscall wrapper - which makes sense for
> > most short getsockopt.
> >
> > (This is different from historic code where the length might be
> > assumed to be 4 regardless of what was passed.)
>
> As the insane legacy cases can only happen for keeping
> compatibility with existing userspace applications,
> we could get the original optval and optlen __user pointers
> out of sockopt_t again via something like:
>
> char __user * __must_check sockopt_get_insame_legacy_optval(sockopt_t *sopt);
> int __user * __must_check sockopt_get_insame_legacy_optlen(sockopt_t *sopt);
>
> And for kernel callers they return NULL and the code should
> turn that into -EINVAL or something similar.

Or better helper macros/inline functions to call the legacy implementations.
something like this:

int sockopt_call_legacy_sock_fn(struct socket *sock, int level, int optname, sockopt_t *sopt,
                                int kernel_errno,
                                int (*legacy_fn)(struct socket *sock, int level,
                                                 int optname, char __user *optval, int __user *optlen))
{
     if (!sopt->legacy.optlen)
          return kernel_errno;

     return legacy_fn(sock, level, optname, sopt->legacy.optval, sopt->legacy.optlen);
}

And a similar sockopt_call_legacy_sk_fn() that takes struct sock instead of struct socket.

That way it would be relatively easy to move the calls of sockopt_call_legacy_{sock,sk}_fn
down the stack to the places where it's really needed in incremental steps.