Re: [RFC] connectat()/bindat() or an alternative design

[Cong Wang <cwang@multikernel.io>]

On Tue, Jun 30, 2026 at 04:22:25PM -0400, John Ericson wrote:
> I'm bumping this and adding new recipients again in light of the
> discussion happening elsewhere in
> <https://lore.kernel.org/all/a49ce818-f38d-41b0-bbf7-80b8aad998b1@app.fastmail.com/>.
> I don't want to count my chickens before they are hatched, but it is
> looking to me like a consensus in that thread is building around the
> ability to opt into intentionally empty/unusable root and working
> directories (at least with nullfs, maybe but less likely with other
> mechanisms instead).
> 
> That new functionality concretizes the motivation for what I am
> proposing in this thread: in such a world, there is little to no point
> binding listening sockets in the file system, because the containing
> directory would have to be conveyed by file descriptor anyways --- might
> as well just directly convey the socket to connect to by file
> descriptor. Likewise, abstract sockets are not appealing, because the
> abstract socket namespace is either too coarse-grained (leaking info in
> the same way root/cwd would), or too cumbersome to keep it from leaking.
> 
> To recap (with some slight changes, like renames), my latest proposal (a
> new version, not either of the two variations in the original email) is
> new syscalls `bind_unix_anon` and `connectat`, supporting a workflow
> like this:
> 
>     /* server */
>     int lfd = socket(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0);
>     int addrfd = bind_unix_anon(
>             lfd,
>             /*flags, for the future*/0);
>     listen(lfd, 64);
> 
>     /* client, handed `addrfd` */
>     int cfd = socket(AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0);
>     connectat(addrfd, cfd, AT_EMPTY_PATH);
> 
> Or, more radically, `bind_unix_anon` and `connectat` could let one skip
> the initial `socket` calls by returning those new sockets directly:

Hm? Why not just setsockopt()? Something like:

struct unix_lookup_ctx {
    int dirfd;
    __u64 resolve_flags;   /* RESOLVE_BENEATH, RESOLVE_NO_SYMLINKS, etc. */
    __u64 op_flags;        /* maybe future */
};

setsockopt(fd, SOL_UNIX, UNIX_NEXT_LOOKUP,
           &ctx, sizeof(ctx));

bind(fd, (struct sockaddr *)&addr, addrlen);
/* or connect(fd, ...) */


Zero new syscall is needed.

Regards,
Cong