Re: [PATCH v3 net-next 6/9] af_unix: Move SOCK_PASS{CRED,PIDFD,SEC} to struct sock.

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Kuniyuki Iwashima wrote:
> From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Date: Thu, 15 May 2025 14:44:14 -0400
> > Kuniyuki Iwashima wrote:
> > > As explained in the next patch, SO_PASSRIGHTS would have a problem
> > > if we assigned a corresponding bit to socket->flags, so it must be
> > > managed in struct sock.
> > > 
> > > Mixing socket->flags and sk->sk_flags for similar options will look
> > > confusing, and sk->sk_flags does not have enough space on 32bit system.
> > > 
> > > Also, as mentioned in commit 16e572626961 ("af_unix: dont send
> > > SCM_CREDENTIALS by default"), SOCK_PASSCRED and SOCK_PASSPID handling
> > > is known to be slow, and managing the flags in struct socket cannot
> > > avoid that for embryo sockets.
> > > 
> > > Let's move SOCK_PASS{CRED,PIDFD,SEC} to struct sock.
> > > 
> > > While at it, other SOCK_XXX flags in net.h are grouped as enum.
> > > 
> > > Note that assign_bit() was atomic, so the writer side is moved down
> > > after lock_sock() in setsockopt(), but the bit is only read once
> > > in sendmsg() and recvmsg(), so lock_sock() is not needed there.
> > 
> > Because the socket lock is already held there?
> 
> No, for example, scm_recv_unix() is called without lock_sock(),
> but it's okay because reading a single bit is always a matter
> of timing, when to snapshot the flag, (unless there is another
> dependency or the bit is read more than once).
> 
> With this, write happens before/after the if block:
> 
>                                <-- write could happen here
>   lock_sock()
>   if (sk->sk_scm_credentials) {
>     do something
>   }
>   lock_unlock()
>                                <-- or here (not related to logic)
> 
> but this is same without lock_sock() if the bit is read only
> once:
> 
>                                <-- write could happen here
>   if (sk->sk_scm_credentials) {
>     do something               <-- or here (not related to logic)
>   }
>                                <-- or here (not related to logic)
> 
> So for SOCK_PASSXXX bits, lock_sock() prevents data-race
> between writers as you pointed out, but it does nothing
> for readers.

Essentially you're saying that a single bit read is a natural
word read, so atomic anyway? I.e., yes this is a data race, safe.
Will KCSAN report on the race regardless?

All other single bit cases in sk_getsockopt use sk_flags
and sock_flag, so are not a good existing example. But the single
bit reads in do_tcp_getsockopt do the same. So I guess it's fine.
Indeed constant_test_bit does nothing special either.

Sounds good, thanks.