netdev - Re: [PATCH net] af_unix: fix lockdep positive in sk_diag_dump

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Message-ID: <CANn89iL6jEVPf3=zoxZSqME6gqRPttat_bZb7yYnRYYPLUcpQw@mail.gmail.com>
Date: Tue, 30 Jan 2024 19:33:38 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, eric.dumazet@...il.com, 
	syzbot <syzkaller@...glegroups.com>, Kuniyuki Iwashima <kuniyu@...zon.com>
Subject: Re: [PATCH net] af_unix: fix lockdep positive in sk_diag_dump_icons()

On Mon, Jan 29, 2024 at 8:05 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> syzbot reported a lockdep splat [1].
>
> Blamed commit hinted about the possible lockdep
> violation, and code used unix_state_lock_nested()
> in an attempt to silence lockdep.
>
> It is not sufficient, because unix_state_lock_nested()
> is already used from unix_state_double_lock().
>
> We need to use a separate subclass.
>
> This patch adds a distinct enumeration to make things
> more explicit.
>
> Also use swap() in unix_state_double_lock() as a clean up.
>
>

...

> +#define unix_state_lock(s)     spin_lock(&unix_sk(s)->lock)
> +#define unix_state_unlock(s)   spin_unlock(&unix_sk(s)->lock)
> +enum unix_socket_lock_class {
> +       U_LOCK_NORMAL,
> +       U_LOCK_SECOND,  /* for double locking, see unix_state_double_lock(). */
> +       U_LOCK_DIAG, /* used while dumping icons, see sk_diag_dump_icons(). */
> +};
> +
> +static void unix_state_lock_nested(struct sock *sk,
> +                                  enum unix_socket_lock_class subclass)

I will add an inline keyword in v2. Not sure why I did not see the
compiler warning.

> +{
> +       spin_lock_nested(&unix_sk(sk)->lock, subclass);
> +}
> +
>  #define peer_wait peer_wq.wait
>