linux-kernel - Re: [PATCH] net/9p: Fix sparse rcu warnings in client.c

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Date:   Thu, 18 Jun 2020 21:08:07 +0200
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Alexander Kapshuk <alexander.kapshuk@...il.com>
Cc:     ericvh@...il.com, lucho@...kov.net, davem@...emloft.net,
        kuba@...nel.org, v9fs-developer@...ts.sourceforge.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net/9p: Fix sparse rcu warnings in client.c

Alexander Kapshuk wrote on Thu, Jun 18, 2020:
> Address sparse nonderef rcu warnings:
> net/9p/client.c:790:17: warning: incorrect type in argument 1 (different address spaces)
> net/9p/client.c:790:17:    expected struct spinlock [usertype] *lock
> net/9p/client.c:790:17:    got struct spinlock [noderef] <asn:4> *
> net/9p/client.c:792:48: warning: incorrect type in argument 1 (different address spaces)
> net/9p/client.c:792:48:    expected struct spinlock [usertype] *lock
> net/9p/client.c:792:48:    got struct spinlock [noderef] <asn:4> *
> net/9p/client.c:872:17: warning: incorrect type in argument 1 (different address spaces)
> net/9p/client.c:872:17:    expected struct spinlock [usertype] *lock
> net/9p/client.c:872:17:    got struct spinlock [noderef] <asn:4> *
> net/9p/client.c:874:48: warning: incorrect type in argument 1 (different address spaces)
> net/9p/client.c:874:48:    expected struct spinlock [usertype] *lock
> net/9p/client.c:874:48:    got struct spinlock [noderef] <asn:4> *
> 
> Signed-off-by: Alexander Kapshuk <alexander.kapshuk@...il.com>

Thanks for this patch.
>From what I can see, there are tons of other parts of the code doing the
same noderef access pattern to access current->sighand->siglock and I
don't see much doing that.
A couple of users justify this by saying SLAB_TYPESAFE_BY_RCU ensures
we'll always get a usable lock which won't be reinitialized however we
access it... It's a bit dubious we'll get the same lock than unlock to
me, so I agree to some change though.

After a second look I think we should use something like the following:

if (!lock_task_sighand(current, &flags))
	warn & skip (or some error, we'd null deref if this happened currently);
recalc_sigpending();
unlock_task_sighand(current, &flags);

As you can see, the rcu_read_lock() isn't kept until the unlock so I'm
not sure it will be enough to please sparse, but I've convinced myself
current->sighand cannot change while we hold the lock and there just are
too many such patterns in the kernel.

Please let me know if I missed something or if there is an ongoing
effort to change how this works; I'll wait for a v2.

-- 
Dominique