linux-kernel - Re: [git pull] vfs fixes

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Date:   Tue, 11 Apr 2017 07:48:40 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [git pull] vfs fixes

On Mon, Apr 10, 2017 at 11:10:19PM -0700, Linus Torvalds wrote:

> It looks odd because the lock part is
> 
>                 if (flags & LOOKUP_RCU)
>                         rcu_read_lock();
> 
> ie it's locked conditionally, and the code in between does not seem to
> return every time LOOKUP_RCU is clear.
> 
> So mind giving this a look? Is it as obviously buggy as I think it is,
> or is there something I'm missing?

It's more obscure than I would like, and can grow into a bug one day, but...
nd_jump_root() can only return non-zero if you have LOOKUP_RCU.  So without
LOOKUP_RCU in flags, this
                if (flags & LOOKUP_RCU)
                        rcu_read_lock();
                set_root(nd);
                if (likely(!nd_jump_root(nd)))
                        return s;
                nd->root.mnt = NULL;
                rcu_read_unlock();
won't get to that rcu_read_unlock() at all - it'll get zero from nd_jump_root()
and proceed to return s;