linux-kernel - Re: [RFC][PATCH] rcu: Use typeof(p) instead of typeof(*p) *

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Date:   Tue, 5 Oct 2021 11:58:17 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Paul <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>, rcu <rcu@...r.kernel.org>,
        netfilter-devel <netfilter-devel@...r.kernel.org>,
        coreteam@...filter.org, netdev <netdev@...r.kernel.org>
Subject: Re: [RFC][PATCH] rcu: Use typeof(p) instead of typeof(*p) *

On Tue, 5 Oct 2021 11:15:12 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> ----- On Oct 5, 2021, at 9:47 AM, rostedt rostedt@...dmis.org wrote:
> [...]
> > #define rcu_dereference_raw(p) \
> > ({ \
> > 	/* Dependency order vs. p above. */ \
> > 	typeof(p) ________p1 = READ_ONCE(p); \
> > -	((typeof(*p) __force __kernel *)(________p1)); \
> > +	((typeof(p) __force __kernel)(________p1)); \
> > })  
> 
> AFAIU doing so removes validation that @p is indeed a pointer, so a user might mistakenly
> try to use rcu_dereference() on an integer, and get away with it. I'm not sure we want to
> loosen this check. I wonder if there might be another way to achieve the same check without
> requiring the structure to be declared, e.g. with __builtin_types_compatible_p ?

Is that really an issue? Because you would be assigning it to an integer.


	x = rcu_dereference_raw(y);

And that just makes 'x' a copy of 'y' and not really a reference to it, thus
if you don't have a pointer, it's just a fancy READ_ONCE(y).

-- Steve