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

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Message-ID: <4dbff8032f874a6f921ba0555c94eeaf@AcuMS.aculab.com>
Date:   Mon, 11 Oct 2021 08:39:35 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mathieu Desnoyers' <mathieu.desnoyers@...icios.com>,
        rostedt <rostedt@...dmis.org>
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" <coreteam@...filter.org>,
        netdev <netdev@...r.kernel.org>
Subject: RE: [RFC][PATCH] rcu: Use typeof(p) instead of typeof(*p) *

From: Mathieu Desnoyers
> Sent: 05 October 2021 16:15
> 
> ----- 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 ?

Could you pass the pointer to something like:
static __always_inline void foo(void *arg) {};

That would fail for integers.
Not sure whether CFI bleats about function pointers though.

	David

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