linux-kernel - Re: [PATCH/RFC 7/7] kernel: Force ACCESS

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CA+55aFxc72VsGTw4yFdeC1Sq65RUjYLKPD1ORnXB2d18WBMzvg@mail.gmail.com>
Date:	Mon, 24 Nov 2014 10:35:39 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	David Howells <dhowells@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	linux-mips <linux-mips@...ux-mips.org>,
	linux-x86_64@...r.kernel.org,
	linux-s390 <linux-s390@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types

On Mon, Nov 24, 2014 at 10:02 AM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> If the goal is to catch non-scalar users, the following is shorter:
> #define ACCESS_ONCE(x) (((typeof(x))0) + *(volatile typeof(x) *)&(x))

Me likey. It probably works well in practice, although I think

 - the "(typeof(x))0)" seems unnecessary and wrong. Why not just "0"?
The typeof is not just longer, but it is incorrect for pointer types
(you can add 0 to a pointer, but you cannot add two pointers together)

 - it does mean that the resulting type ends up being upgraded to
"int", for the usual C type reasons.

Note that the "upgraded to 'int'" is true with or without the
"(typeof(x))0". If you add two 'char' values, the addition is still
done in 'int'.

Maybe you *meant* that typeof to fix the second problem, like so:

  (typeof(x)) (0 + *(volatile typeof(x) *)&(x))

Hmm? That casts the result of the addition, not the zero.

             Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/