linux-kernel - Re: [PATCH] Fix off-by-one bug in mbind() syscall implementation

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Message-ID: <20100726094931.GA17756@basil.fritz.box>
Date:	Mon, 26 Jul 2010 11:49:31 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Andre Przywara <andre.przywara@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] Fix off-by-one bug in mbind() syscall implementation

On Mon, Jul 26, 2010 at 11:28:18AM +0200, Andre Przywara wrote:
> When the mbind() syscall implementation processes the node mask
> provided by the user, the last node is accidentally masked out.
> This is present since the dawn of time (aka Before Git), I guess
> nobody realized that because libnuma as the most prominent user of
> mbind() uses large masks (sizeof(long)) and nobody cared if the
> 64th node is not handled properly. But if the user application
> defers the masking to the kernel and provides the number of valid bits
> in maxnodes, there is always the last node missing.
> However this also affect the special case with maxnodes=0, the manpage
> reads that mbind(ptr, len, MPOL_DEFAULT, &some_long, 0, 0); should
> reset the policy to the default one, but in fact it returns EINVAL.
> This patch just removes the decrease-by-one statement, I hope that
> there is no workaround code in the wild that relies on the bogus
> behavior.

Actually libnuma and likely most existing users rely on it.

The only way to change it would be to add new system calls.

-Andi
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