linux-kernel - Re: [BUG] binfmt_elf: get_user() called in vma_dump_size() after set_fs(KERNEL

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Message-Id: <20090206.141831.248558063.davem@davemloft.net>
Date:	Fri, 06 Feb 2009 14:18:31 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	roland@...hat.com
Cc:	akpm@...ux-foundation.org, gerald.schaefer@...ibm.com,
	linux-kernel@...r.kernel.org, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com, torvalds@...ux-foundation.org
Subject: Re: [BUG] binfmt_elf: get_user() called in vma_dump_size() after
 set_fs(KERNEL_DS)

From: Roland McGrath <roland@...hat.com>
Date: Fri,  6 Feb 2009 14:07:17 -0800 (PST)

> set_fs is quite cheap at least on most machines.  So a pair of set_fs calls
> around that get_user call doesn't seem so bad.  OTOH, on the machines where
> this actually matters at all (maybe just sparc, arm, s390?) it is
> presumably (much?) more costly.  But it seems like the best solution, and
> certainly is straightforward.

On sparc set_fs() is just a privileged register write, so pretty cheap
and definitely less expensive than get_user_pages() :-)
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