linux-kernel - Re: [PATCH] getsockopt() early argument sanity checking

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Date:	Sun, 20 Aug 2006 20:57:18 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	linux-kernel@...r.kernel.org, Netdev <netdev@....sgi.com>,
	Solar Designer <solar@...nwall.com>
Subject: Re: [PATCH] getsockopt() early argument sanity checking

Arjan wrote:

>> We're on UP.  sys_getsockopt() does get_user() (due to the patch) and
>> makes sure that the passed *optlen is sane.  Even if this get_user()
>> sleeps, the value it returns in "len" is what's currently in memory at
>> the time of the get_user() return (correct?)  Then an underlying
>> *getsockopt() function does another get_user() on optlen (same address),
>> without doing any other user-space data accesses or anything else that
>> could sleep first.  Is it possible that this second get_user()
>> invocation would sleep?  I think not since it's the same address that
>> we've just read a value from, we did not leave kernel space, and we're
>> on UP (so no other processor could have changed the mapping).  So the
>> patch appears to be sufficient for this special case (which is not
>> unlikely).
>
>this reasoning goes out the window with kernel preemption of course ;)
>  
>
Or O_DIRECT? I'm not sure what's easier to time, a kernel preemption or 
a DMA to the user address.

--
    Manfred

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