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

[Manfred Spraul <manfred@colorfullife.com>]

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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