Re: file locking fix for 3.2

[Al Viro]

On Sat, Dec 24, 2011 at 04:50:12PM -0500, J. Bruce Fields wrote:

>     locks: fix null dereference on lease-break failure path
>     
>     Commit 778fc546f749c588aa2f6cd50215d2715c374252 "locks: fix tracking of
>     inprogress lease breaks" introduced a null dereference on failure to
>     allocate memory.
>     
>     This means an open (without O_NONBLOCK set) on a file with a lease
>     applied (generally only done when Samba or nfsd (with v4) is running)
>     could crash if a kmalloc() fails.

NULL?  AFAICS, lease_alloc() returns ERR_PTR() on failure...  I really
don't like the look of that code, TBH - at the very least it needs to
be commented a lot.  E.g. the rules for calling or not calling ->lm_break()
are really not obvious - AFAICS, we do that if
	i_have_this_lease || (mode & O_NONBLOCK)
is true *or* if allocation has succeeded.  The former condition is what'll
end up with -EWOULDBLOCK; I can understand not wanting to return that in
preference to -ENOMEM, but...  Do we want to skip ->lm_break() stuff only
in case of allocation failures that won't be overridden by -EWOULDBLOCK?
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