No real reason, except that there are simply two versions of the patches
floating around: the original (that added the "access_ok()"), and the one
I actually applied to the development tree (in which I hand-edited the
patch to remove the old "!base" comparison, since I thought that one was
just wrong).
Which one you prefer is probably a matter of taste, and which one you
ended up picking up is probably a matter of just luck. They are basically
the same, with the only difference being whether you think NULL is special
or not. I don't personally think it should be, and if people want to put
data at their linear address zero, they may have reasons for it (vm86 mode
is one such reason, things like wine might be another - basically some
environments may have legacy reasons to think that address 0 can be
something else than just NULL).
For any *normal* program, this should be totally immaterial, since address
0 won't be mapped, and you'll just take an EFAULT later. And from the
security standpoint in this particular case, address 0 is a perfectly
normal user address, so there is nothing special about it.
[ What _is_ special about address 0 is that if the kernel forgets some
NULL pointer check, we'd want to see an oops, not a user space access.
But that's really a totally different class of bug, and quite frankly,
if you want to disallow people mmap'ing at offset zero, it's an mmap()
issue, not anything else. See the new SECURITY_DEFAULT_MMAP_MIN_ADDR
thing for that. ]
In this particular case, maybe some -stable person might have felt that
they just didn't want to change semantics for the NULL pointer, or maybe
they didn't even notice that what I committed to the development tree was
slightly changed. It _really_ doesn't matter.
Linus
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