On Wed, Sep 10, 2014 at 04:26:46PM +0200, Jiri Kosina wrote:
> On Wed, 10 Sep 2014, Theodore Ts'o wrote:
>
> > I'd much rather depending on better testing and static checkers to fix
> > them, since kfree *is* a hot path.
>
> BTW if we stretch this argument a little bit more, we should also kill the
> ZERO_OR_NULL_PTR() check from kfree() and make it callers responsibility
> to perform the checking only if applicable ... we are currently doing a
> lot of pointless checking in cases where caller would be able to guarantee
> that the pointer is going to be non-NULL.
What you're saying is that we should remove the ZERO_SIZE_PTR
completely. ZERO_SIZE_PTR is a very useful idiom and also it's too late
to remove it because everything depends on it.
Returning ZERO_SIZE_PTR is not an error. Callers shouldn't test for it.
It works like this:
1) User space says "copy zero items to somewhere."
2) The kernel says "here is a zero size pointer"
3) We do some stuff like:
copy_from_user(zero_pointer, src, 0)
or:
for (i = 0; i < 0; i++)
4) The caller frees the ZERO_SIZE_PTR.
5) We return success.
If we get rid of it then we're start returning -ENOMEM all over the
place and that breaks userspace. Or we introduce zero as a special case
for every kmalloc.
You would think there would be a lot of bugs with ZERO_SIZE_POINTERs
but they seem fairly rare to me. There are some where we allocate a
zero length string and then put a NUL terminator at the end.
regards,
dan carpenter
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