Re: [RFC 0/4] CONFIG_STABLE to switch off development checks

Dave Kleikamp
2007-06-04T01:03:53Z

On Sat, 2007-06-02 at 09:28 -0700, Jeremy Fitzhardinge wrote:
> Dave Kleikamp wrote:
> > I'm on Christoph's side here. I don't think it makes sense for any code
> > to ask to allocate zero bytes of memory and expect valid memory to be
> > returned.
> >
>
> Yes, everyone agrees on that. If you do kmalloc(0), its never OK to
> dereference the result. The question is whether kmalloc(0) should complain.

Yeah, I see that you aren't necessarily asking for valid memory, just
something that appears valid. I'm still of the mind that if code is
asking for a zero-length allocation, it's raising a flag that it's not
taking some corner case into account. But I think I'm just
regurgitating what Christoph is arguing.

> > Would a compromise be to return a pointer to some known invalid region?
> > This way the kmalloc(0) call would appear successful to the caller, but
> > any access to the memory would result in an exception.
> >
>
> Yes, that's what Christoph has posted.

Oh. I went back and re-read the thread and it looks like you proposed
this already. I don't see where Christoph did, or agreed, but maybe I
missed something.

> I'm slightly concerned about
> kmalloc() returning the same non-NULL address multiple times, but it
> seems sound otherwise.

If the caller is asking for 0 bytes, it shouldn't be doing anything with
the returned address except checking for a NULL return. But then, it's
hard to predict everything that calling code might be doing, such as
allocating buffers and creating a hash based on their addresses. Of
course, if there's code that would have a problem with it, I think it's
a further argument that it would be better off avoiding the calling
kmalloc(0) in the first place.

Shaggy
--
David Kleikamp
IBM Linux Technology Center