Re: [patch 1/7] slab: introduce kzfree()

[Nick Piggin]

On Tuesday 24 February 2009 01:51:05 Hugh Dickins wrote:
> On Tue, 24 Feb 2009, Nick Piggin wrote:
> > Well, the buffer is only non-modified in the case of one of the
> > allocators (SLAB). All others overwrite some of the data region
> > with their own metadata.
> >
> > I think it is OK to use const, though. Because k(z)free has the
> > knowledge that the data will not be touched by the caller any
> > longer.
>
> Sorry, you're not adding anything new to the thread here.
>
> Yes, the caller is surrendering the buffer, so we can get
> away with calling the argument const; and Linus argues that's
> helpful in the case of kfree (to allow passing a const pointer
> without having to cast it).

(Yes, not that I agree his argument is strong enough to be able
to call libc's definition wrong)

> My contention is that kzfree(const void *ptr) is nonsensical
> because it says please zero this buffer without modifying it.
>
> But the change has gone in, I seem to be the only one still
> bothered by it, and I've conceded that the "z" might stand
> for zap rather than zero.
>
> So it may be saying please hide the contents of this buffer,
> rather than please zero it.  And then it can be argued that
> the modification is an implementation detail which happens
> (like other housekeeping internal to the sl?b allocator)
> only after the original buffer has been freed.
>
> Philosophy.

Hmm, well it better if kzfree is defined to zap rather than zero
anyway. zap is a better definition because it theoretically allows
the implementation to do something else (poision it with some
other value; mark it as zapped and don't reallocate it without
zeroing it; etc). And also it doesn't imply that the caller still
cares about what it actually gets filled with.


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