Re: [PATCH v5 2/2] mm, treewide: Rename kzfree() to kfree_sensitive()

[Waiman Long]

On 6/16/20 2:09 PM, Andrew Morton wrote:
> On Tue, 16 Jun 2020 11:43:11 -0400 Waiman Long <longman@xxxxxxxxxx> wrote:
>
> > As said by Linus:
> >
> >    A symmetric naming is only helpful if it implies symmetries in use.
> >    Otherwise it's actively misleading.
> >
> >    In "kzalloc()", the z is meaningful and an important part of what the
> >    caller wants.
> >
> >    In "kzfree()", the z is actively detrimental, because maybe in the
> >    future we really _might_ want to use that "memfill(0xdeadbeef)" or
> >    something. The "zero" part of the interface isn't even _relevant_.
> >
> > The main reason that kzfree() exists is to clear sensitive information
> > that should not be leaked to other future users of the same memory
> > objects.
> >
> > Rename kzfree() to kfree_sensitive() to follow the example of the
> > recently added kvfree_sensitive() and make the intention of the API
> > more explicit. In addition, memzero_explicit() is used to clear the
> > memory to make sure that it won't get optimized away by the compiler.
> >
> > The renaming is done by using the command sequence:
> >
> >    git grep -w --name-only kzfree |\
> >    xargs sed -i 's/\bkzfree\b/kfree_sensitive/'
> >
> > followed by some editing of the kfree_sensitive() kerneldoc and adding
> > a kzfree backward compatibility macro in slab.h.
> >
> > ...
> >
> > --- a/include/linux/slab.h
> > +++ b/include/linux/slab.h
> > @@ -186,10 +186,12 @@ void memcg_deactivate_kmem_caches(struct mem_cgroup *, struct mem_cgroup *);
> >    */
> >   void * __must_check krealloc(const void *, size_t, gfp_t);
> >   void kfree(const void *);
> > -void kzfree(const void *);
> > +void kfree_sensitive(const void *);
> >   size_t __ksize(const void *);
> >   size_t ksize(const void *);
> >   
> > +#define kzfree(x)	kfree_sensitive(x)	/* For backward compatibility */
> > +
> What was the thinking here?  Is this really necessary?
>
> I suppose we could keep this around for a while to ease migration.  But
> not for too long, please.
It should be there just for 1 release cycle. I have broken out the btrfs 
patch to the btrfs list and I didn't make the kzfree to kfree_sensitive 
conversion there as that patch was in front in my patch list. So 
depending on which one lands first, there can be a window where the 
compilation may fail without this workaround. I am going to send out 
another patch in the next release cycle to remove it.

Cheers,
Longman