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

Andrew Morton
2020-06-16T18:09:46Z

On Tue, 16 Jun 2020 11:43:11 -0400 Waiman Long <longman@redhat.com> 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.