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

Waiman Long
2020-06-16T18:36:15Z

On 6/16/20 2:09 PM, Andrew Morton wrote:
> 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.
>
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