linux-kernel - Re: [PATCH v5 2/2] mm, treewide: Rename kzfree() to kfree

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Message-ID: <65002c1e-5e31-1f4e-283c-186e06e55ef0@redhat.com>
Date:   Tue, 16 Jun 2020 14:36:15 -0400
From:   Waiman Long <longman@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Joe Perches <joe@...ches.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        "Jason A . Donenfeld" <Jason@...c4.com>, linux-mm@...ck.org,
        keyrings@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-amlogic@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
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        ecryptfs@...r.kernel.org, kasan-dev@...glegroups.com,
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Subject: Re: [PATCH v5 2/2] mm, treewide: Rename kzfree() to kfree_sensitive()

On 6/16/20 2:09 PM, Andrew Morton wrote:
> On Tue, 16 Jun 2020 11:43:11 -0400 Waiman Long <longman@...hat.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