linux-kernel - Re: [PATCH 1/2] mm, treewide: Rename kzfree() to kfree

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Date:   Tue, 14 Apr 2020 02:32:03 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Waiman Long <longman@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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>
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Subject: Re: [PATCH 1/2] mm, treewide: Rename kzfree() to kfree_sensitive()

On 4/13/20 3:15 PM, Waiman Long 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. 

Seems reasonable to me. One bikeshed, that you can safely discard and 
ignore as a mere bikeshed: kfree_memzero or kfree_scrub or 
kfree_{someverb} seems like a better function name, as it describes what 
the function does, rather than "_sensitive" that suggests something 
about the data maybe but who knows what that entails. If you disagree, 
not a big deal either way.

 > In addition, memzero_explicit() is used to clear the
 > memory to make sure that it won't get optimized away by the compiler.

This had occurred to me momentarily a number of years ago, but I was 
under the impression that the kernel presumes extern function calls to 
always imply a compiler barrier, making it difficult for the compiler to 
reason about what happens in/after kfree, in order to be able to 
optimize out the preceding memset. With LTO, that rule obviously 
changes. I guess new code should be written with cross-object 
optimizations in mind now a days? [Meanwhile, it would be sort of 
interesting to teach gcc about kfree to enable additional scary 
optimizations...]