musl - Re: [C23 string conversion 1/3] C23: add the new memset

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Message-ID: <20230526121829.4b9a9538@inria.fr>
Date: Fri, 26 May 2023 12:18:29 +0200
From: Jₑₙₛ Gustedt <jens.gustedt@...ia.fr>
To: Joakim Sindholt <opensource@...sha.com>
Cc: musl@...ts.openwall.com
Subject: Re: [C23 string conversion 1/3] C23: add the new
 memset_explicit function

Joakim,

on Fri, 26 May 2023 11:52:36 +0200 you (Joakim Sindholt
<opensource@...sha.com>) wrote:

> I don't see how this is in any way useful. It's certainly not part of
> the standard, which only says:
> 
> > The intention is that the memory store is always performed (i.e.,
> > never elided), regardless of optimizations. This is in contrast to
> > calls to the memset function (7.26.6.1)  

There has been a long discussion in WG14 about this what is even
possible to say here. The clear intent in all discussions was to have
something that best inhibits all sorts of information leak.

What you are citing is just a footnote. The normative text says:

     The purpose of this function is to make sensitive information
     stored in the object inaccessible

So this is ist the expressed intent.

This is clearly a QoI issue. I think that indeed a sequential
reordering barrier is the minimal quality that implementations can
provide. But since this is not time critical, we might be able to
provide a bit more, with modest cost, such as synchronization with
other threads, and such as deleting the information where even the
object was located in the first place.

Thanks
Jₑₙₛ

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