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From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Thu, 1 Dec 2022 11:34:03 +0000
Message-ID: <CAH6eHdTq=BQGkAY=1w0QvXghkVmE-SRYEfip9vMqn-dO6twYZA@mail.gmail.com>
Subject: Re: Avoiding stack buffer clear being optimised out
To: Jonny Grant <jg@jguk.org>
Cc: gcc-help <gcc-help@gcc.gnu.org>
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On Thu, 1 Dec 2022 at 11:31, Jonathan Wakely wrote:
>
> On Thu, 1 Dec 2022 at 10:44, Jonny Grant wrote:
> > Thank you Jonathan and David for your replies.
> >
> > That "noipa" looks to have sorted this issue
> >
> > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
> >
> > That page also suggests "noinline" attribute which seems to suggest I'd need to add asm (""); in each wrapper of memset()
>
> I already used the noinline attribute in my example above.
>
> > I'd much rather have memset_s - Jonathan, do you think GCC could add some built-in functions for memset_s ?     __builtin_memset_s() would be great.
>
> No.
>
> But C2x adds a memset_explicit function that does what you want, so
> that should arrive in glibc soonish.
> I thought it had been added, but was searching the C2x draft for
> "memset_secure" and other incorrect names.
>
> https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2631.htm was the
> proposal adding it.

Ah, and that proposal links to an example implementation which uses
glibc's explicit_bzero which already exists (since glibc 2.25, and
also in some BSDs) and does what you want.
https://github.com/ojeda/secure_clear/blob/master/example-implementation/secure_clear.h