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Date: Tue, 26 Jun 2018 16:43:41 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] proposal adding explicit_bzero

On Fri, Jun 15, 2018 at 01:37:43PM +0000, David CARLIER wrote:
> Hi dear lists,
> this is my first message so forgive me if this idea had already been rejected.

It's definitely not rejected outright, and I think the consensus is
to adopt it. But..

> From c0a16cf96b96b009097d6ed656a2a7b8969e8399 Mon Sep 17 00:00:00 2001
> From: David Carlier <dcarlier@...lias.info>
> Date: Fri, 15 Jun 2018 13:30:09 +0000
> Subject: [PATCH] string: adding simple explicit_bzero implementation.
> 
> glibc implementing it and modern security based code starting
> using it widely, here a simple implementation using memory barrier.
> ---
>  include/string.h            | 1 +
>  src/string/explicit_bzero.c | 8 ++++++++
>  2 files changed, 9 insertions(+)
>  create mode 100644 src/string/explicit_bzero.c
> 
> diff --git a/include/string.h b/include/string.h
> index ce1dc300..795a2abc 100644
> --- a/include/string.h
> +++ b/include/string.h
> @@ -82,6 +82,7 @@ void *memccpy (void *__restrict, const void *__restrict, int, size_t);
>  char *strsep(char **, const char *);
>  size_t strlcat (char *, const char *, size_t);
>  size_t strlcpy (char *, const char *, size_t);
> +void explicit_bzero (void *, size_t);
>  #endif
>  
>  #ifdef _GNU_SOURCE
> diff --git a/src/string/explicit_bzero.c b/src/string/explicit_bzero.c
> new file mode 100644
> index 00000000..47dba3c7
> --- /dev/null
> +++ b/src/string/explicit_bzero.c
> @@ -0,0 +1,8 @@
> +#define _BSD_SOURCE
> +#include <string.h>
> +
> +void explicit_bzero(void *d, size_t n)
> +{
> +	memset(d, 0, n);
> +	__asm__ volatile("": "r="(d) :: "memory");
> +}
> -- 

The constraint here looks wrong. Normally = is written before the
type, not after; I'm not sure if all compiler versions accept the
unusual form with it after. But more importantly you have it as an
output constraint, where it's essentially a dead store, such that the
asm block does nothing to make explicit_bzero force the memset to
happen.

I think you meant for the constraint to be an input constraint "r"(d).
Does that sound right?

Rich

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