Re: [PATCH] proposal adding explicit_bzero

[David CARLIER <devnexen@gmail.com>]

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Yes true sorry :-|.

Kind regards.

On Tue, 26 Jun 2018 at 21:43, Rich Felker <dalias@libc.org> wrote:

> 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@afilias.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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