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Message-ID: <20170705000303.6936aefe@inria.fr>
Date: Wed, 5 Jul 2017 00:03:03 +0200
From: Jens Gustedt <jens.gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: Re: more on missing volatile qualifications
Hello Rich,
On Tue, 4 Jul 2017 17:19:09 -0400 Rich Felker <dalias@...c.org> wrote:
> On Sun, Jun 25, 2017 at 01:06:29PM +0200, Jens Gustedt wrote:
> > There is a reading of the C standard that says that volatile only
> > has implications if an object itself is such qualified, having a
> > volatile qualified lvalue access isn't enough. I don't think that
> > any current compiler does such weird things, but who knows where
> > optimisers will go in the future.
>
> Indeed. GCC seems committed to treating "accesses through volatile
> lvalue" as volatile, but I'd rather not depend on it. Perhaps we
> should add a primitive to atomic.h for loading the value of atomics so
> that we never access them directly; then volatile would not matter.
While I agree for the principle, identifying all these places and
replace them with a primitive might not be so easy.
Luckily most of these variables a called "lock" :)
I'll look into it and see what I can do. But I will probably not be
able to come up with sensible platform specific choices for an
"a_load" primitive and set it to "a_cas(p, 0, 0)" or so, to have a
start.
> > AFAICS for the third finding in sigaction.c this would not be an
> > issue. Since in addition this is something dealing with signal
> > stuff, I still think that volatile would be in order, here.
>
> The line:
>
> memcpy(set, handler_set, sizeof handler_set);
>
> is not valid if handler_set is made volatile;
Yes, so said the compiler, too.
> we'd have to write out
> the code to copy it. Not a big deal though, and more correct anyway;
> using memcpy to copy something that's semantically atomic is sloppy.
Ok, so let's do this properly.
> Unfortunately since I don't want to encode knowledge of the naming of
> sigset_t internals here, we'd probably need a loop to copy to a
> non-volatile array the same as handler_set, then memcpy from there to
> set.
Yes, unfortunately. To do that we'd also need an "a_load_l" primitive,
or we'd have to change handler_set to an array of unsigned, which is
far more intrusive.
Jens
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