linux-kernel - Re: [PATCH v4 08/11] READ_ONCE: Drop pointer qualifiers when reading from scalar types

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Message-ID: <6cbc8ae1-8eb1-a5a0-a584-2081fca1c4aa@rasmusvillemoes.dk>
Date:   Wed, 22 Apr 2020 12:25:03 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Cc:     linux-arch@...r.kernel.org, kernel-team@...roid.com,
        Mark Rutland <mark.rutland@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v4 08/11] READ_ONCE: Drop pointer qualifiers when reading
 from scalar types

On 21/04/2020 17.15, Will Deacon wrote:
> Passing a volatile-qualified pointer to READ_ONCE() is an absolute
> trainwreck for code generation: the use of 'typeof()' to define a
> temporary variable inside the macro means that the final evaluation in
> macro scope ends up forcing a read back from the stack. When stack
> protector is enabled (the default for arm64, at least), this causes
> the compiler to vomit up all sorts of junk.
> 
> Unfortunately, dropping pointer qualifiers inside the macro poses quite
> a challenge, especially since the pointed-to type is permitted to be an
> aggregate, and this is relied upon by mm/ code accessing things like
> 'pmd_t'. Based on numerous hacks and discussions on the mailing list,
> this is the best I've managed to come up with.

Hm, maybe this can be brought to work, only very lightly tested. It
basically abuses what -Wignored-qualifiers points out:

  warning: type qualifiers ignored on function return type

Example showing the idea:

const int c(void);
volatile int v(void);

int hack(int x, int y)
{
	typeof(c()) a = x;
	typeof(v()) b = y;

	a += b;
	b += a;
	a += b;
	return a;
}

Since that compiles, a cannot be const-qualified, and the generated code
certainly suggests that b is not volatile-qualified. So something like

#define unqual_type(x) _unqual_type(x, unique_id_dance)
#define _unqual_type(x, id) typeof( ({
  typeof(x) id(void);
  id();
}) )

and perhaps some _Pragma("GCC diagnostic push")/_Pragma("GCC diagnostic
ignored -Wignored-qualifiers")/_Pragma("GCC diagnostic pop") could
prevent the warning (which is in -Wextra, so I don't think it would
appear in a normal build anyway).

No idea how well any of this would work across gcc versions or with clang.

Rasmus