linux-kernel - RE: [PATCH v2] x86/asm: fix assembly constraints in bitops

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Date:   Tue, 2 Apr 2019 11:45:20 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexander Potapenko' <glider@...gle.com>,
        "paulmck@...ux.ibm.com" <paulmck@...ux.ibm.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        "jyknight@...gle.com" <jyknight@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>
Subject: RE: [PATCH v2] x86/asm: fix assembly constraints in bitops

From: Alexander Potapenko
> Sent: 02 April 2019 12:28
> 
> 1. Use memory clobber in bitops that touch arbitrary memory
> 
> Certain bit operations that read/write bits take a base pointer and an
> arbitrarily large offset to address the bit relative to that base.

Although x86_64 can use a signed 64bit bit number, looking at arm and arm64
they use 'int nr' throughout as do the generic functions.
Maybe x86 ought to be consistent here.
I doubt negative bit numbers are expected to work?

Did you try telling gcc that a big buffer (250MB is the limit for 32bit)
from the pointer might be changed?
That ought to be softer than a full 'memory' clobber as it should
only affect memory that could be accessed through the pointer.

....
> -#define BITOP_ADDR(x) "+m" (*(volatile long *) (x))
> +#define RLONG_ADDR(x) "m" (*(volatile long *) (x))
> +#define WBYTE_ADDR(x) "+m" (*(volatile char *) (x))
> 
> -#define ADDR				BITOP_ADDR(addr)
> +#define ADDR				RLONG_ADDR(addr)

Is it worth just killing ADDR ?
(as a different patch)

	David

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