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SubjectRE: [PATCH v2] x86/asm: fix assembly constraints in bitops
Date
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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