Re: [PATCH 3/3] asm-generic, x86: Add bitops instrumentation for KASAN

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 29, 2019 at 12:16:31PM +0200, Marco Elver wrote:
> On Wed, 29 May 2019 at 12:01, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, May 29, 2019 at 11:20:17AM +0200, Marco Elver wrote:
> > > For the default, we decided to err on the conservative side for now,
> > > since it seems that e.g. x86 operates only on the byte the bit is on.
> >
> > This is not correct, see for instance set_bit():
> >
> > static __always_inline void
> > set_bit(long nr, volatile unsigned long *addr)
> > {
> >         if (IS_IMMEDIATE(nr)) {
> >                 asm volatile(LOCK_PREFIX "orb %1,%0"
> >                         : CONST_MASK_ADDR(nr, addr)
> >                         : "iq" ((u8)CONST_MASK(nr))
> >                         : "memory");
> >         } else {
> >                 asm volatile(LOCK_PREFIX __ASM_SIZE(bts) " %1,%0"
> >                         : : RLONG_ADDR(addr), "Ir" (nr) : "memory");
> >         }
> > }
> >
> > That results in:
> >
> >         LOCK BTSQ nr, (addr)
> >
> > when @nr is not an immediate.
> 
> Thanks for the clarification. Given that arm64 already instruments
> bitops access to whole words, and x86 may also do so for some bitops,
> it seems fine to instrument word-sized accesses by default. Is that
> reasonable?

Eminently -- the API is defined such; for bonus points KASAN should also
do alignment checks on atomic ops. Future hardware will #AC on unaligned
[*] LOCK prefix instructions.

(*) not entirely accurate, it will only trap when crossing a line.
    https://lkml.kernel.org/r/1556134382-58814-1-git-send-email-fenghua.yu@intel.com