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

[Dmitry Vyukov]

On Wed, May 29, 2019 at 12:30 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, May 29, 2019 at 12:16:31PM +0200, Marco Elver wrote:
> > On Wed, 29 May 2019 at 12:01, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxx

Interesting. Does an address passed to bitops also should be aligned,
or alignment is supposed to be handled by bitops themselves?

This probably should be done as a separate config as not related to
KASAN per se. But obviously via the same
{atomicops,bitops}-instrumented.h hooks which will make it
significantly easier.