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

[Dmitry Vyukov]

On Wed, May 29, 2019 at 1:23 PM Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
> On 5/29/19 1:57 PM, Dmitry Vyukov wrote:
> > 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?
> >
>
> It should be aligned. This even documented in Documentation/core-api/atomic_ops.rst:
>
>         Native atomic bit operations are defined to operate on objects aligned
>         to the size of an "unsigned long" C data type, and are least of that
>         size.  The endianness of the bits within each "unsigned long" are the
>         native endianness of the cpu.
>
>
> > 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.
> >
>
> Agreed.

Thanks. I've filed https://bugzilla.kernel.org/show_bug.cgi?id=203751
for checking alignment with all the points and references, so that
it's not lost.