Re: [PATCH 2/3] asm-generic, x86: wrap atomic operations

Dmitry Vyukov
2017-03-21T18:07:48Z

On Tue, Mar 21, 2017 at 11:41 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> On Tue, Mar 21, 2017 at 12:25:06PM +0300, Andrey Ryabinin wrote:
>> On 03/20/2017 08:17 PM, Mark Rutland wrote:
>> > Hi,
>> >
>> > On Tue, Mar 14, 2017 at 08:24:13PM +0100, Dmitry Vyukov wrote:
>> >> /**
>> >> - * atomic_read - read atomic variable
>> >> + * arch_atomic_read - read atomic variable
>> >> * @v: pointer of type atomic_t
>> >> *
>> >> * Atomically reads the value of @v.
>> >> */
>> >> -static __always_inline int atomic_read(const atomic_t *v)
>> >> +static __always_inline int arch_atomic_read(const atomic_t *v)
>> >> {
>> >> - return READ_ONCE((v)->counter);
>> >> + /*
>> >> + * We use READ_ONCE_NOCHECK() because atomic_read() contains KASAN
>> >> + * instrumentation. Double instrumentation is unnecessary.
>> >> + */
>> >> + return READ_ONCE_NOCHECK((v)->counter);
>> >> }
>> >
>> > Just to check, we do this to avoid duplicate reports, right?
>> >
>> > If so, double instrumentation isn't solely "unnecessary"; it has a
>> > functional difference, and we should explicitly describe that in the
>> > comment.
>> >
>> > ... or are duplicate reports supressed somehow?
>>
>> They are not suppressed yet. But I think we should just switch kasan
>> to single shot mode, i.e. report only the first error. Single bug
>> quite often has multiple invalid memory accesses causing storm in
>> dmesg. Also write OOB might corrupt metadata so the next report will
>> print bogus alloc/free stacktraces.
>> In most cases we need to look only at the first report, so reporting
>> anything after the first is just counterproductive.
>
> FWIW, that sounds sane to me.
>
> Given that, I agree with your comment regarding READ_ONCE{,_NOCHECK}().
>
> If anyone really wants all the reports, we could have a boot-time option
> to do that.

I don't mind changing READ_ONCE_NOCHECK to READ_ONCE. But I don't have
strong preference either way.

We could do:
#define arch_atomic_read_is_already_instrumented 1
and then skip instrumentation in asm-generic if it's defined. But I
don't think it's worth it.

There is no functional difference, it's only an optimization (now
somewhat questionable). As Andrey said, one can get a splash of
reports anyway, and it's the first one that is important. We use KASAN
with panic_on_warn=1 so we don't even see the rest.