Re: [PATCH -rcu] asm-generic, kcsan: Add KCSAN instrumentation for bitops

[Marco Elver]

On Fri, 17 Jan 2020 at 14:14, Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Fri, 17 Jan 2020 at 13:25, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> >
> > On Wed, Jan 15, 2020 at 9:50 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> > > On Wed, 15 Jan 2020 at 20:55, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > > On Wed, Jan 15, 2020 at 8:51 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> > > > > On Wed, 15 Jan 2020 at 20:27, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > > Are there any that really just want kasan_check_write() but not one
> > > > of the kcsan checks?
> > >
> > > If I understood correctly, this suggestion would amount to introducing
> > > a new header, e.g. 'ksan-checks.h', that provides unified generic
> > > checks. For completeness, we will also need to consider reads. Since
> > > KCSAN provides 4 check variants ({read,write} x {plain,atomic}), we
> > > will need 4 generic check variants.
> >
> > Yes, that was the idea.
> >
> > > I certainly do not feel comfortable blindly introducing kcsan_checks
> > > in all places where we have kasan_checks, but it may be worthwhile
> > > adding this infrastructure and starting with atomic-instrumented and
> > > bitops-instrumented wrappers. The other locations you list above would
> > > need to be evaluated on a case-by-case basis to check if we want to
> > > report data races for those accesses.
> >
> > I think the main question to answer is whether it is more likely to go
> > wrong because we are missing checks when one caller accidentally
> > only has one but not the other, or whether they go wrong because
> > we accidentally check both when we should only be checking one.
> >
> > My guess would be that the first one is more likely to happen, but
> > the second one is more likely to cause problems when it happens.
>
> Right, I guess both have trade-offs.
>
> > > As a minor data point, {READ,WRITE}_ONCE in compiler.h currently only
> > > has kcsan_checks and not kasan_checks.
> >
> > Right. This is because we want an explicit "atomic" check for kcsan
> > but we want to have the function inlined for kasan, right?
>
> Yes, correct.
>
> > > My personal preference would be to keep the various checks explicit,
> > > clearly opting into either KCSAN and/or KASAN. Since I do not think
> > > it's obvious if we want both for the existing and potentially new
> > > locations (in future), the potential for error by blindly using a
> > > generic 'ksan_check' appears worse than potentially adding a dozen
> > > lines or so.
> > >
> > > Let me know if you'd like to proceed with 'ksan-checks.h'.
> >
> > Could you have a look at the files I listed and see if there are any
> > other examples that probably a different set of checks between the
> > two, besides the READ_ONCE() example?
>
> All the user-copy related code should probably have kcsan_checks as well.
>
> > If you can't find any, I would prefer having the simpler interface
> > with just one set of annotations.
>
> That's fair enough. I'll prepare a v2 series that first introduces the
> new header, and then applies it to the locations that seem obvious
> candidates for having both checks.

I've sent a new patch series which introduces instrumented.h:
   http://lkml.kernel.org/r/20200120141927.114373-1-elver@xxxxxxxxxx

Thanks,
-- Marco