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

[Marco Elver <elver@google.com>]

On Fri, 17 Jan 2020 at 13:25, Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Wed, Jan 15, 2020 at 9:50 PM Marco Elver <elver@google.com> wrote:
> > On Wed, 15 Jan 2020 at 20:55, Arnd Bergmann <arnd@arndb.de> wrote:
> > > On Wed, Jan 15, 2020 at 8:51 PM Marco Elver <elver@google.com> wrote:
> > > > On Wed, 15 Jan 2020 at 20:27, Arnd Bergmann <arnd@arndb.de> 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.

Thanks,
-- Marco