Re: [RFC PATCH 22/30] Code tagging based fault injection

Dmitry Vyukov
2022-09-01T08:43:49Z

On Wed, 31 Aug 2022 at 19:30, Kent Overstreet
<kent.overstreet@linux.dev> wrote:
> > > From: Kent Overstreet <kent.overstreet@linux.dev>
> > >
> > > This adds a new fault injection capability, based on code tagging.
> > >
> > > To use, simply insert somewhere in your code
> > >
> > > dynamic_fault("fault_class_name")
> > >
> > > and check whether it returns true - if so, inject the error.
> > > For example
> > >
> > > if (dynamic_fault("init"))
> > > return -EINVAL;
> >
> > Hi Suren,
> >
> > If this is going to be used by mainline kernel, it would be good to
> > integrate this with fail_nth systematic fault injection:
> > https://elixir.bootlin.com/linux/latest/source/lib/fault-inject.c#L109
> >
> > Otherwise these dynamic sites won't be tested by testing systems doing
> > systematic fault injection testing.
>
> That's a discussion we need to have, yeah. We don't want two distinct fault
> injection frameworks, we'll have to have a discussion as to whether this is (or
> can be) better enough to make a switch worthwhile, and whether a compatibility
> interface is needed - or maybe there's enough distinct interesting bits in both
> to make merging plausible?
>
> The debugfs interface for this fault injection code is necessarily different
> from our existing fault injection - this gives you a fault injection point _per
> callsite_, which is huge - e.g. for filesystem testing what I need is to be able
> to enable fault injection points within a given module. I can do that easily
> with this, not with our current fault injection.
>
> I think the per-callsite fault injection points would also be pretty valuable
> for CONFIG_FAULT_INJECTION_USERCOPY, too.
>
> OTOH, existing kernel fault injection can filter based on task - this fault
> injection framework doesn't have that. Easy enough to add, though. Similar for
> the interval/probability/ratelimit stuff.
>
> fail_function is the odd one out, I'm not sure how that would fit into this
> model. Everything else I've seen I think fits into this model.
>
> Also, it sounds like you're more familiar with our existing fault injection than
> I am, so if I've misunderstood anything about what it can do please do correct
> me.

What you are saying makes sense. But I can't say if we want to do a
global switch or not. I don't know how many existing users there are
(by users I mean automated testing b/c humans can switch for one-off
manual testing).

However, fail_nth that I mentioned is orthogonal to this. It's a
different mechanism to select the fault site that needs to be failed
(similar to what you mentioned as "interval/probability/ratelimit
stuff"). fail_nth allows to fail the specified n-th call site in the
specified task. And that's the only mechanism we use in
syzkaller/syzbot.
And I think it can be supported relatively easily (copy a few lines to
the "does this site needs to fail" check).

I don't know how exactly you want to use this new mechanism, but I
found fail_nth much better than any of the existing selection
mechanisms, including what this will add for specific site failing.

fail_nth allows to fail every site in a given test/syscall one-by-one
systematically. E.g. we can even have strace-like utility that repeats
the given test failing all sites in to systematically:
$ fail_all ./a_unit_test
This can be integrated into any CI system, e.g. running all LTP tests with this.

For file:line-based selection, first, we need to get these file:line
from somewhere; second, lines are changing over time so can't be
hardcoded in tests; third, it still needs to be per-task, since
unrelated processes can execute the same code.

One downside of fail_nth, though, is that it does not cover background
threads/async work. But we found that there are so many untested
synchronous error paths, that moving to background threads is not
necessary at this point.

> Interestingly: I just discovered from reading the code that
> CONFIG_FAULT_INJECTION_STACKTRACE_FILTER is a thing (hadn't before because it
> depends on !X86_64 - what?). That's cool, though.