linux-kernel - Re: Collecting both remote and "local" coverage with KCOV

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Date:   Thu, 19 Nov 2020 08:24:40 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Alexander Bulekov <alxndr@...edu>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Collecting both remote and "local" coverage with KCOV

On Wed, Nov 18, 2020 at 3:21 AM Alexander Bulekov <alxndr@...edu> wrote:
>
> On 201116 1805, Andrey Konovalov wrote:
> > On Mon, Nov 16, 2020 at 9:35 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
> > >
> > > On Mon, Nov 16, 2020 at 3:39 AM Alexander Bulekov <alxndr@...edu> wrote:
> > > >
> > > > Hello,
> > > > I'm trying to collect coverage over the syscalls issued by my process,
> > > > as well as the kthreads spawned as a result of these syscalls
> > > > (eg coverage over vhost ioctls and the worker kthread). Is there a way
> > > > to collect coverage with both KCOV_REMOTE_ENABLE(with common_handle) and
> > > > KCOV_ENABLE, simultaneously?
> > > >
> > > > Based on the code it seems that these two modes are mutually
> > > > exclusive within a single task, but I don't think this is mentioned in
> > > > the Documentation, so I want to make sure I'm not missing something.
> > >
> > > Hi Alex,
> > >
> > > Yes, it's probably not supported within a single task. The easiest way
> > > to verify is to try it ;)
> > >
> > > It is possible to collect both coverages, but you will need 2 threads
> > > (one just to set up remote KCOV).
> > >
> > > Unless I am missing any fundamental limitations, I would say it would
> > > be reasonable to support this within a single task as well.
> >
> > I think the reason we did that initially, is because we don't care
> > about normal coverage for USB emitting pseudo-syscalls. Filed a bug
> > for this: https://bugzilla.kernel.org/show_bug.cgi?id=210225
>
> I'm interested in adding support for this. Looking through the code, I
> can think of ~two approaches:
>
> 1.) Allow the same kcov fd to be used to track coverage with both
> KCOV_REMOTE_ENABLE and KCOV_ENABLE. If we try to use the same coverage
> bitmap for both the remote and the local coverage, I think the local
> part would have to deal with the kcov_remote_lock. If the local part
> continues to write directly into the user-space coverage-area, as it
> does now, it seems it would require locking for each __sanitizer_cov
> call.  Alternatively, the local and the remote parts could write into
> different coverage-bitmaps, but I'm not sure if there is a neat way to
> do this.

This has 2 problems:
 - performance (__sanitizer_cov is by far the most performance
critical part of kernel with KCOV=y)
 - recurions, locks are also traced, so it's not that we really can
call anything there

> 2.) Allow multiple kcov fds to be used by the same task. In the task,
> keep a linked-list of pointers to kcov objects (remote or local). For
> each __sanitizer_... call, walk the linked list and check if any of the
> kcov objects match the requirements (trace_cmp/trace_pc/remote). This
> would also have the side-effect of enabling simultaneous PC and CMP
> tracing. Of course, it seems that this would add some overhead (in the
> case of a single open fd, there would be extra pointer dereferences to
> get the area[], size, etc).

Walking linked list in __sanitizer_... has the same performance
problems, but I think we don't really need to do it.
Assuming we have at most 1 KCOV that traces the task itself we can
continue keeping it cached in task_struct:
https://elixir.bootlin.com/linux/v5.10-rc4/source/include/linux/sched.h#L1254
and __sanitizer_... will continue using these fields.

For the kcov pointer in task struct:
https://elixir.bootlin.com/linux/v5.10-rc4/source/include/linux/sched.h#L1257
we either have a linked list, or 1 pointer for local tracking and a
separate list for remote kcov's:
struct kcov *kcov; // local tracing
struct kcov *remote_kcovs; // remote tracing, can be more than 1
Whichever is better I am not sure, it seems that some functions would
benefit from a single list (KCOV_DISABLE), while others would benefit
from separate fields (KCOV_ENABLE).
Maybe the simplest code will be if we use both approaches -- put all
kcov's into a list, but also cache the local kcov into a separate
field? Then KCOV_DISABLE could just walk the list, but KCOV_ENABLE can
continue checking 1 field.