linux-kernel - Re: [PATCH 0/5] kcov: PREEMPT

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Date:   Mon, 6 Sep 2021 18:28:24 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Marco Elver <elver@...gle.com>
Cc:     kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Clark Williams <williams@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/5] kcov: PREEMPT_RT fixup + misc

On 2021-09-06 18:13:11 [+0200], Marco Elver wrote:
> Thanks for sorting this out. Given syzkaller is exercising all of
> KCOV's feature, I let syzkaller run for a few hours with PROVE_LOCKING
> (and PROVE_RAW_LOCK_NESTING) on, and looks fine:
> 
>     Acked-by: Marco Elver <elver@...gle.com>
>     Tested-by: Marco Elver <elver@...gle.com>

awesome.

> > One thing I noticed and have no idea if this is right or not:
> > The code seems to mix long and uint64_t for the reported instruction
> > pointer / position in the buffer. For instance
> > __sanitizer_cov_trace_pc() refers to a 64bit pointer (in the comment)
> > while the area pointer itself is (long *). The problematic part is that
> > a 32bit application on a 64bit pointer will expect a four byte pointer
> > while kernel uses an eight byte pointer.
> 
> I think the code is consistent in using 'unsigned long' for writing
> regular pos/IP (except write_comp_data(), which has a comment about
> it). The mentions of 64-bit in comments might be inaccurate though.
> But I think it's working as expected:
> 
> - on 64-bit kernels, pos/IP can be up to 64-bit;
> - on 32-bit kernels, pos/IP can only be up to 32-bit.
> 
> User space necessarily has to know about the bit-ness of its kernel,
> because the coverage information is entirely dependent on the kernel
> image. I think the examples in documentation weren't exhaustive in
> this regard. At least that's my take -- Dmitry or Andrey would know
> for sure (Dmitry is currently on vacation, but hopefully can clarify
> next week).

okay.

> Thanks,
> -- Marco

Sebastian