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Date:   Thu, 8 Dec 2022 09:00:17 +0100
From:   Stephen Kitt <steve@sk2.org>
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     linux-kernel@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>,
        linux-pci@vger.kernel.org, Jan Rueth <rueth@comsys.rwth-aachen.de>
Subject: Re: [PATCH] PCI/ASPM: Call pcie_aspm_sanity_check() as late as
 possible
Message-ID: <20221208090017.132db7bd@heffalump.sk2.org>
In-Reply-To: <20221207215608.GA1471870@bhelgaas>
References: <20221006115950.821736-1-steve@sk2.org>
        <20221207215608.GA1471870@bhelgaas>
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Hi Bjorn,

On Wed, 7 Dec 2022 15:56:08 -0600, Bjorn Helgaas <helgaas@kernel.org> wrote:
> On Thu, Oct 06, 2022 at 01:59:50PM +0200, Stephen Kitt wrote:
> > In pcie_aspm_init_link_state(), a number of checks are made to
> > determine whether the function should proceed, before the result of
> > the call to pcie_aspm_sanity_check() is actually used. The latter
> > function doesn't change any state, it only reports a result, so
> > calling it later doesn't make any difference to the state of the
> > devices or the information we have about them. But having the call
> > early reportedly can cause null-pointer dereferences; see
> > https://unix.stackexchange.com/q/322337 for one example with
> > pcie_aspm=3Doff (this was reported in 2016, but the relevant code hasn't
> > changed since then). =20
>=20
> Thanks, Stephen!
>=20
> That stackexchange report doesn't have much information, but it looks
> similar to this old report from Jan Rueth, which I'm sorry to say I
> never got resolved:
>=20
>   https://bugzilla.kernel.org/show_bug.cgi?id=3D187731
>   https://lore.kernel.org/all/4cec62c2-218a-672b-8c12-d44e8df56aae@comsys=
.rwth-aachen.de/#t
>=20
> And Jan's patch is almost identical to yours :)
>=20
> I hope to get this resolved, but I don't have time to work on it
> before the upcoming merge window, which will probably open Sunday.
> And then it's holiday time, so it may be January before I get back to
> it.  I'm just dropping the links here as breadcrumbs for picking this
> back up.

Thanks for the update! I was somewhat bemused by the dereference here, I=E2=
=80=99m
reassured to see I=E2=80=99m not the only one. Unfortunately I don=E2=80=99=
t have hardware
which exhibits this problem, I submitted the patch because it seemed
reasonably sensible even though as you say there is probably something else
going on here. Of course if this approach is useful, Jan=E2=80=99s patch sh=
ould go in
rather than mine.

Anyway, it=E2=80=99s been six years, so a few more weeks won=E2=80=99t make=
 any difference
;-).

Enjoy the holiday season!

Regards,

Stephen

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