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Date:   Wed, 7 Dec 2022 15:56:08 -0600
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Stephen Kitt <steve@sk2.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: <20221207215608.GA1471870@bhelgaas>
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[+cc Jan]

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=off (this was reported in 2016, but the relevant code hasn't
> changed since then).

Thanks, Stephen!

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:

  https://bugzilla.kernel.org/show_bug.cgi?id=187731
  https://lore.kernel.org/all/4cec62c2-218a-672b-8c12-d44e8df56aae@comsys.rwth-aachen.de/#t

And Jan's patch is almost identical to yours :)

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.

Bjorn

> This moves the call to pcie_aspm_sanity_check() just before the result
> is actually used, giving all the other checks a chance to run first.
> 
> Signed-off-by: Stephen Kitt <steve@sk2.org>
> ---
>  drivers/pci/pcie/aspm.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index a8aec190986c..38df439568b7 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -889,7 +889,7 @@ static void pcie_aspm_update_sysfs_visibility(struct pci_dev *pdev)
>  void pcie_aspm_init_link_state(struct pci_dev *pdev)
>  {
>  	struct pcie_link_state *link;
> -	int blacklist = !!pcie_aspm_sanity_check(pdev);
> +	int blacklist;
>  
>  	if (!aspm_support_enabled)
>  		return;
> @@ -923,6 +923,7 @@ void pcie_aspm_init_link_state(struct pci_dev *pdev)
>  	 * upstream links also because capable state of them can be
>  	 * update through pcie_aspm_cap_init().
>  	 */
> +	blacklist = !!pcie_aspm_sanity_check(pdev);
>  	pcie_aspm_cap_init(link, blacklist);
>  
>  	/* Setup initial Clock PM state */
> 
> base-commit: 833477fce7a14d43ae4c07f8ddc32fa5119471a2
> -- 
> 2.30.2
>