linux-kernel - Re: [PATCH] PCI/ACPI: do not reference a pci device after it has been released

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Message-ID: <20220428155858.GA14614@bhelgaas>
Date:   Thu, 28 Apr 2022 10:58:58 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     bhelgaas@...gle.com, rafael@...nel.org,
        linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
        linux-pci@...r.kernel.org, linux-acpi@...r.kernel.org,
        whitehat002 <hackyzh002@...il.com>
Subject: Re: [PATCH] PCI/ACPI: do not reference a pci device after it has
 been released

On Thu, Apr 28, 2022 at 04:28:53PM +0200, Greg Kroah-Hartman wrote:
> In acpi_get_pci_dev(), the debugging message for when a PCI bridge is
> not found uses a pointer to a pci device whose reference has just been
> dropped.  The chance that this really is a device that is now been
> removed from the system is almost impossible to happen, but to be safe,
> let's print out the debugging message based on the acpi root device
> which we do have a valid reference to at the moment.

This code was added by 497fb54f578e ("ACPI / PCI: Fix NULL pointer
dereference in acpi_get_pci_dev() (rev. 2)").  Not sure if it's worth
a Fixes: tag.

acpi_get_pci_dev() is used by only five callers, three of which are
video/backlight related.  I'm always skeptical of one-off interfaces
like this, but I don't know enough to propose any refactoring or other
alternatives.

I'll leave this for Rafael, but if I were applying I would silently
touch up the subject to match convention:

  PCI/ACPI: Do not reference PCI device after it has been released

> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Len Brown <lenb@...nel.org>
> Cc: linux-pci@...r.kernel.org
> Cc: linux-acpi@...r.kernel.org
> Reported-by: whitehat002 <hackyzh002@...il.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> ---
>  drivers/acpi/pci_root.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/acpi/pci_root.c b/drivers/acpi/pci_root.c
> index 6f9e75d14808..ecda378dbc09 100644
> --- a/drivers/acpi/pci_root.c
> +++ b/drivers/acpi/pci_root.c
> @@ -303,7 +303,8 @@ struct pci_dev *acpi_get_pci_dev(acpi_handle handle)
>  		 * case pdev->subordinate will be NULL for the parent.
>  		 */
>  		if (!pbus) {
> -			dev_dbg(&pdev->dev, "Not a PCI-to-PCI bridge\n");
> +			dev_dbg(&root->device->dev,
> +				"dev %d, function %d is not a PCI-to-PCI bridge\n", dev, fn);

This should use "%02x.%d" to be consistent with the dev_set_name() in
pci_setup_device().

>  			pdev = NULL;
>  			break;
>  		}
> -- 
> 2.36.0
>