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Subject: Re: [PATCH v7 1/2] iommu - Enable debugfs exposure of IOMMU driver
 internals
To:     Greg KH <greg@kroah.com>
Cc:     iommu@lists.linux-foundation.org, joro@8bytes.org,
        linux-kernel@vger.kernel.org
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 <20180524091807.GA18148@kroah.com>
From:   Gary R Hook <gary.hook@amd.com>
Message-ID: <68d7f040-fc7a-5eb3-342f-687fc6e591f2@amd.com>
Date:   Tue, 5 Jun 2018 11:48:44 -0500
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On 05/24/2018 04:18 AM, Greg KH wrote:
> On Mon, May 14, 2018 at 12:20:20PM -0500, Gary R Hook wrote:
>> Provide base enablement for using debugfs to expose internal data of an
>> IOMMU driver. When called, create the /sys/kernel/debug/iommu directory.
>>
>> Emit a strong warning at boot time to indicate that this feature is
>> enabled.
>>
>> This function is called from iommu_init, and creates the initial DebugFS
>> directory. Drivers may then call iommu_debugfs_new_driver_dir() to
>> instantiate a device-specific directory to expose internal data.
>> It will return a pointer to the new dentry structure created in
>> /sys/kernel/debug/iommu, or NULL in the event of a failure.
>>
>> Since the IOMMU driver can not be removed from the running system, there
>> is no need for an "off" function.
>>
>> Signed-off-by: Gary R Hook <gary.hook@amd.com>
>> ---
>>   drivers/iommu/Kconfig         |   10 ++++++
>>   drivers/iommu/Makefile        |    1 +
>>   drivers/iommu/iommu-debugfs.c |   72 +++++++++++++++++++++++++++++++++++++++++
>>   drivers/iommu/iommu.c         |    2 +
>>   include/linux/iommu.h         |   11 ++++++
>>   5 files changed, 96 insertions(+)
>>   create mode 100644 drivers/iommu/iommu-debugfs.c
>>
>> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
>> index f3a21343e636..2eab6a849a0a 100644
>> --- a/drivers/iommu/Kconfig
>> +++ b/drivers/iommu/Kconfig
>> @@ -60,6 +60,16 @@ config IOMMU_IO_PGTABLE_ARMV7S_SELFTEST
>>   
>>   endmenu
>>   
>> +config IOMMU_DEBUGFS
>> +	bool "Export IOMMU internals in DebugFS"
>> +	depends on DEBUG_FS
>> +	help
>> +	  Allows exposure of IOMMU device internals. This option enables
>> +	  the use of debugfs by IOMMU drivers as required. Devices can,
>> +	  at initialization time, cause the IOMMU code to create a top-level
>> +	  debug/iommu directory, and then populate a subdirectory with
>> +	  entries as required.
> 
> You need a big "DO NOT ENABLE THIS OPTION UNLESS YOU REALLY REALLY KNOW
> WHAT YOU ARE DOING!!!" line here.  To match up with your crazy kernel
> log warning.
> 
> Otherwise distros will turn this on, I guarantee it.

Apologies for not seeing this. Notes from Greg have been going to junk 
mail >:-(

I will add this text to the Kconfig item.

> 
>> +
>>   config IOMMU_IOVA
>>   	tristate
>>   
>> diff --git a/drivers/iommu/Makefile b/drivers/iommu/Makefile
>> index 1fb695854809..74cfbc392862 100644
>> --- a/drivers/iommu/Makefile
>> +++ b/drivers/iommu/Makefile
>> @@ -2,6 +2,7 @@
>>   obj-$(CONFIG_IOMMU_API) += iommu.o
>>   obj-$(CONFIG_IOMMU_API) += iommu-traces.o
>>   obj-$(CONFIG_IOMMU_API) += iommu-sysfs.o
>> +obj-$(CONFIG_IOMMU_DEBUGFS) += iommu-debugfs.o
>>   obj-$(CONFIG_IOMMU_DMA) += dma-iommu.o
>>   obj-$(CONFIG_IOMMU_IO_PGTABLE) += io-pgtable.o
>>   obj-$(CONFIG_IOMMU_IO_PGTABLE_ARMV7S) += io-pgtable-arm-v7s.o
>> diff --git a/drivers/iommu/iommu-debugfs.c b/drivers/iommu/iommu-debugfs.c
>> new file mode 100644
>> index 000000000000..bb1bf2d0ac51
>> --- /dev/null
>> +++ b/drivers/iommu/iommu-debugfs.c
>> @@ -0,0 +1,72 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * IOMMU debugfs core infrastructure
>> + *
>> + * Copyright (C) 2018 Advanced Micro Devices, Inc.
>> + *
>> + * Author: Gary R Hook <gary.hook@amd.com>
>> + */
>> +
>> +#include <linux/pci.h>
>> +#include <linux/iommu.h>
>> +#include <linux/debugfs.h>
>> +
>> +static struct dentry *iommu_debugfs_dir;
>> +
>> +/**
>> + * iommu_debugfs_setup - create the top-level iommu directory in debugfs
>> + *
>> + * Provide base enablement for using debugfs to expose internal data of an
>> + * IOMMU driver. When called, this function creates the
>> + * /sys/kernel/debug/iommu directory.
>> + *
>> + * Emit a strong warning at boot time to indicate that this feature is
>> + * enabled.
>> + *
>> + * This function is called from iommu_init; drivers may then call
>> + * iommu_debugfs_new_driver_dir() to instantiate a vendor-specific
>> + * directory to be used to expose internal data.
>> + */
>> +void iommu_debugfs_setup(void)
>> +{
>> +	if (!iommu_debugfs_dir) {
>> +		iommu_debugfs_dir = debugfs_create_dir("iommu", NULL);
>> +		if (iommu_debugfs_dir) {
> 
> No need to care about the return value, just keep going.  Nothing you
> should, or should not, do depending on the return value of a debugfs
> call.

Sorry. Some habits are hard to break. It just seemed that if there's no 
iommu debugfs directory, nothing else needed to be done. But plowing 
ahead works for me. Thank you.

> 
>> +			pr_warn("\n");
>> +			pr_warn("*************************************************************\n");
>> +			pr_warn("**     NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE    **\n");
>> +			pr_warn("**                                                         **\n");
>> +			pr_warn("**  IOMMU DebugFS SUPPORT HAS BEEN ENABLED IN THIS KERNEL  **\n");
>> +			pr_warn("**                                                         **\n");
>> +			pr_warn("** This means that this kernel is built to expose internal **\n");
>> +			pr_warn("** IOMMU data structures, which may compromise security on **\n");
>> +			pr_warn("** your system.                                            **\n");
>> +			pr_warn("**                                                         **\n");
>> +			pr_warn("** If you see this message and you are not debugging the   **\n");
>> +			pr_warn("** kernel, report this immediately to your vendor!         **\n");
>> +			pr_warn("**                                                         **\n");
>> +			pr_warn("**     NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE    **\n");
>> +			pr_warn("*************************************************************\n");
>> +		}
>> +	}
>> +}
>> +
>> +/**
>> + * iommu_debugfs_new_driver_dir - create a vendor directory under debugfs/iommu
>> + * @vendor: name of the vendor-specific subdirectory to create
>> + *
>> + * This function is called by an IOMMU driver to create the top-level debugfs
>> + * directory for that driver.
>> + *
>> + * Return: upon success, a pointer to the dentry for the new directory.
>> + *         NULL in case of failure.
>> + */
>> +struct dentry *iommu_debugfs_new_driver_dir(const char *vendor)
>> +{
>> +	struct dentry *d_new;
>> +
>> +	d_new = debugfs_create_dir(vendor, iommu_debugfs_dir);
>> +
>> +	return d_new;
> 
> This function can be reduced to 1 line.  But really, why do you need it
> at all?

My intention was to not expose the base dentry. Which means that drivers 
need to call in to this file to create a vendor-specific subdirectory. 
At least, that was my take-away from the discussion. But you dislike 
this approach to the code, so I'm open to suggestions. At the very 
least, it's now a one liner.