linux-kernel - Re: [Patch v2 2/3] firmware: dmi_scan: add SBMIOS entry and DMI tables

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Date:	Tue, 21 Apr 2015 16:24:27 +0200
From:	Jean Delvare <jdelvare@...e.de>
To:	Ivan Khoronzhuk <ivan.khoronzhuk@...ballogic.com>
Cc:	linux-kernel@...r.kernel.org, matt.fleming@...el.com,
	ard.biesheuvel@...aro.org, grant.likely@...aro.org,
	linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
	mikew@...gle.com, dmidecode-devel@...gnu.org,
	leif.lindholm@...aro.org, msalter@...hat.com, roy.franz@...aro.org
Subject: Re: [Patch v2 2/3] firmware: dmi_scan: add SBMIOS entry and DMI
 tables

Hi Ivan,

On Mon, 20 Apr 2015 13:19:46 +0300, Ivan Khoronzhuk wrote:
> Some utils, like dmidecode and smbios, need to access SMBIOS entry
> table area in order to get information like SMBIOS version, size, etc.
> Currently it's done via /dev/mem. But for situation when /dev/mem
> usage is disabled, the utils have to use dmi sysfs instead, which
> doesn't represent SMBIOS entry and adds code/delay redundancy when direct
> access for table is needed.
> 
> So this patch creates dmi/tables and adds SMBIOS entry point to allow
> utils in question to work correctly without /dev/mem. Also patch adds
> raw dmi table to simplify dmi table processing in user space, as
> proposed by Jean Delvare.
> 
> Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@...ballogic.com>
> ---
>  .../ABI/testing/sysfs-firmware-dmi-tables          | 22 ++++++
>  drivers/firmware/dmi-sysfs.c                       | 17 +++--
>  drivers/firmware/dmi_scan.c                        | 82 ++++++++++++++++++++++
>  include/linux/dmi.h                                |  2 +
>  4 files changed, 114 insertions(+), 9 deletions(-)
>  create mode 100644 Documentation/ABI/testing/sysfs-firmware-dmi-tables
> (...)
> +static int __init dmi_init(void)
> +{
> +	int ret;
> +	u8 *dmi_table = NULL;
> +	struct kobject *tables_kobj = NULL;
> +
> +	if (!dmi_available) {
> +		ret = -ENODATA;
> +		goto err;
> +	}
> +
> +	/*
> +	 * Set up dmi directory at /sys/firmware/dmi. This entry should stay
> +	 * even after farther error, as it can be used by other modules like
> +	 * dmi-sysfs.
> +	 */
> +	dmi_kobj = kobject_create_and_add("dmi", firmware_kobj);
> +	tables_kobj = kobject_create_and_add("tables", dmi_kobj);

I'm afraid you can't do that. kobject_create_and_add() doesn't check if
the parent is NULL and will happily create "tables" at the root of
sysfs if for any reason the previous call to kobject_create_and_add()
failed and returned NULL. I agree it is unlikely and would be cleaned
up immediately, but still, instantiating an object at the wrong place,
even temporarily, is wrong.

> +	if (!(dmi_kobj && tables_kobj)) {
> +		ret = -ENOMEM;
> +		goto err;
> +	}

I'd rather go with:

	if (!(dmi_kobj = kobject_create_and_add("dmi", firmware_kobj))
	 || !(tables_kobj = kobject_create_and_add("tables", dmi_kobj))) {

I know that checkpatch complains about this construct, but in many
cases it is the right thing to do.

Another possible approach is simply:

	dmi_kobj = kobject_create_and_add("dmi", firmware_kobj);
	if (dmi_kobj)
		tables_kobj = kobject_create_and_add("tables", dmi_kobj);
	if (!tables_kobj) {

> +
> +	bin_attr_smbios_entry_point.size = smbios_entry_point_size;
> +	bin_attr_smbios_entry_point.private = smbios_entry_point;
> +	ret = sysfs_create_bin_file(tables_kobj, &bin_attr_smbios_entry_point);
> +	if (ret)
> +		goto err;
> +
> +	dmi_table = dmi_remap(dmi_base, dmi_len);
> +	if (!dmi_table) {
> +		ret = -ENOMEM;
> +		goto err;
> +	}
> +
> +	bin_attr_DMI.size = dmi_len;
> +	bin_attr_DMI.private = dmi_table;
> +	ret = sysfs_create_bin_file(tables_kobj, &bin_attr_DMI);
> +	if (!ret)
> +		return 0;
> +
> +err:
> +	pr_err("dmi: Firmware registration failed.\n");
> +
> +	if (tables_kobj) {
> +		sysfs_remove_bin_file(tables_kobj,
> +				      &bin_attr_smbios_entry_point);
> +		kobject_del(tables_kobj);
> +		kobject_put(tables_kobj);
> +	}
> +
> +	if (dmi_table)
> +		dmi_unmap(dmi_table);

I'm not happy with this single error label. This forces you to
initialize all your pointers to NULL and then to check for them to find
out what needs to be done in the error path. With multiple error labels,
you would know exactly what needs to be done, this is more efficient.

> +
> +	return ret;
> +}

Everything else looks good. No need to resend, I'll fix up the above
myself in place or as an incremental patch.

Thanks,
-- 
Jean Delvare
SUSE L3 Support
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