Re: [RFC PATCH 4/5] sysfs: Introduce a mechanism to hide static attribute_groups

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jan 30, 2024 at 01:24:08AM -0800, Dan Williams wrote:
> Add a mechanism for named attribute_groups to hide their directory at
> sysfs_update_group() time, or otherwise skip emitting the group
> directory when the group is first registered. It piggybacks on
> is_visible() in a similar manner as SYSFS_PREALLOC, i.e. special flags
> in the upper bits of the returned mode. To use it, specify a symbol
> prefix to DEFINE_SYSFS_GROUP_VISIBLE(), and then pass that same prefix
> to SYSFS_GROUP_VISIBLE() when assigning the @is_visible() callback:
> 
> 	DEFINE_SYSFS_GROUP_VISIBLE($prefix)
> 
> 	struct attribute_group $prefix_group = {
> 		.name = $name,
> 		.is_visible = SYSFS_GROUP_VISIBLE($prefix),
> 	};
> 
> SYSFS_GROUP_VISIBLE() expects a definition of $prefix_group_visible()
> and $prefix_attr_visible(), where $prefix_group_visible() just returns
> true / false and $prefix_attr_visible() behaves as normal.
> 
> The motivation for this capability is to centralize PCI device
> authentication in the PCI core with a named sysfs group while keeping
> that group hidden for devices and platforms that do not meet the
> requirements. In a PCI topology, most devices will not support
> authentication, a small subset will support just PCI CMA (Component
> Measurement and Authentication), a smaller subset will support PCI CMA +
> PCIe IDE (Link Integrity and Encryption), and only next generation
> server hosts will start to include a platform TSM (TEE Security
> Manager).
> 
> Without this capability the alternatives are:
> 
> * Check if all attributes are invisible and if so, hide the directory.
>   Beyond trouble getting this to work [1], this is an ABI change for
>   scenarios if userspace happens to depend on group visibility absent any
>   attributes. I.e. this new capability avoids regression since it does
>   not retroactively apply to existing cases.
> 
> * Publish an empty /sys/bus/pci/devices/$pdev/tsm/ directory for all PCI
>   devices (i.e. for the case when TSM platform support is present, but
>   device support is absent). Unfortunate that this will be a vestigial
>   empty directory in the vast majority of cases.
> 
> * Reintroduce usage of runtime calls to sysfs_{create,remove}_group()
>   in the PCI core. Bjorn has already indicated that he does not want to
>   see any growth of pci_sysfs_init() [2].
> 
> * Drop the named group and simulate a directory by prefixing all
>   TSM-related attributes with "tsm_". Unfortunate to not use the naming
>   capability of a sysfs group as intended.
> 
> In comparison, there is a small potential for regression if for some
> reason an @is_visible() callback had dependencies on how many times it
> was called. Additionally, it is no longer an error to update a group
> that does not have its directory already present, and it is no longer a
> WARN() to remove a group that was never visible.
> 
> Link: https://lore.kernel.org/all/2024012321-envious-procedure-4a58@gregkh/ [1]
> Link: https://lore.kernel.org/linux-pci/20231019200110.GA1410324@bhelgaas/ [2]
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
>  fs/sysfs/group.c      |   45 ++++++++++++++++++++++++++++-------
>  include/linux/sysfs.h |   63 ++++++++++++++++++++++++++++++++++++++++---------
>  2 files changed, 87 insertions(+), 21 deletions(-)

You beat me to this again :)

I have tested this patch, and it looks good, I'll send out my series
that uses it for a different subsystem as well.

I guess I can take this as a static tag for others to pull from for this
rc development cycle?

thanks,

greg k-h