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

[Dan Williams <dan.j.williams@intel.com>]

Greg KH wrote:
> 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 :)

Pardon the spam, was just showing it in context of the patchset I was
developing.

> 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?

That works for me. Thanks Greg!