[RFC PATCH v1 1/7] fs: Add inode_get_ino() and implement get_ino() for NFS

[Paul Moore]

On Thu, Oct 17, 2024 at 1:05 PM Jeff Layton <jlayton at kernel.org> wrote:
> On Thu, 2024-10-17 at 11:15 -0400, Paul Moore wrote:
> > On Thu, Oct 17, 2024 at 10:58 AM Christoph Hellwig <hch at infradead.org> wrote:
> > > On Thu, Oct 17, 2024 at 10:54:12AM -0400, Paul Moore wrote:
> > > > Okay, good to know, but I was hoping that there we could come up with
> > > > an explicit list of filesystems that maintain their own private inode
> > > > numbers outside of inode-i_ino.
> > >
> > > Anything using iget5_locked is a good start.  Add to that file systems
> > > implementing their own inode cache (at least xfs and bcachefs).
> >
> > Also good to know, thanks.  However, at this point the lack of a clear
> > answer is making me wonder a bit more about inode numbers in the view
> > of VFS developers; do you folks care about inode numbers?  I'm not
> > asking to start an argument, it's a genuine question so I can get a
> > better understanding about the durability and sustainability of
> > inode->i_no.  If all of you (the VFS folks) aren't concerned about
> > inode numbers, I suspect we are going to have similar issues in the
> > future and we (the LSM folks) likely need to move away from reporting
> > inode numbers as they aren't reliably maintained by the VFS layer.
> >
>
> Like Christoph said, the kernel doesn't care much about inode numbers.
>
> People care about them though, and sometimes we have things in the
> kernel that report them in some fashion (tracepoints, procfiles, audit
> events, etc.). Having those match what the userland stat() st_ino field
> tells you is ideal, and for the most part that's the way it works.
>
> The main exception is when people use 32-bit interfaces (somewhat rare
> these days), or they have a 32-bit kernel with a filesystem that has a
> 64-bit inode number space (NFS being one of those). The NFS client has
> basically hacked around this for years by tracking its own fileid field
> in its inode.

When I asked if the VFS dev cared about inode numbers this is more of
what I was wondering about.  Regardless of if the kernel itself uses
inode numbers for anything, it does appear that users do care about
inode numbers to some extent, and I wanted to know if the VFS devs
viewed the inode numbers as a first order UAPI interface/thing, or if
it was of lesser importance and not something the kernel was going to
provide much of a guarantee around.  Once again, I'm not asking this
to start a war, I'm just trying to get some perspective from the VFS
dev side of things.

> A lot of the changes can probably be automated via coccinelle. I'd
> probably start by turning all of the direct i_ino accesses into static
> inline wrapper function calls. The hard part will be parceling out that
> work into digestable chunks. If you can avoid "flag day" changes, then
> that's ideal.  You'd want a patch per subsystem so you can collect
> ACKs.
>
> The hardest part will probably be the format string changes. I'm not
> sure you can easily use coccinelle for that, so that may need to be
> done by hand or scripted with python or something.

Out of curiosity, is this on anyone's roadmap?  I've already got
enough work in security land to keep myself occupied until I'm hit by
that mythical bus, so I can't volunteer in good conscience, but I (and
many others in security land) would be grateful for a single,
consistent way to fetch inode numbers :)

-- 
paul-moore.com