Re: [PATCH v3 00/12] vfs: change inode->i_ino from unsigned long to u64

[Mimi Zohar <zohar@linux.ibm.com>]

On Mon, 2026-03-09 at 15:33 -0400, Jeff Layton wrote:
> On Mon, 2026-03-09 at 15:00 -0400, Mimi Zohar wrote:
> > On Mon, 2026-03-09 at 13:59 -0400, Jeff Layton wrote:
> > > On Mon, 2026-03-09 at 13:47 -0400, Mimi Zohar wrote:
> > > > [ I/O socket time out.  Trimming the To list.]
> > > > 
> > > > On Wed, 2026-03-04 at 10:32 -0500, Jeff Layton wrote:
> > > > > This version squashes all of the format-string changes and the i_ino
> > > > > type change into the same patch. This results in a giant 600+ line patch
> > > > > at the end of the series, but it does remain bisectable.  Because the
> > > > > patchset was reorganized (again) some of the R-b's and A-b's have been
> > > > > dropped.
> > > > > 
> > > > > The entire pile is in the "iino-u64" branch of my tree, if anyone is
> > > > > interested in testing this.
> > > > > 
> > > > >     https://git.kernel.org/pub/scm/linux/kernel/git/jlayton/linux.git/
> > > > > 
> > > > > Original cover letter follows:
> > > > > 
> > > > > ----------------------8<-----------------------
> > > > > 
> > > > > Christian said [1] to "just do it" when I proposed this, so here we are!
> > > > > 
> > > > > For historical reasons, the inode->i_ino field is an unsigned long,
> > > > > which means that it's 32 bits on 32 bit architectures. This has caused a
> > > > > number of filesystems to implement hacks to hash a 64-bit identifier
> > > > > into a 32-bit field, and deprives us of a universal identifier field for
> > > > > an inode.
> > > > > 
> > > > > This patchset changes the inode->i_ino field from an unsigned long to a
> > > > > u64. This shouldn't make any material difference on 64-bit hosts, but
> > > > > 32-bit hosts will see struct inode grow by at least 4 bytes. This could
> > > > > have effects on slabcache sizes and field alignment.
> > > > > 
> > > > > The bulk of the changes are to format strings and tracepoints, since the
> > > > > kernel itself doesn't care that much about the i_ino field. The first
> > > > > patch changes some vfs function arguments, so check that one out
> > > > > carefully.
> > > > > 
> > > > > With this change, we may be able to shrink some inode structures. For
> > > > > instance, struct nfs_inode has a fileid field that holds the 64-bit
> > > > > inode number. With this set of changes, that field could be eliminated.
> > > > > I'd rather leave that sort of cleanups for later just to keep this
> > > > > simple.
> > > > > 
> > > > > Much of this set was generated by LLM, but I attributed it to myself
> > > > > since I consider this to be in the "menial tasks" category of LLM usage.
> > > > > 
> > > > > [1]: https://lore.kernel.org/linux-fsdevel/20260219-portrait-winkt-959070cee42f@brauner/
> > > > > 
> > > > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > > 
> > > > Jeff, missing from this patch set is EVM.  In hmac_add_misc() EVM copies the
> > > > i_ino and calculates either an HMAC or file meta-data hash, which is then
> > > > signed. 
> > > > 
> > > > 
> > > 
> > > Thanks Mimi, good catch.
> > > 
> > > It looks like we should just be able to change the ino field to a u64
> > > alongside everything else. Something like this:
> > > 
> > > diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
> > > index c0ca4eedb0fe..77b6c2fa345e 100644
> > > --- a/security/integrity/evm/evm_crypto.c
> > > +++ b/security/integrity/evm/evm_crypto.c
> > > @@ -144,7 +144,7 @@ static void hmac_add_misc(struct shash_desc *desc, struct inode *inode,
> > >                           char type, char *digest)
> > >  {
> > >         struct h_misc {
> > > -               unsigned long ino;
> > > +               u64 ino;
> > >                 __u32 generation;
> > >                 uid_t uid;
> > >                 gid_t gid;
> > > 
> > 
> > Agreed.
> > 
> > > 
> > > That should make no material difference on 64-bit hosts. What's the
> > > effect on 32-bit? Will they just need to remeasure everything or would
> > > the consequences be more dire? Do we have any clue whether anyone is
> > > using EVM in 32-bit environments?
> > 
> > All good questions. Unfortunately I don't know the answer to most of them. What
> > we do know: changing the size of the i_ino field would affect EVM file metadata
> > verification and would require relabeling the filesystem.  Even packages
> > containing EVM portable signatures, which don't include or verify the i_ino
> > number, would be affected.
> > 
> 
> Ouch. Technically, I guess this is ABI...
> 
> While converting to u64 seems like the ideal thing to do, the other
> option might be to just keep this as an unsigned long for now.
> 
> No effect on 64-bit, but that could keep things working 32-bit when the
> i_ino casts properly to a u32. ext4 would be fine since they don't
> issue inode numbers larger than UINT_MAX. xfs and btrfs are a bit more
> iffy, but worst case they'd just need to be relabeled (which is what
> they'll need to do anyway).
> 
> If we do that, then we should probably add a comment to this function
> explaining why it's an unsigned long.

Agreed.

> 
> Thoughts?

My concern would be embedded/IoT devices, but I don't have any insight into who
might be using it on 32 bit.

Mimi