[PATCH v3 0/3] fs/nfsd: accept a backdated timestamp from a delegation holder

[Jeff Layton <jlayton@kernel.org>]

This version fixes a couple of issues that Sashiko's review pointed out.

The main problem is that an attribute delegation has strict rules about
updates can be done, and that prevents applications that backdate
timestamps on files after writing them, (e.g. tar -x, cp -p, etc.) from
setting them properly.

We could fix this on the client by making it always issue an RPC for the
SETATTR, but that would harm performance in these common use-cases.
This set allows the server to accept some of these backdated mtime
updates, by changing how the ctime is handled.

It'd be nice to see this in v7.4.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
Changes in v3:
- Compare after the clamp and truncate in inode_set_ctime_deleg(), not
  before, so a clamped update stamps too
- Compare a SETATTR report against the inode before clamping, so an
  untouched file with a future time keeps it
- Set dl_setattr only once nfsd_setattr() has applied the update, and
  hold the stateid reference until then
- Note the SETATTR/DELEGRETURN race on dl_setattr in a comment
- Link to v2: https://lore.kernel.org/r/20260902-delegts-v2-0-383cb289ce88@kernel.org

Changes in v2:
- Split the change into three patches
- Stamp now in inode_set_ctime_deleg() for a stale update, so nfsd need
  not compare
- Drop a SETATTR report that matches the inode, so an untouched file
  keeps its change attribute
- Set dl_setattr only on the branch that carries a c/mtime update
- Compare against the inode, not the grant-time snapshots (Chuck)
- Set the ctime whenever a CB_GETATTR moves the mtime
- Take i_rwsem before the CB_GETATTR comparison
- Drop the unused dl_atime, dl_mtime and dl_ctime, and make
  nfsd4_vet_deleg_time() static
- Link to v1: https://lore.kernel.org/r/20260901-delegts-v1-1-9937f7ee4370@kernel.org

---
Jeff Layton (3):
      fs: stamp the current time for a stale delegated ctime update
      nfsd: accept a backdated timestamp from a delegation holder
      nfsd: compare CB_GETATTR times against the inode

 fs/inode.c          | 25 +++++++++------
 fs/nfsd/nfs4proc.c  | 92 +++++++++++++++++++++++++++++++++++++++++------------
 fs/nfsd/nfs4state.c | 49 +++++++++++++++++-----------
 fs/nfsd/state.h     |  8 -----
 4 files changed, 119 insertions(+), 55 deletions(-)
---
base-commit: ac579868af0c900d8fd9c8cfe9e10db46ccc5a75
change-id: 20260901-delegts-e91a648f1427

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>