[PATCH 7.2.y] nfsd: fix all-zero stateid in CB_OFFLOAD for async COPY

[Lubos Rendek]

[ There is no upstream commit: this fixes the 7.2.y backport of
  d0beaee498e1. Mainline fixed the problem as part of the async COPY
  rework in commit f307b4f7ed17 ("nfsd: split nfsd4_copy into transient
  and durable async copy objects"), which is too risky to backport. ]

Since commit 14b978e8d05c ("nfsd: fix stale s2s_cp_stateids IDR entry
for async COPY"), nfsd4_copy() calls dup_copy_fields() before the copy
stateid is created and then writes the new stateid only into the COPY
result. The async copy's own cp_res.cb_stateid stays zero, so
nfsd4_send_cb_offload() sends CB_OFFLOAD with an all-zero stateid. The
client cannot match it to its pending COPY and waits forever in
nfs42_proc_copy().

Also copy the new stateid into async_copy->cp_res.cb_stateid. Calling
dup_copy_fields() after nfs4_init_copy_state() instead is not an
option, because dup_copy_fields() also copies cp_stateid and would
overwrite the stateid that the backport moved to the async copy.

Tested on 7.2.10 with an NFSv4.2 server-side COPY between two
exports: with this change the copy completes and CB_OFFLOAD carries
the same stateid as the COPY reply. Without it, cp(1) hangs on 7.2.4
and 7.2.9; the affected code in nfsd4_copy() is the same in 7.2.10.

Fixes: 14b978e8d05c ("nfsd: fix stale s2s_cp_stateids IDR entry for async COPY")
Closes: https://lore.kernel.org/stable/7cc824c4-632d-49df-ae5e-cfcbb5bf76ad@xxxxxxxxxxx/
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Lubos Rendek <l.rendek@xxxxxxxxxxx>
---
 fs/nfsd/nfs4proc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index 939749709..7c2955796 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -2243,6 +2243,8 @@ nfsd4_copy(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 			goto out_dec_async_copy_err;
 		memcpy(&result->cb_stateid, &async_copy->cp_stateid.cs_stid,
 			sizeof(result->cb_stateid));
+		memcpy(&async_copy->cp_res.cb_stateid, &async_copy->cp_stateid.cs_stid,
+		       sizeof(async_copy->cp_res.cb_stateid));
 		if ((READ_ONCE(copy->nf_dst->nf_file->f_mode) &
 			       FMODE_NOCMTIME) != 0)
 			async_copy->attr_update = true;
-- 
2.47.3