From 250ec14932d5cfe102f68a57892bb566eee7f83e Mon Sep 17 00:00:00 2001 From: Chris Mason Date: Sat, 30 May 2026 09:19:23 -0400 Subject: [PATCH] nfsd: fix partial-write detection in nfsd_direct_write nfsd_direct_write() walks a list of write segments and, after each vfs_iocb_iter_write(), tries to detect a short write so the loop can stop before placing the next segment at a wrong file offset: host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter); if (host_err < 0) return host_err; *cnt += host_err; if (host_err < segments[i].iter.count) break; /* partial write */ vfs_iocb_iter_write() runs the iter through ->write_iter(), which advances the iter by the number of bytes written. By the time the check runs, segments[i].iter.count is the residual, not the original request length: before write_iter: iter.count == original_len after write_iter: iter.count == original_len - host_err The condition then reduces to host_err < original_len - host_err, so the break fires only when less than half of the segment was written. Any short write completing between 50% and 99% of the segment slips through; the loop advances to the next segment with kiocb->ki_pos only bumped by the short amount, writing the next segment's payload at the wrong offset and over-reporting *cnt to the NFS client. Snapshot the segment's byte count before the write and compare host_err against that snapshot so any short write breaks the loop. Fixes: 06c5c97293e3 ("NFSD: Implement NFSD_IO_DIRECT for NFS WRITE") Cc: stable@vger.kernel.org Assisted-by: kres:claude-opus-4-7 Reported-by: Chris Mason Signed-off-by: Chris Mason Link: https://patch.msgid.link/20260530-nfsd-fixes-v2-7-f27e8eb4d974@kernel.org Signed-off-by: Chuck Lever --- fs/nfsd/vfs.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c index a3940548136f..0dcfaab95021 100644 --- a/fs/nfsd/vfs.c +++ b/fs/nfsd/vfs.c @@ -1380,6 +1380,7 @@ nfsd_direct_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct file *file = nf->nf_file; unsigned int nsegs, i; ssize_t host_err; + size_t expected; nsegs = nfsd_write_dio_iters_init(nf, rqstp->rq_bvec, nvecs, kiocb, *cnt, segments); @@ -1401,11 +1402,13 @@ nfsd_direct_write(struct svc_rqst *rqstp, struct svc_fh *fhp, kiocb->ki_flags |= IOCB_DONTCACHE; } + expected = iov_iter_count(&segments[i].iter); + host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter); if (host_err < 0) return host_err; *cnt += host_err; - if (host_err < segments[i].iter.count) + if (host_err < (ssize_t)expected) break; /* partial write */ }