Apologies for the git-send-email misfire.. still not sure what happened, but will sort it out for future.
On Tue, Sep 08, 2026 at 12:34:35PM -0400, Mike Snitzer wrote: > While qualifying NFSD's NFSD_IO_DIRECT write path with byte-level data > verification, it was found that the ITER_BVEC payloads nfsd submits > expose silent data corruption in two bio-based block drivers and > two defects in nfsd itself. Example problematic payloads is the first > fragment starts mid-page because the RPC header precedes it in the > receive buffer, and fragment lengths need not be sector multiples. > bio_iov_bvec_set() passes such an array to the queue as-is; nothing > below it validates per-bvec sector alignment. > > Patches 1-2 fix silent corruption (write completes successfully, data > lands wrong) and are stable candidates: > > - brd re-derives each segment's device position from > bio->bi_iter.bi_sector, which bio_advance_iter_single() advances by > whole sectors only, so a sub-sector segment length skews everything > that follows. > > - zram hardwires is_partial_io() to false on 4K-page kernels, sending > sub-page bvecs down a whole-page path that ignores bv_offset/bv_len > entirely, and has the same sector-cursor skew. > > Both are verified with a synthetic-bio reproducer (stamped pattern, > write, read back, compare) across mid-page and page-aligned > geometries. > > Patches 3-4 fix nfsd: the filecache never fetches DIO alignment > attributes on the supplied-file acquire branch, so every WRITE to a > file created via NFSv4 OPEN(CREATE) is refused direct I/O for the > file's cached lifetime; and nfsd's statx-based DIO gate is weaker than > bio_split_io_at()'s split-time checks, so an admitted iterator can > still be rejected by the block layer -- retry the segment buffered > instead of failing a valid WRITE with NFS4ERR_INVAL. > > One open question for the iomap/block maintainers: should ITER_BVEC > direct I/O with sub-sector bvec boundaries be validated or bounced > centrally rather than trusted to every driver's iteration? An audit > of in-tree bio-based drivers found the same bi_sector-derived position > pattern in dm-io, dm-log-writes, dm-writecache (pmem path) and > dm-integrity -- unreachable through nfsd today only because dm queues > advertise dma_alignment >= 511, which nfsd's alignment gate refuses. > > Tested with the reproducer matrix on brd, zram and nvme-loop at 4K and > 16K page size (aarch64) and 4K (x86_64), plus 30-connection NFS write > rigs comparing source against export byte-for-byte: clean with the > fixes, corrupting or erroring without them. > > Mike Snitzer (3): > brd: iterate the bio by byte position, not bi_sector > zram: handle sub-page bvec segments without corrupting data > nfsd: fall back to buffered I/O when a direct write gets -EINVAL > > David Flynn (1): > nfsd: fetch direct I/O alignment for files handed to the filecache > > drivers/block/brd.c | 29 +++++++++++++++++++++++------ > drivers/block/zram/zram_drv.c | 36 +++++++++++++++++------------------ > fs/nfsd/filecache.c | 4 ++-- > fs/nfsd/vfs.c | 29 +++++++++++++++++++++++++++++ > 4 files changed, 77 insertions(+), 31 deletions(-) > > -- > 2.52.0
