md/raid5: reject zero-sector reshape chunks

Sashiko reported that RAID5 can accept a reshape chunk size that becomes
zero sectors. chunk_size_store() stores the sysfs byte value as n >> 9, so
writing a value below 512 bytes sets mddev->new_chunk_sectors to zero.
RAID5 then accepted that pending reshape geometry and raid5_start_reshape()
installed it into conf->chunk_sectors, letting reshape code divide by zero.

Reject zero-sector chunks both in check_reshape(), where normal sysfs
requests are validated, and in raid5_start_reshape(), so assembly/resume
paths also cannot install zero chunk geometry.

Test script: in QEMU, create a plain three-disk RAID5 array with 64K
chunks, write/read back a small pattern, write 1 to
/sys/block/md0/md/chunk_size, add a fourth disk, and run mdadm --grow
--raid-devices=4 --backup-file=... . The script scans dmesg for divide
error/Oops/KASAN signatures.

Bad kernel, eb29914412c3:

  echo 1 > /sys/block/md0/md/chunk_size
  mdadm --grow /dev/md0 --raid-devices=4 --backup-file=/root/md0-grow.bak

  Oops: divide error: 0000 [#1] SMP KASAN NOPTI
  RIP: raid5_get_active_stripe+0x863/0xc10
  Call Trace:
   raid5_sync_request
   md_do_sync
   md_thread
  Kernel panic - not syncing: Fatal exception

Fixed kernel: echo 1 > /sys/block/md0/md/chunk_size bash: echo: write
error: Invalid argument chunk_write_rc=1 grow_rc=skipped RESULT:
REJECTED_ZERO_CHUNK_NO_OOPS

Tested-by: Mykola Marzhan <mykola@meshstor.io>
Link: https://patch.msgid.link/20260802195038.164272-5-yukuai@kernel.org
Signed-off-by: Yu Kuai <yukuai@fygo.io>

0 files changed, 0 insertions, 0 deletions