[PATCH 0/4] ksmbd: implement Continuously Available (CA) share support

[Yunseong Kim <yunseong.kim@est.tech>]

Enable the Continuously Available (CA) share infrastructure in ksmbd,
allowing SMB clients to request and use persistent handles on configured shares.

Background
==========

KSMBD already has a complete in-memory durable handle v1/v2
implementation including the persistent handle data model
(fp->is_persistent), protocol parsing (DH2Q with
SMB2_DHANDLE_FLAG_PERSISTENT), and reconnect validation (DH2C). However,
the server never advertises SMB2_GLOBAL_CAP_PERSISTENT_HANDLES or sets
SMB2_SHARE_CAP_CONTINUOUS_AVAILABILITY, making this code path unreachable
by clients.

Per MS-SMB2, a client can only request a persistent handle on a share
that advertises SMB2_SHARE_CAP_CONTINUOUS_AVAILABILITY. This creates a
chicken-and-egg: we cannot test persistent handles without first
advertising the CA capability.

Implementation Approach
=======================

I chose a top-down approach: enable the CA share capability first,
validate with real clients using Samba smbtorture, and then layer on-disk
state persistence in a follow-up series. This allows us to:

  1. Get end-to-end protocol validation immediately with smbtorture
  2. Discover client behavioral expectations before building persistence
  3. Deliver incremental value (fencing + write-through + disconnect
     resilience) before the harder persistence work

Note: at this stage, handles survive connection/session loss but NOT full
server crash. On-disk state persistence and startup recovery will be
addressed in a follow-up series.

Patches
=======

  Patch 1: Advertise SMB2_GLOBAL_CAP_PERSISTENT_HANDLES in negotiate
            response when durable handles are enabled globally.

  Patch 2: Set SMB2_SHARE_CAP_CONTINUOUS_AVAILABILITY in tree connect
            response for shares with 'continuous availability = yes'.

  Patch 3: Add STATUS_FILE_NOT_AVAILABLE fencing: block conflicting
            opens from other clients while a persistent handle is in
            disconnected state (MS-SMB2 3.3.5.9).

  Patch 4: Enforce FILE_WRITE_THROUGH on all opens to CA shares to
            ensure data commits to stable storage (MS-SMB2 3.3.5.9).

The corresponding ksmbd-tools patch that adds the 'continuous availability'
share configuration parameter is submitted:

  https://github.com/namjaejeon/ksmbd-tools/pull/205

Testing
=======

Build the kernel with debug sanitizers enabled:

  $ vng --build --force \
      --configitem CONFIG_SMB_SERVER=y \
      --configitem CONFIG_SMB_SERVER_CHECK_CAP_NET_ADMIN=y \
      --configitem CONFIG_SMB_SERVER_KERBEROS5=y \
      --configitem CONFIG_CIFS=y \
      --configitem CONFIG_CIFS_XATTR=y \
      --configitem CONFIG_CIFS_POSIX=y \
      --configitem CONFIG_CIFS_ALLOW_INSECURE_LEGACY=y \
      --configitem CONFIG_CIFS_DEBUG=y \
      --configitem CONFIG_DEBUG_KERNEL=y \
      --configitem CONFIG_KASAN=y \
      --configitem CONFIG_KASAN_GENERIC=y \
      --configitem CONFIG_KASAN_INLINE=y \
      --configitem CONFIG_UBSAN=y \
      --configitem CONFIG_LOCKDEP=y \
      --configitem CONFIG_LOCK_DEBUGGING_SUPPORT=y \
      --configitem CONFIG_PROVE_LOCKING=y \
      --configitem CONFIG_DEBUG_SPINLOCK=y

Run the CA-specific smbtorture tests inside virtme-ng:

  $ vng --memory 4G --exec '
      mkdir -p /tmp/ksmbd_share /tmp/ksmbd_ca /tmp/ksmbd_conf
      chmod 777 /tmp/ksmbd_share /tmp/ksmbd_ca
      useradd -u 4242 -M fuzz 2>/dev/null || true
      ksmbd.adduser -C ksmbd-sandbox.config \
          -P /tmp/ksmbd_conf/ksmbdpwd.db -a fuzz -p fuzz
      ksmbd.mountd -C ksmbd-sandbox.config \
          -P /tmp/ksmbd_conf/ksmbdpwd.db -n &
      sleep 3
      T=/path/to/samba/bin/smbtorture
      $T //127.0.0.1/ca_share -U fuzz%fuzz \
          smb2.durable-v2-open.persistent-open-oplock
      $T //127.0.0.1/ca_share -U fuzz%fuzz \
          smb2.durable-v2-open.persistent-open-lease
      $T //127.0.0.1/ca_share -U fuzz%fuzz \
          smb2.durable-v2-open.reopen1
      dmesg | grep -E "BUG:|KASAN:|UBSAN:|WARNING:.*ksmbd"
  '

Key smbtorture tests for CA validation:

  smb2.durable-v2-open.persistent-open-oplock
    - Connects to CA share, verifies SMB2_SHARE_CAP_CONTINUOUS_AVAILABILITY
    - Opens file with DH2Q + SMB2_DHANDLE_FLAG_PERSISTENT + batch oplock
    - Verifies server grants persistent handle (response has flag 0x02)
    - Tests various oplock level combinations with persistent flag

  smb2.durable-v2-open.persistent-open-lease
    - Same as above but with lease (RWH) instead of oplock
    - Disconnects TCP, reconnects with DH2C, verifies handle survives
    - Tests lease level combinations with persistent flag

  smb2.durable-v2-open.reopen1
    - Basic durable v2 disconnect/reconnect lifecycle
    - Opens file with batch oplock + DH2Q
    - Disconnects session, establishes new session
    - Reconnects via DH2C with CreateGuid matching
    - Verifies file access resumes after reconnect

  smb2.durable-v2-open.purge-disconnected-rwh-with-rwh-open
    - Tests conflict resolution: a new RWH open from a different client
      should purge (close) a disconnected durable handle with RWH lease
    - Verifies the server correctly resolves lease conflicts

Results on KASAN/UBSAN/LOCKDEP kernel:

    smb2.durable-v2-open.persistent-open-oplock: PASS
    smb2.durable-v2-open.persistent-open-lease:  PASS
    smb2.durable-v2-open.reopen1:                PASS
    smb2.durable-v2-open.purge-disconnected-rwh-with-rwh-open: PASS

    Zero KASAN/UBSAN/lockdep findings

Signed-off-by: Yunseong Kim <yunseong.kim@est.tech>
---
Yunseong Kim (4):
      ksmbd: advertise SMB2_GLOBAL_CAP_PERSISTENT_HANDLES
      ksmbd: set SMB2_SHARE_CAP_CONTINUOUS_AVAILABILITY in tree connect
      ksmbd: add fencing for disconnected persistent handles
      ksmbd: enforce write-through on CA share opens

 fs/smb/server/smb2ops.c   | 10 +++++-----
 fs/smb/server/smb2pdu.c   | 33 +++++++++++++++++++++++++++++----
 fs/smb/server/vfs_cache.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 fs/smb/server/vfs_cache.h |  2 ++
 4 files changed, 81 insertions(+), 9 deletions(-)
---
base-commit: 4c6320e0ad400d4ee41cfde614c05a0d87f54e1b
change-id: 20260824-b4-ca-ace9405ce4b0

Best regards,
--  
Yunseong Kim <yunseong.kim@est.tech>