[PATCH v3 0/3] 9p: handle long directory entry names in readdir

[Haobin Wu <853555@gmail.com>]

A host directory entry whose name is longer than 255 bytes currently
makes 9p's readdir fail with -EIO and hides every entry after it, which
is how this was noticed on WSL (Plan 9 shares of Windows drives).

Patch 1 drops the copy into the fixed 256-byte p9_dirent::d_name and
uses the string p9pdu_vreadf() already allocated, so the listing no
longer aborts.

Patch 2 then skips entries whose name is longer than NAME_MAX in the
9p2000.L readdir instead of returning them to userspace, as discussed
with Dominique and Jan on v1: the VFS only enforces PATH_MAX in
verify_dirent_name(), and nothing can operate on such a name afterwards
anyway.

Patch 3 applies the same limit to the legacy 9p2000/9p2000.u readdir.
It is a separate commit because it changes behaviour there: such names
used to be listed.

On patch 3, to be explicit about why this is a behaviour change: the
256-byte limit lives in p9dirent_read(), whose only caller is the
9p2000.L path. v9fs_dir_readdir() parses with p9stat_read(), where
p9_wstat::name comes from the 's' conversion (kmalloc(len + 1), no
NAME_MAX check), so legacy really did emit over-long names.

Changes since v2:
- Patch 2: say why the entry is skipped in the debug message and log it
  at P9_DEBUG_ERROR, the level of the strscpy() message it replaces
  (Christian, Dominique).
- New patch 3: same NAME_MAX limit in v9fs_dir_readdir() for legacy
  9p2000(.u), as its own commit (Christian, Dominique).
- Dropped the Assisted-by: trailers.

Changes since v1:
- Use my real name in From/Signed-off-by (Dominique).
- Split into two patches (Dominique).
- Reworded the subject and commit message of patch 1 to describe the
  existing readdir path and clarify that no allocation is added
  (Dominique).
- New patch 2 skipping entries longer than NAME_MAX (Dominique, Jan).
- Dropped the bouncing sripathik@in.ibm.com address from Cc.

v2: https://lore.kernel.org/all/20260916135403.15789-1-853555@gmail.com/
v1: https://lore.kernel.org/all/20260826064819.52523-1-853555@gmail.com/

Haobin Wu (3):
  9p: skip intermediate directory entry name copy in p9dirent_read()
  9p: skip directory entries with names longer than NAME_MAX
  9p: skip over-long directory entry names for legacy 9p2000 too

 fs/9p/vfs_dir.c         | 31 +++++++++++++++++++++++++------
 include/net/9p/client.h |  2 +-
 net/9p/protocol.c       | 12 ++----------
 3 files changed, 28 insertions(+), 17 deletions(-)


base-commit: 028ef9c96e96197026887c0f092424679298aae8
-- 
2.54.0 (Apple Git-157)