Re: [PATCH] eventpoll: return -ENOIOCTLCMD for unknown ioctl commands

On Thu, Sep 24, 2026 at 02:57:47PM -0400, hengyul@cs.unc.edu wrote:

From: Hengyu Liang <redacted>

Before commit 18e2bf0edf4d ("eventpoll: Add epoll ioctl for
epoll_params"), epoll files had no ioctl handler, so ioctl() on an epoll
file descriptor failed with ENOTTY. That commit introduced the
EPIOCSPARAMS and EPIOCGPARAMS commands, but ep_eventpoll_ioctl() returns
-EINVAL for any other command, so since v6.9 every other ioctl() on an
epoll file descriptor fails with EINVAL instead of ENOTTY.

Documentation/driver-api/ioctl.rst says that an ioctl handler must
return -ENOTTY or -ENOIOCTLCMD for an unknown command, and that
returning -EINVAL there is wrong. Returning -ENOIOCTLCMD was also the
intent of the original series, whose changelog since v3 [1] says "when
an unknown ioctl is received, -ENOIOCTLCMD is returned instead of
-EINVAL as the ioctl documentation requires", and ep_eventpoll_bp_ioctl()
does return -ENOIOCTLCMD for unknown commands. However,
ep_eventpoll_ioctl() only passes EPIOCSPARAMS and EPIOCGPARAMS to it and
handles all other commands in its own default case, which returns
-EINVAL, so that path is never reached.

This is visible to userspace. For example, isatty(), ttyname() and
tcgetattr() on an epoll file descriptor set errno to EINVAL, while they
set ENOTTY for any other file descriptor that does not refer to a
terminal, as they also did for epoll file descriptors before v6.9.

Return -ENOIOCTLCMD from the default case, which the VFS turns into
-ENOTTY, and update the epoll_busy_poll selftest, which expected EINVAL
for an unknown command.

[1] https://lore.kernel.org/r/20240125225704.12781-1-jdamato@fastly.com (local)

Fixes: 18e2bf0edf4d ("eventpoll: Add epoll ioctl for epoll_params")
Signed-off-by: Hengyu Liang <redacted>
---
 fs/eventpoll.c                                | 2 +-
 tools/testing/selftests/net/epoll_busy_poll.c | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

Reviewed-by: Joe Damato <redacted>