Günther Noack abe6acfa7d
fs: Return ENOTTY directly if FS_IOC_GETUUID or FS_IOC_GETFSSYSFSPATH fail
These IOCTL commands should be implemented by setting attributes on the
superblock, rather than in the IOCTL hooks in struct file_operations.

By returning -ENOTTY instead of -ENOIOCTLCMD, we instruct the fs/ioctl.c
logic to return -ENOTTY immediately, rather than attempting to call
f_op->unlocked_ioctl() or f_op->compat_ioctl() as a fallback.

Why this is safe:

Before this change, fs/ioctl.c would unsuccessfully attempt calling the
IOCTL hooks, and then return -ENOTTY.  By returning -ENOTTY directly, we
return the same error code immediately, but save ourselves the fallback
attempt.

Motivation:

This simplifies the logic for these IOCTL commands and lets us reason about
the side effects of these IOCTLs more easily.  It will be possible to
permit these IOCTLs under LSM IOCTL policies, without having to worry about
them getting dispatched to problematic device drivers (which sometimes do
work before looking at the IOCTL command number).

Link: https://lore.kernel.org/all/cnwpkeovzbumhprco7q2c2y6zxzmxfpwpwe3tyy6c3gg2szgqd@vfzjaw5v5imr/
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20240405214040.101396-2-gnoack@google.com
Acked-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Christian Brauner <brauner@kernel.org>
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Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.
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