mm/swap: reject swapon() on filesystem-level encrypted files

ext4 and f2fs don't prevent filesystem-level encrypted files from being
set up directly as swap files.  In this case, encryption is bypassed.

No one should be doing this, vs.  the methods of encrypted swap that
actually do work (such as swapping to a dm-crypt device, or swapping to a
loopback device on top of a filesystem-level encrypted file).

Nevertheless, to prevent user error, make swapon() explicitly reject this
case.  Document this behavior in fscrypt.rst as well.

Link: https://lore.kernel.org/20260803180426.3123-1-ebiggers@kernel.org
Fixes: 9bd8212f981e ("ext4 crypto: add encryption policy and password salt support")
Fixes: f424f664f0e8 ("f2fs crypto: add encryption policy and password salt support")
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Baoquan He <baoquan.he@linux.dev>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions