On 07/31/26 at 11:14am, Eric Biggers wrote:
> On Fri, Jul 31, 2026 at 11:11:02PM +0800, Baoquan He wrote:
> > On 07/30/26 at 11:48am, Eric Biggers wrote:
> > > 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.
> > >
> > > Fixes: 9bd8212f981e ("ext4 crypto: add encryption policy and password
> > > salt support")
> > > Fixes: f424f664f0e8 ("f2fs crypto: add encryption policy and password
> > > salt support")
> > > Cc: [email protected]
> > > Signed-off-by: Eric Biggers <[email protected]>
> > > ---
> > > Documentation/filesystems/fscrypt.rst | 4 ++++
> > > mm/swapfile.c | 4 ++++
> > > 2 files changed, 8 insertions(+)
> > >
> > > diff --git a/Documentation/filesystems/fscrypt.rst
> > > b/Documentation/filesystems/fscrypt.rst
> > > index c0dd35f1af12..cba1989777da 100644
> > > --- a/Documentation/filesystems/fscrypt.rst
> > > +++ b/Documentation/filesystems/fscrypt.rst
> > > @@ -1238,6 +1238,10 @@ astute users may notice some differences in
> > > behavior:
> > >
> > > - DAX (Direct Access) is not supported on encrypted files.
> > >
> > > +- Encrypted files cannot be used directly as swap files. To swap to
> > > + an encrypted file, set up a loopback device on top of it.
> > > + Alternatively, encrypted swap can use a dm-crypt device instead.
> > > +
> > > - The maximum length of an encrypted symlink is 2 bytes shorter than
> > > the maximum length of an unencrypted symlink. For example, on an
> > > EXT4 filesystem with a 4K block size, unencrypted symlinks can be up
> > > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > > index 78b49b0658ad..e4991da81b5f 100644
> > > --- a/mm/swapfile.c
> > > +++ b/mm/swapfile.c
> > > @@ -3650,6 +3650,10 @@ SYSCALL_DEFINE2(swapon, const char __user *,
> > > specialfile, int, swap_flags)
> > > error = -EBUSY;
> > > goto bad_swap_unlock_inode;
> > > }
> > > + if (IS_ENCRYPTED(inode)) {
> > > + error = -EINVAL;
> > > + goto bad_swap_unlock_inode;
> > > + }
> >
> > Sounds reasonable to me.
> >
> > One minor suggestion: it might be helpful to emit a pr_warn_once()
> > explaining *why* the swapon was rejected and pointing users to the
> > dm-crypt / loopback alternatives, so the -EINVAL isn't opaque.
> > Optional though -- the check itself is what matters.
>
> How's this?
>
> pr_warn_once("Filesystem-level encrypted swapfile '%s' is unsupported.
> Create a loop device over it, or use dm-crypt\n", name->name);
Looks good to me, thx.
>
> > And this patch updates fscrypt.rst. Would it be good to also
> > update the swapon(8) man page to document the new -EINVAL
> > condition?
> >
> > Other than these,
> >
> > Reviewed-by: Baoquan He <[email protected]>
>
> swapon(8) is for the util-linux program, not the syscall, so it doesn't
> document error codes specifically. It does have a NOTES section that
> incompletely documents filesystem-specific constraints on swap files.
> It's missing most of the current constraints. But sure, I'll send a
> patch that adds a note to there about this one. That will be a
> util-linux patch.
Sounds great, thanks.
