Re: [RFC PATCH 1/4] fs: Add FS_XFLAG_COMPRESSED & FS_XFLAG_ENCRYPTED for FS_IOC_FS[GS]ETXATTR API

["Pali Rohár" <pali@kernel.org>]

On Sunday 16 February 2025 21:17:55 Amir Goldstein wrote:
> On Sun, Feb 16, 2025 at 7:34 PM Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > On Sun, Feb 16, 2025 at 05:40:26PM +0100, Pali Rohár wrote:
> > > This allows to get or set FS_COMPR_FL and FS_ENCRYPT_FL bits via FS_IOC_FSGETXATTR/FS_IOC_FSSETXATTR API.
> > >
> > > Signed-off-by: Pali Rohár <pali@kernel.org>
> >
> > Does this really allow setting FS_ENCRYPT_FL via FS_IOC_FSSETXATTR, and how does
> > this interact with the existing fscrypt support in ext4, f2fs, ubifs, and ceph
> > which use that flag?
> 
> As far as I can tell, after fileattr_fill_xflags() call in
> ioctl_fssetxattr(), the call
> to ext4_fileattr_set() should behave exactly the same if it came some
> FS_IOC_FSSETXATTR or from FS_IOC_SETFLAGS.
> IOW, EXT4_FL_USER_MODIFIABLE mask will still apply.
> 
> However, unlike the legacy API, we now have an opportunity to deal with
> EXT4_FL_USER_MODIFIABLE better than this:
>         /*
>          * chattr(1) grabs flags via GETFLAGS, modifies the result and
>          * passes that to SETFLAGS. So we cannot easily make SETFLAGS
>          * more restrictive than just silently masking off visible but
>          * not settable flags as we always did.
>          */
> 
> if we have the xflags_mask in the new API (not only the xflags) then
> chattr(1) can set EXT4_FL_USER_MODIFIABLE in xflags_mask
> ext4_fileattr_set() can verify that
> (xflags_mask & ~EXT4_FL_USER_MODIFIABLE == 0).
> 
> However, Pali, this is an important point that your RFC did not follow -
> AFAICT, the current kernel code of ext4_fileattr_set() and xfs_fileattr_set()
> (and other fs) does not return any error for unknown xflags, it just
> ignores them.
> 
> This is why a new ioctl pair FS_IOC_[GS]ETFSXATTR2 is needed IMO
> before adding support to ANY new xflags, whether they are mapped to
> existing flags like in this patch or are completely new xflags.
> 
> Thanks,
> Amir.

But xflags_mask is available in this new API. It is available if the
FS_XFLAG_HASEXTFIELDS flag is set. So I think that the ext4 improvement
mentioned above can be included into this new API.

Or I'm missing something?