Re: [RFC PATCH v3 3/5] ntfs3: make crtime user-settable using ptime

[Amir Goldstein]

On Tue, Nov 3, 2026 at 10:49 AM Pali Rohár <pali@xxxxxxxxxx> wrote:
>
> On Tuesday 03 November 2026 09:23:32 Amir Goldstein wrote:
> > On Tue, Nov 3, 2026 at 2:19 AM Andreas Dilger <adilger@xxxxxxxxx> wrote:
> > >
> > > On Nov 2, 2026, at 09:38, Pali Rohár <pali@xxxxxxxxxx> wrote:
> > > >
> > > > On Monday 02 November 2026 16:09:41 Amir Goldstein wrote:
> > > >> On Mon, Nov 2, 2026 at 3:24 PM Pali Rohár <pali@xxxxxxxxxx> wrote:
> > > >>>
> > > >>> On Sunday 01 November 2026 02:55:49 koraynilay wrote:
> > > >>>> @@ -154,6 +158,11 @@ int ntfs_fileattr_set(const struct mnt_idmap *idmap, struct dentry *dentry,
> > > >>>>      if (flags & FS_APPEND_FL)
> > > >>>>              new_fl |= S_APPEND;
> > > >>>>
> > > >>>> +     if (fa->fsx_xflags & FS_XFLAG_PTIME) {
> > > >>>> +             ni->i_crtime.tv_sec = fa->fsx_ptime.tv_sec;
> > > >>>> +             ni->i_crtime.tv_nsec = fa->fsx_ptime.tv_nsec;
> > > >>>> +     }
> > > >>>> +
> > > >>>>      inode_set_flags(inode, new_fl, S_IMMUTABLE | S_APPEND);
> > > >>>>
> > > >>>>      /* Save nodump flag to return in ntfs_getattr. */
> > > >>>> @@ -275,8 +284,9 @@ int ntfs_getattr(const struct mnt_idmap *idmap, const struct path *path,
> > > >>>>      if (unlikely(is_bad_ni(ni)))
> > > >>>>              return -EINVAL;
> > > >>>>
> > > >>>> -     stat->result_mask |= STATX_BTIME;
> > > >>>> +     stat->result_mask |= (STATX_BTIME | STATX_PTIME);
> > > >>>>      stat->btime = ni->i_crtime;
> > > >>>> +     stat->ptime = ni->i_crtime;
> > > >>>
> > > >>> For me this patch looks dangerous. Somehow I would expect that the
> > > >>> "btime" will never change for existing inode.
> > > >>>
> > > >>> But because the ptime is mapped to i_crtime and ptime is settable from
> > > >>> userspace, with this patch it is directly possible to change btime.
> > > >>>
> > > >>>
> > > >>>
> > > >>> I was thinking more about it... Is not this just misunderstanding of how
> > > >>> the security model around the times is working on different systems?
> > > >>> Because on NTFS there is stored just one field which says when the file
> > > >>> was created. There are not two fields like "time set by user and time
> > > >>> set by system".
> > > >>>
> > > >>> Windows security model allows anybody with WRITE_ATTRIBUTES access to
> > > >>> change that create time. Linux security model does not allow to change it.
> > > >>>
> > > >>> NTFS in this view is just a filesystem like any other (ext4, btrfs, ...).
> > > >>> In my opinion if the ext4 or btrfs filesystem is correctly implemented
> > > >>> on Windows systems, then Windows security model should handle it in the
> > > >>> same way like for NTFS or exFAT. So if something grants WRITE_ATTRIBUTES
> > > >>> access, then Windows should allow to change also btime on ext4 or btrfs.
> > > >>>
> > > >>>
> > > >>>
> > > >>> On the other hand, I understand the use case to have two creation times.
> > > >>> One time which would never be changed and could be used for any kind of
> > > >>> atomic / synchronization by userspace application logic (this btime).
> > > >>> And another time for human which can be changed at any time to any value
> > > >>> which user wants (this ptime).
> > > >>>
> > > >>> In any case, I would suggest that filesystem driver would not report to
> > > >>> userspace unsupported values and would not try to invent other values
> > > >>> and lying about them.
> > > >>
> > > >> But all those fs with mutable creation time already report it with STATX_BTIME
> > > >> so what can we do about this?
> > > >
> > > > Well, what I wanted above to point out is that the "mutability" of
> > > > creation time is not feature of filesystem, but in my opinion rather
> > > > property of the security model on the running operating system.
> > > >
> > > > Maybe better question is: how ext4 btimes differs from the ntfs btime?
> > > > In my opinion there is no much difference, both of them has meaning
> > > > "time when was the file created".
> > > >
> > > >> If userspace requests both STATX_BTIME|STATX_PTIME in mask arg,
> > > >> those fs could technically return only STATX_PTIME and it should not regress
> > > >> existing programs.
> > > >>
> > > >> But this is still a bit surprising behavior is it not?
> > > >>
> > > >> Thanks,
> > > >> Amir.
> > > >
> > > > I'm not sure what to do with it.
> > > >
> > > > If we allow to change btime of ntfs then for consistency we should also
> > > > allow to change btime of ext4.
> > > >
> > > > Maybe another idea... define new capability which would allow to change
> > > > btime (on any filesystem)? This would at least make the consistency.
> >
> > Pali, I think this is a very bad idea.
>
> Ok, if the point is that ext4 btime must stay immutable then it is a bad idea.
>
> > This has nothing to do with capabilities.
> > The Windows and MacOS filesystems have allowed setting their
> > "creation time" forever
>
> FreeBSD for some fs allow it too (see below).
>
> > and for all the other Linux filesystems that had
> > "birth time" it was immutable, so let's not mix filesystem features
> > with capabilities.
>
> I think that it is important to first define behavior and invariants of
> these "btime" and "ptime" kernel properties.
>
> If we want to be consistent then there are two possible options:
>
> 1. If we choose that the STATX_BTIME is immutable then no kernel fs
>    driver should allow to change.
>
> 2. If we choose that the "NTFS create time" is reported via STATX_BTIME
>    then STATX_BTIME is not immutable at all.
>
> These two options cannot be chosen together because option 1 would
> implie that the modifiable "NTFS create time" cannot be reported over
> STATX_BTIME.
>
> We are have currently STATX_BTIME immutable and "NTFS create time"
> reported via STATX_BTIME.
>
> If we want to stay _consistent_ across fs drivers then adding API for
> changing "NTFS create time" would break one of those two things.
>
> In my opinion we should stay consistent across filesystem drivers and
> provide unified behavior of APIs independently of used filesystem.
> Otherwise userspace applications would have to write fs specific code
> for doing their use cases (and we can stay with fs-specific xattrs, no
> need to create unified APIs).
>
> Does it make sense? Or it is still a bad idea to think about it in this way?
>

My "bad idea" comment was only referring to capability.

w.r.t the STATX_BTIME ambiguity there is another option we may want to
consider: Don't publish STATX_PTIME via statx() at all.

One of the reasons that developers have pointed to fileattr as the right API
to host the creation time as well as the DOS attributes is that it has a history
of carrying filesystem specific attributes.

Not every attribute available in lsattr is also available in statx().
If projid is not available in statx() then the settable creation time
doesn't need to be in statx() either.

The file_[sg]etattr() syscalls provides a symmetric API to set/get it.

The same could be true to HIDDEN/ARCHIVE/SYSTEM flags if those
are added to fileattr.

Do we need to stop publishing NTFS creation time in STATX_BTIME?
I think it's too late for that and nobody is going to gain anything
from doing that.

Thanks,
Amir.