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

[Pali Rohár]

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?

> >
> > I think the ext4 and xfs maintainers have repeatedly said that btime/crtime
> > attribute should not be modified by users.  There was a long discussion on
> > this from Ted as well, and the reality is that "creation" time is not worth
> > the bits that it is written on,
> 
> This is an opinion of individual people, who did not lay out technical
> arguments.
> I have concrete technical arguments in favor of first class creation time for
> ext4/xfs from my experience with production systems that need it and use xattr
> to store it (by smbd), but I am not going to list them here because
> this is not a
> thread about implementing mutable creation time for ext4/xfs.
> 
> > since it is not preserved by any tools, has
> 
> Facts check:
> 
> # NEWS for rsync 3.2.3 (6 Aug 2020)
> 
> ## Changes in this version:
> ...
>  - Added [`--crtimes`](rsync.1#opt) (`-N`) option for preserving the file's
>    create time (I believe that this is macOS only at the moment).
> 
> Incidentally, I have an implementation for --crtimes for cifs [1], but this
> is not extendable to support all the filsystems which support settable crtime.

Another two facts check:

FreeBSD has also API for changing "birth (creation) times".
It is documented in utimes(2) manpage:
https://man.freebsd.org/cgi/man.cgi?query=utimes&sektion=2

So I guess that there has to be some tool which is using this API and
therefore these "birth (creation) times" are preserved.


NFS4 has time_create attribute (FATTR4_TIME_CREATE=50) which is
per-fs-object read-write attribute and defined (in RFC8881 5.8.2.40.) as:
  The time of creation of the object. This attribute does not have any
  relation to the traditional UNIX file attribute "ctime" or "change time".
Since NFSv4.1 it can be even specified during creating a new file/dir
which seems to be for backup-restore. And permissions (RFC8881 6.2.1.3.1.):
  ACE4_WRITE_ATTRIBUTES
  Operation(s) affected: SETATTR of time_access_set, time_backup,
    time_create, time_modify_set, mimetype, hidden, system
  Discussion: Permission to change the times associated with a file or
    directory to an arbitrary value. Also permission to change the
    mimetype, hidden, and system attributes. A user having ACE4_WRITE_DATA
    or ACE4_WRITE_ATTRIBUTES will be allowed to set the times associated
    with a file to the current server time.

I'm not sure if there is some backup/restore tool which do it over NFS4
but the NFS4 description seems to be very clear that the network protocol
itself is already prepared for these use cases. And permissions mostly
matches the SMB / NTFS / Windows security model.

> > a "vague" meaning (is it when the *file* was created, when the *content* was
> > created, do edits update it, is it preserved across filesystems, archives,
> > etc.) and adding it into the filesystem is misplaced effort.
> 
> Sorry now you are talking nonsense. Meaning has nothing to do with it.
> This is a property of filesystems (plural) which have documented APIs
> and documented network protocols - it is what it is.
> 
> If you are looking for a meaning, it is the first publication date, whether
> it means something to you is irrelevant - what's relevant is that customers
> use it and want to preserve it in backups, whatever its meaning is.

I agree here. I also heard lot of times that users (customers) want to
preserve create time in backups and be able to restore it. And that
Windows is already able to do it, Linux not yet.

Maybe what is a "vague" here is where that "create time" is stored and
from which (API) source is GUI (or CLI) application showing it to user.

> >
> > Storing it as an xattr has some reasonable chance of being preserved across
> > tar/cp/filesystems, and doesn't introduce a herculean effort of changing
> > all of the system calls and on-disk formats for every filesystem.
> >

I think the part "being preserved across tar/cp/filesystems" is
relevant. If a completely new API is introduced into Linux then it means
that every existing backup tool would not be able to preserve this
property and also would not be able to restore it.

I'm not sure if we can do anything with this issue, but it is important
to know about this limitation.

About the part "introduce a herculean effort of changing all of the
system calls and on-disk formats for every filesystem.". In my opinion
it is better to do that cleanup (maybe even in all stat-related
syscalls) if it makes things easier and prevents another technical
issues.

> 
> As I said I can provide data for why the xattr approach taken by smbd is
> limited and inferior, but this is not the place to get into it (there are hints
> in the rsync commit message below).
> 
> Thanks,
> Amir.
> 
> [1] https://github.com/amir73il/rsync/commits/cifs-crtimes/

I would say that xattr does not solve this issue too.