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

[Amir Goldstein]

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.
This has nothing to do with capabilities.
The Windows and MacOS filesystems have allowed setting their
"creation time" forever 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 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.

> 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.

>
> 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.
>

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/