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

[koraynilay]

On Tue Nov 3, 2026 at 4:10 PM CET, Amir Goldstein wrote:
> 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.

btime currently is also a fliesystem specific implementation tho, and
there was already a transition from stat to statx when btime was added,
so I think ptime should be included in statx, especially since it should
be treated as one of the standard timestamps from now on (or at least,
that's what I hope to achieve eventually).
The only difference to the currently standard timestamps would be that
a/mtime are managed by utimensat, ctime/btime by the filesystems
themselves (as they're more "internal" and less "user-facing") and ptime
would be managed by file_[gs]etattr() (since changing utimensat wouldn't
make sense as said in the april thread).

Thanks,
koraynilay

> 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.

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