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

[koraynilay]

On Tue Nov 3, 2026 at 4:25 PM CET, Pali Rohár wrote:
> On Tuesday 03 November 2026 16:10:24 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.
>> 
>> 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.
>
> Kernel NTFS driver changed more times, so I would not say that it is too
> late to stop publishing NTFS creation time in STATX_BTIME, and start
> publishing it in STATX_PTIME.
>
>
> Anyway, I have another idea (if stopping publishing of NTFS is not possible):
> Let fs drivers to fill ptime and btime members only when fs itself
> supports particular field (ext4 would fill only btime, nfts would fill
> only ptime; btrfs with patch would fill both). For userspace STATX_BTIME
> request, VFS will copy ptime to btime if the btime is not provided.
> For userspace STATX_PTIME request or STATX_BTIME|STATX_PTIME request,
> the VFS will not do this copy. Or We can introduce new flag
> STATX_NO_FAKE_BTIME (just random name for now) and this would cause that
> the ptime would not be copied to btime (so it would not be needed to
> misuse STATX_BTIME|STATX_PTIME). The logic would be unified in VFS
> layer, not in the fs drivers.

I feel like this would be a bit of an over-complication/not clear
enough, as we would merge a lot more of the meaning of btime and ptime,
while you would never return mtime when userspace asks for  ctime (I
think?).

But maybe it could make sense as a compromise to not break userspace too
much.


Best,
koraynilay

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