Re: [RFC PATCH v3 1/5] vfs: add provenance time (ptime)

[koraynilay]

On Tue Nov 3, 2026 at 4:14 PM CET, Amir Goldstein wrote:
> On Tue, Nov 3, 2026 at 12:13 PM koraynilay <koray.fra@xxxxxxxxx> wrote:
>>
>> On Mon Nov 2, 2026 at 3:00 PM CET, Amir Goldstein wrote:
>> > On Sun, Nov 1, 2026 at 2:57 AM koraynilay <koray.fra@xxxxxxxxx> wrote:
>> >>
>> >> Add provenance time (ptime), indicating the time when a file or
>> >> directory was created. This is user-settable via file_setattr() and
>> >> retrievable via statx(2) (using STATX_PTIME) or file_getattr().
>> >>
>> >> The semantics of when to preserve this new timestamp are the same as
>> >> mtime, the idea being that there is a correspondence like this:
>> >>
>> >> ctime <-> btime
>> >> mtime <-> ptime
>> >>
>> >> so every operation (copying, moving, ...) that currently preserves mtime
>> >> should also preserve ptime.
>> >>
>> >> Furthermore, in order to avoid the legacy FS_IOC_FSSETXATTR accepting
>> >> FS_XFLAG_PTIME, add an FS_FILE_ATTR_XFLAGS_MASK mask to support new
>> >> xflags, and use it in fileattr_to_file_attr()/file_attr_to_fileattr() to
>> >> allow ptime to be used with the file_setattr()/file_getattr() syscalls.
>> >>
>> >> [ This is a re-implementation of the initial patch[1] by Sean Smith
>> >>   <defendthedisabled@xxxxxxxxx>, following the advice[2] from Darrick J.
>> >>   Wong <djwong@xxxxxxxxxx> of using file_setattr() instead of updating
>> >>   utimensat() ]
>> >>
>> >> [1]: https://lore.kernel.org/20260405195007.1306-2-DefendTheDisabled@xxxxxxxxx
>> >> [2]: https://lore.kernel.org/20260407014129.GC6192@frogsfrogsfrogs
>> >>
>> >> Assisted-by: Gemini:3.8-flash-high antigravity-cli-1.4.7
>> >> Signed-off-by: koraynilay <koray.fra@xxxxxxxxx>
>> >> ---
>> >>  fs/file_attr.c            | 33 +++++++++++++++++++++++++++++++--
>> >>  fs/stat.c                 |  2 ++
>> >>  include/linux/fileattr.h  |  5 +++++
>> >>  include/linux/stat.h      |  1 +
>> >>  include/uapi/linux/fs.h   |  8 +++++++-
>> >>  include/uapi/linux/stat.h |  7 ++++++-
>> >>  6 files changed, 52 insertions(+), 4 deletions(-)
>> >>
>> >> diff --git a/fs/file_attr.c b/fs/file_attr.c
>> >> index 833e03810b24..c0645cb22bf3 100644
>> >> --- a/fs/file_attr.c
>> >> +++ b/fs/file_attr.c
>> >> @@ -102,7 +102,7 @@ EXPORT_SYMBOL(vfs_fileattr_get);
>> >>  static void fileattr_to_file_attr(const struct file_kattr *fa,
>> >>                                   struct file_attr *fattr)
>> >>  {
>> >> -       __u32 mask = FS_XFLAGS_MASK;
>> >> +       __u32 mask = FS_FILE_ATTR_XFLAGS_MASK;
>> >>
>> >>         memset(fattr, 0, sizeof(struct file_attr));
>> >>         fattr->fa_xflags = fa->fsx_xflags & mask;
>> >> @@ -110,6 +110,8 @@ static void fileattr_to_file_attr(const struct file_kattr *fa,
>> >>         fattr->fa_nextents = fa->fsx_nextents;
>> >>         fattr->fa_projid = fa->fsx_projid;
>> >>         fattr->fa_cowextsize = fa->fsx_cowextsize;
>> >> +       fattr->fa_ptime_sec = fa->fsx_ptime.tv_sec;
>> >> +       fattr->fa_ptime_nsec = fa->fsx_ptime.tv_nsec;
>> >>  }
>> >>
>> >>  /**
>> >> @@ -141,15 +143,20 @@ EXPORT_SYMBOL(copy_fsxattr_to_user);
>> >>  static int file_attr_to_fileattr(const struct file_attr *fattr,
>> >>                                  struct file_kattr *fa)
>> >>  {
>> >> -       __u64 mask = FS_XFLAGS_MASK;
>> >> +       __u64 mask = FS_FILE_ATTR_XFLAGS_MASK;
>> >
>> > That is not the correct way to extend this API.
>> > This fixed mask was added as a placeholder for
>> > "We intend to add support for more xflags to selective filesystems":
>> > https://lore.kernel.org/linux-fsdevel/20250329143312.1350603-2-amir73il@xxxxxxxxx/
>> >
>> > Please see this POC patch for filesystems to publish their supported
>> > xflags mask:
>> > https://lore.kernel.org/linux-fsdevel/20250329143312.1350603-3-amir73il@xxxxxxxxx/
>>
>> Thanks, I did not think of adding a dynamic mask like that, I will look
>> into it and implement it for v4.
>>
>> > This will allow callers of file_getattr() to know if the filesystem
>> > supports ptime
>> > without trying to set it and failing.
>>
>> So after adding fsx_xflags_mask to file_kattr, every filesystem would
>> populate fsx_xflags_mask with all the returned (== supported) FS_XFLAG_*
>> in their own specific .fileattr_get, correct?
>
> Not all filesystems. Just the ones that wish to extend the constant
> legacy FS_XFLAGS_MASK.
> So just the ones that you change to support XFLAGS_PTIME

Ah ok yeah that makes more sense, so like the fileattr_has_fsx_mask()
(that checks if fa has fsx, ignoring the suppored/allowed ones specified
as second arg) that I added, but in the opposite way: "I advertise only
the extra fsx (so only 0 or FS_XFLAG_PTIME rn) that I support".

Thanks,
koraynilay

> Thanks,
> Amir.

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