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    SubjectRe: Revised statx(2) man page for review [and AT_EMPTY_PATH question]
    Hi David,

    On 26 April 2017 at 17:10, David Howells <dhowells@redhat.com> wrote:
    > Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote:
    >
    >> > This indicates what stx_attributes the VFS and filesystem actually support.
    >> >
    >> >> __s32 tv_nsec; /* Nanoseconds before or since tv_sec */
    >> >
    >> > If you're going to do Dmitry's suggestion, then this needs to be __u32 and you
    >> > should remove "before or".
    >>
    >> I think the question is rather: what is going to be done to the API?
    >> Will it be changed as Dmitry suggests?
    >
    > I've forwarded Dmitry's patch to this effect.

    The man page now corresponds.

    >> Having two ways to do something is odd, and redundant. Note
    >> of the other APIs that provide this functionality do so
    >> in both ways, AFAIK. It's not a big problem, but certainly
    >> strange. If you settle on having just one, then I'd say
    >> choose AT_EMPTY_PATH.
    >
    > If I choose that, I presume I would have to give EINVAL if the path is NULL or
    > anything other than ""?

    AFAICS, just set lookup_flags to include LOOKUP_EMPTY and
    getname_flags() does the rest. (Essentially, AT_EMPTY_PATH is a safety
    catch for an empty path: if the path is nonempty, it is interpreted as
    usual, othewise if it is empty, you get ENOENT unless AT_EMPTY_PATH is
    also set.

    >> Under ERRORS I added:
    >>
    >> .TP
    >> .B EINVAL
    >> Reserved flag specified in
    >> .IR mask .
    >>
    >> Okay?
    >
    > That's fine.

    Thanks.

    >> >> It should be noted that the kernel may return fields that
    >> >> weren't requested and may fail to return fields that were
    >> >> requested, depending on what the backing filesystem supports.
    >> >
    >> > Maybe add "and can be safely ignored" in there somewhere since this seems to
    >> > be upsetting people.
    >>
    >> You say "in there somewhere", but it's not quite clear to me which piece
    >> this applies to. Could you propose a wording please.
    >
    > Can you do footnotes in roff?
    >
    > It should be noted that the kernel may return fields that
    > weren't requested[*] and may fail to return fields that were
    > requested, depending on what the backing filesystem supports.
    >
    > [*] These can be safely ignored.
    >
    > Or maybe:
    >
    > It should be noted that the kernel may return fields that
    > weren't requested and may fail to return fields that were
    > requested, depending on what the backing filesystem supports.
    > Fields that are given values despite being unrequested can just
    > be ignored.

    I took the second approach.

    >> >> If a filesystem does not support a field or if it has an unrep‐
    >> >> resentable value (for instance, a file with an exotic type),
    >> >> then the mask bit corresponding to that field will be cleared
    >> >> in stx_mask even if the user asked for it and a dummy value
    >> >> will be filled in for compatibility purposes if one is avail‐
    >> >> able (e.g., a dummy UID and GID may be specified to mount under
    >> >> some circumstances).
    >> >
    >> > I don't promise a dummy value for any "extended" field other than zero.
    >>
    >> I don't know what you mean to say here. Do you mean some
    >> text in the page should change?
    >
    > The paragraph promises a "dummy value will be filled in for compatibility
    > purposes if one is available", but doesn't place any restriction on the fields
    > towhich this applies. This is only true of the basic stat fields; all other
    > fields will be cleared if not set.
    >
    > Maybe we can just leave this as is. We're not promising a dummy field will
    > *always* be emplaced. We can always say that they're just not available for
    > extended fields if someone asks.
    >
    > Maybe the best thing to do is to simply add "and cleared otherwise." to the
    > end of the paragraph.

    Two points:
    * You do realize the text about "dummy values" was your original text?
    * Adding "and cleared otherwise" to end of the paragraph doesn't make
    sense. I'll leave the text as is, but if you want to propose a more
    complete phrasing, let me know.

    >> > Should this list either be in alphabetical order or offset-in-struct order?
    >>
    >> Probably the same order as the struct.
    >
    > Sounds good.

    Already done.

    >> Added. But, what does "no usable value here" mean? (The relationship
    >> between stx_attributes_mask and stx_attributes still isn't
    >> so clear to me.
    >
    > It's not so obvious with the bits that are currently defined. But I have a
    > patch that adds Windows attribute bits also (for cifs, ntfs, fat, ...). What
    > does it mean, say, if the archive bit is clear? Does it mean that archive
    > isn't set in the fs or that the fs doesn't support it?
    >
    > Further, I have plans to add a 'setattrx' syscall that takes a statx struct
    > and calls notify_change() with its contents in the kernel. If I do that, I
    > need to indicate to notify_change() what changes should be effected. stx_mask
    > covers most of the fields, but not stx_attributes. Some of these attributes
    > would be alterable.
    >
    > Would you prefer it to be reverted for the moment?

    To what does "it" refer?

    Anyway, I think we do need some better text describing these two
    fields and the difference between them. Can you come up with
    something?

    Cheers,

    Michael



    --
    Michael Kerrisk
    Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
    Linux/UNIX System Programming Training: http://man7.org/training/

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