Re: Revised statx(2) man page for review [and AT_EMPTY_PATH question]

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hello David,

Thanks for your comments. Some questions below.

On 04/26/2017 01:35 PM, David Howells wrote:
> 
> Michael Kerrisk (man-pages) <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> 
>>        Note: There is no glibc wrapper for renameat2(); see NOTES.
> 
> statx, not renameat2.

Already reported, and fixed.

> 
>>                __u64 stx_blocks;      /* Number of 512B blocks allocated */
> 
> The following needs to be added in here:
> 
> 		__u64	stx_attributes_mask; /* Mask to show what's supported in stx_attributes */

Done.

> 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?

>>        statx() uses pathname, dirfd, and  flags  identify  the  target
>>        file in one of the following ways:
> 
> "to identify"

Already reported, and fixed.

>>        A pathname interpreted relative to a directory file descriptor
> 
> I think this is too wordy for a heading and it almost seems to merge into the
> description paragraph since it almost ends at the same right margin.  How
> about:
> 
> 	A directory-relative pathname

Yes. Changed as you suggest.

>>        By file descriptor
>>               If pathname is NULL, then the target  file  is  the  one
>>               referred  to  by  the  file descriptor dirfd.  dirfd may
>>               refer to any type of file, not just a  directory.   (The
>>               AT_EMPTY_PATH  flag  described  below  provides  similar
>>               functionality.)
>>
>>               ┌───────────────────────────────┐
>>               │FIXME                                                │
>>               ├───────────────────────────────┤
>>               │It appears that there  are  two  different  ways  of │
>>               │doing  the  same  thing:  specifying  the file to be │
>>               │stat'ed via a file descriptor.  Either,  we  specify │
>>               │'pathname'  as  NULL, or we specify 'pathname' as an │
>>               │empty string and  include  the  AT_EMPTY_PATH  flag. │
>>               │What's  the  rationale  for having two ways of doing │
>>               │this?                                                │
>>               └───────────────────────────────┘
> 
> AT_EMPTY_PATH wasn't there back in 2010.  I could eliminate the:
> 
> 	statx(fd, NULL, 0, ...);
> 
> option in favour of:
> 
> 	statx(fd, "", AT_EMPTY_PATH, ...);
> 
> Any thoughts either way, Al?
> 
> It would seem that AT_EMPTY_PATH should be redundant, though, since you can
> just set the pathname pointer to NULL.

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.

>>        The mask argument to statx() is used to tell the  kernel  which
>>        fields  the  caller is interested in.  mask is an ORed combina‐
>>        tion of the following constants:
>>
>>            STATX_TYPE          Want stx_mode & S_IFMT
>> 	     ...
>>            STATX_ALL           [All currently available fields]
>>
>>        Note the kernel does not reject values in mask other  than  the
>>        above.   Instead, it simply informs the caller which values are
>>        supported by this kernel and filesystem via the  statx.stx_mask
>>        field.  Therefore, do not simply set mask to UINT_MAX (all bits
>>        set), as one or more bits may, in the future, be used to  spec‐
>>        ify an extension to the buffer.
> 
> Is it worth mentioning STATX__RESERVED here, I wonder?  It's not something
> that you can actually use (you'll get EINVAL if you try), and will be renamed
> should it become used.

Under ERRORS I added:

.TP
.B EINVAL
Reserved flag specified in
.IR mask .

Okay?

>>        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.
> 
>>        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?

>>        Apart from stx_mask (which is described above), the  fields  in
>>        the statx structure are:
>>
>>        stx_mode
>>               The file type and mode.  See inode(7) for details.
>> 	 ...
> 
> Should this list either be in alphabetical order or offset-in-struct order?

Probably the same order as the struct.
> This needs to be added:
> 
> 	stx_attributes_mask
> 	      A mask indicating which bits in stx_attributes are supported by
> 	      the VFS and the filesystem.

Added.

>>    File attributes
>>        The stx_attributes field contains a  set  of  ORed  flags  that
>>        indicate additional attributes of the file:
> 
> Probably should mention stx_attributes_mask here also.  Perhaps:
> 
>        The stx_attributes field contains a set of ORed flags that
>        indicate additional attributes of the file.  Note that any
>        attribute that is not indicated as supported by
>        stx_attributes_mask has no usable value here.  The bits in
>        stx_attributes_mask correspond bit-by-bit to stx_attributes.

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.

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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