Re: [PATCH RFC 2/3] open.2: add O_EMPTYPATH documentation

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hello Aleksa,

You write "5.FOO" in these patches. When do you expect these changes to 
land in the kernel?

On 10/3/19 4:55 PM, Aleksa Sarai wrote:
> Some of the wording around empty paths in path_resolution(7) also needed
> to be reworked since it's now legal (if you pass O_EMPTYPATH).
> 
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  man2/open.2            | 42 +++++++++++++++++++++++++++++++++++++++++-
>  man7/path_resolution.7 | 17 ++++++++++++++++-
>  2 files changed, 57 insertions(+), 2 deletions(-)
> 
> diff --git a/man2/open.2 b/man2/open.2
> index b0f485b41589..7217fe056e5e 100644
> --- a/man2/open.2
> +++ b/man2/open.2
> @@ -48,7 +48,7 @@
>  .\" FIXME . Apr 08: The next POSIX revision has O_EXEC, O_SEARCH, and
>  .\" O_TTYINIT.  Eventually these may need to be documented.  --mtk
>  .\"
> -.TH OPEN 2 2018-04-30 "Linux" "Linux Programmer's Manual"
> +.TH OPEN 2 2019-10-03 "Linux" "Linux Programmer's Manual"

No need to update the timestamp. I have scripts that handle this
automatically.

>  .SH NAME
>  open, openat, creat \- open and possibly create a file
>  .SH SYNOPSIS
> @@ -421,6 +421,21 @@ was followed by a call to
>  .BR fdatasync (2)).
>  .IR "See NOTES below" .
>  .TP
> +.BR O_EMPTYPATH " (since Linux 5.FOO)"
> +If \fIpathname\fP is an empty string, re-open the the file descriptor given as

In general, I prefer the general form

.I pathname

over \fIpathname\fP. 

If you would be willing to cahnge that, it would  save me a little work.
(And likewise throughout the rest of the patch.)

> +the \fIdirfd\fP argument to
> +.BR openat (2).
> +This can be used with both ordinary (file and directory) and \fBO_PATH\fP file
> +descriptors, but cannot be used with
> +.BR AT_FDCWD
> +(or as an argument to plain
> +.BR open (2).) When re-opening an \fBO_PATH\fP file descriptor, the same "link

There's a formatting problem here which can be fixed by inserting a 
newline before "When".

> +mode" restrictions apply as with re-opening through
> +.BR proc (5)
> +(see
> +.BR path_resolution "(7) and " symlink (7)
> +for more details.)
> +.TP
>  .B O_EXCL
>  Ensure that this call creates the file:
>  if this flag is specified in conjunction with
> @@ -668,6 +683,13 @@ with
>  (or via procfs using
>  .BR AT_SYMLINK_FOLLOW )
>  even if the file is not a directory.
> +You can even "re-open" (or upgrade) an
> +.BR O_PATH
> +file descriptor by using
> +.BR O_EMPTYPATH
> +(see the section for
> +.BR O_EMPTYPATH
> +for more details.)
>  .IP *
>  Passing the file descriptor to another process via a UNIX domain socket
>  (see
> @@ -958,6 +980,15 @@ is not allowed.
>  (See also
>  .BR path_resolution (7).)
>  .TP
> +.B EBADF
> +.I pathname
> +was an empty string (and
> +.B O_EMPTYPATH
> +was passed) with
> +.BR open (2)
> +(instead of
> +.BR openat (2).)
> +.TP
>  .B EDQUOT
>  Where
>  .B O_CREAT
> @@ -1203,6 +1234,15 @@ The following additional errors can occur for
>  .I dirfd
>  is not a valid file descriptor.
>  .TP
> +.B EBADF
> +.I pathname
> +was an empty string (and
> +.B O_EMPTYPATH
> +was passed), but the provided
> +.I dirfd
> +was an invalid file descriptor (or was
> +.BR AT_FDCWD .)
> +.TP
>  .B ENOTDIR
>  .I pathname
>  is a relative pathname and
> diff --git a/man7/path_resolution.7 b/man7/path_resolution.7
> index 46f25ec4cdfa..85dd354e9a93 100644
> --- a/man7/path_resolution.7
> +++ b/man7/path_resolution.7
> @@ -22,7 +22,7 @@
>  .\" the source, must acknowledge the copyright and authors of this work.
>  .\" %%%LICENSE_END
>  .\"
> -.TH PATH_RESOLUTION 7 2017-11-26 "Linux" "Linux Programmer's Manual"
> +.TH PATH_RESOLUTION 7 2019-10-03 "Linux" "Linux Programmer's Manual"
>  .SH NAME
>  path_resolution \- how a pathname is resolved to a file
>  .SH DESCRIPTION
> @@ -198,6 +198,21 @@ successfully.
>  Linux returns
>  .B ENOENT
>  in this case.
> +.PP
> +As of Linux 5.FOO, an empty path argument can be used to indicate the "re-open"
> +an existing file descriptor if
> +.B O_EMPTYPATH
> +is passed as a flag argument to
> +.BR openat (2),
> +with the
> +.I dfd
> +argument indicating which file descriptor to "re-open". This is approximately
> +equivalent to opening
> +.I /proc/self/fd/$fd

.IR /proc/self/fd/$fd ,

> +where
> +.I $fd
> +is the open file descriptor to be "re-opened".
> +

No blank line here.

>  .SS Permissions
>  The permission bits of a file consist of three groups of three bits; see
>  .BR chmod (1)
> 

Thanks,

Michael

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