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    SubjectRe: [PATCH] open(2): document O_PATH
    Date
    "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:

    > Hi Mike,
    >
    > [Al, Aneesh, there is a question for you below]
    >
    > On Sun, Apr 29, 2012 at 3:25 PM, Mike Frysinger <vapier@gentoo.org> wrote:
    >> Signed-off-by: Mike Frysinger <vapier@gentoo.org>
    >> ---
    >>  man2/open.2 |   16 +++++++++++++++-
    >>  1 file changed, 15 insertions(+), 1 deletion(-)
    >>
    >> diff --git a/man2/open.2 b/man2/open.2
    >> index a655fae..61689cf 100644
    >> --- a/man2/open.2
    >> +++ b/man2/open.2
    >> @@ -47,7 +47,6 @@
    >>  .\" FIXME . Apr 08: The next POSIX revision has O_EXEC, O_SEARCH, and
    >>  .\" O_TTYINIT.  Eventually these may need to be documented.  --mtk
    >>  .\" FIXME Linux 2.6.33 has O_DSYNC, and a hidden __O_SYNC.
    >> -.\" FIXME: Linux 2.6.39 added O_PATH
    >>  .\"
    >>  .TH OPEN 2 2012-02-27 "Linux" "Linux Programmer's Manual"
    >>  .SH NAME
    >> @@ -428,6 +427,21 @@ For a discussion of the effect of
    >>  in conjunction with mandatory file locks and with file leases, see
    >>  .BR fcntl (2).
    >>  .TP
    >> +.B O_PATH
    >> +The path is opened for accessing its file attributes only.  Since the file
    >> +itself is not opened, most operations (such as
    >> +.BR read (2)
    >> +or
    >> +.BR write (2))
    >> +will return
    >> +.BR EBADF .
    >> +You may however use functions that operate on the file descriptor itself
    >> +such as
    >> +.BR close (2),
    >> +functions that duplicate file descriptors, and as the dirfd argument with
    >> +all the *at style of functions (e.g.
    >> +.BR openat (2)).
    >> +.TP
    >>  .B O_SYNC
    >>  The file is opened for synchronous I/O.
    >>  Any
    >
    > Thanks for the prod. Adding this has been on my list for a while.
    > There's actually quite a lot more to say, and I've written the patch
    > below. Could you check it over.
    >
    > Al, Aneesh, there is one question for you in a FIXME below. Could you
    > take a look please?
    >
    > Thanks,
    >
    > Michael
    >
    >
    > --- a/man2/open.2
    > +++ b/man2/open.2
    > @@ -428,6 +427,66 @@ For a discussion of the effect of
    > in conjunction with mandatory file locks and with file leases, see
    > .BR fcntl (2).
    > .TP
    > +.BR O_PATH " (since Linux 2.6.39)"
    > +.\" commit 1abf0c718f15a56a0a435588d1b104c7a37dc9bd
    > +Obtain a file descriptor that is used only for fetching file attributes.
    > +The file itself is not opened, and most file operations (e.g.,
    > +.BR read (2),
    > +.BR write (2))
    > +fail with the error
    > +.BR EBADF .
    > +The following operations
    > +.I can
    > +be performed on the resulting file descriptor:
    > +.RS
    > +.IP * 3
    > +Closing the file descriptor
    > +.RB ( close (2)).
    > +.\" FIXME Commit 1abf0c718f15a56a0a435588d1b104c7a37dc9bdcw
    > +.\" message says that closing the file descriptor does not affect
    > +.\" POSIX locks or dnotify.
    > +.\" However, my testing shows that it DOES affect dnotify (and inotify).
    > +.\" Does close() affect POSIX locks?
    > +.IP *


    IIUC what an O_PATH descritor doesn't do is to flush dnotify markers

    if (likely(!(filp->f_mode & FMODE_PATH))) {
    dnotify_flush(filp, id);
    locks_remove_posix(filp, id);
    }

    I don't know much about markers, but as per fsnotify_backend.h

    /*
    * a mark is simply an object attached to an in core inode which allows an
    * fsnotify listener to indicate they are either no longer interested in events
    * of a type matching mask or only interested in those events.
    *
    * these are flushed when an inode is evicted from core and may be flushed
    * when the inode is modified (as seen by fsnotify_access). Some fsnotify users
    * (such as dnotify) will flush these when the open fd is closed and not at
    * inode eviction or modification.
    */
    struct fsnotify_mark {

    It also doesn't remove posix locks. I tested this with a test prg

    struct flock flock;
    flock.l_type = F_WRLCK;
    flock.l_whence = SEEK_SET;
    flock.l_start = 0;
    flock.l_len = 0;
    fd = open(argv[1], O_RDWR);
    fcntl(fd, F_SETLKW, &flock);
    fd = open(argv[1], O_PATH);
    close(fd);

    The close doesn't result in lock release.

    > +Duplicating the file descriptor
    > +.RB ( dup (2),
    > +.BR fcntl (2)
    > +.BR F_DUPFD ,
    > +etc.).
    > +.IP *

    -aneesh

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