[Bug 134971] New: Clarify kill(2) man-page for permission to "send" signal 0

[Bug 134971] New: Clarify kill(2) man-page for permission to "send" signal 0

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=134971

            Bug ID: 134971
           Summary: Clarify kill(2) man-page for permission to "send"
                    signal 0
           Product: Documentation
           Version: unspecified
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: man-pages
          Assignee: documentation_man-pages-ztI5WcYan/vQLgFONoPN62D2FQJk+8+b@public.gmane.org
          Reporter: jrw32982-/E1597aS9LQAvxtiuMwx3w@public.gmane.org
        Regression: No

The description of errors relating to signal 0 leaves room for
misinterpretation.

The kill(2) man-page says that for signal 0, no signal is sent, mentions that
error checking is performed and that signal 0 can be used to check for process
existence.  Then it says the a process must have perms to send a signal. 
Later, for EPERM, it says that the process doesn't have perms to send the
signal.

But for signal 0, no signal is sent.  What's not 100% clear is that, *even for
signal 0*, a process must have perms (CAP_KILL or matching user ID).  I
misinterpreted it to mean that any process could send signal 0 to any other
process (regardless of user ID), but could not send a non-zero signal to a
process without the appropriate perms.  But this is wrong.  The reason for the
confusion is the word "send/sent" not being used 100% consistently.

Maybe the wording could be enhanced to say "For a process to have permission to
send a signal, including signal 0, it must either be ...".

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[Bug 134971] Clarify kill(2) man-page for permission to "send" signal 0

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=134971

Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
         Resolution|---                         |CODE_FIX

--- Comment #1 from Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
Hello John,

I agree the page could be clearer. What I did was change the paragraph
describing sig==0 to:

       If sig is 0, then no signal is sent, but existence and  permission
       checks  are  still  performed;  this  can be used to check for the
       existence of a process ID or process group ID that the  caller  is
       permitted to signal.

I believe that makes the point clear, so I'll close this for now. Please reopen
if you think there's still a problem.

Thanks,

Michael

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[Bug 134971] Clarify kill(2) man-page for permission to "send" signal 0

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=134971

--- Comment #2 from John Wiersba <jrw32982-/E1597aS9LQAvxtiuMwx3w@public.gmane.org> ---
Thanks, Michael!  That does make it clearer!



      From: "bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r@public.gmane.org"
<bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r@public.gmane.org>
 To: jrw32982-/E1597aS9LQAvxtiuMwx3w@public.gmane.org 
 Sent: Saturday, July 16, 2016 6:24 AM
 Subject: [Bug 134971] Clarify kill(2) man-page for permission to "send" signal
0

https://bugzilla.kernel.org/show_bug.cgi?id=134971

Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> changed:

          What    |Removed                    |Added
----------------------------------------------------------------------------
            Status|NEW                        |RESOLVED
                CC|                            |mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
        Resolution|---                        |CODE_FIX

--- Comment #1 from Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
Hello John,

I agree the page could be clearer. What I did was change the paragraph
describing sig==0 to:

      If sig is 0, then no signal is sent, but existence and  permission
      checks  are  still  performed;  this  can be used to check for the
      existence of a process ID or process group ID that the  caller  is
      permitted to signal.

I believe that makes the point clear, so I'll close this for now. Please reopen
if you think there's still a problem.

Thanks,

Michael

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