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

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/[email protected]
          Reporter: jrw32982-/[email protected]
        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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https://bugzilla.kernel.org/show_bug.cgi?id=134971

Michael Kerrisk <[email protected]> changed:

What |Removed |Added
----------------------------------------------------------------------------

             Status|NEW                         |RESOLVED
                 CC|                            |[email protected]
         Resolution|---                         |CODE_FIX

--- Comment #1 from Michael Kerrisk <[email protected]> --- 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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https://bugzilla.kernel.org/show_bug.cgi?id=134971

--- Comment #2 from John Wiersba <jrw32982-/[email protected]> --- Thanks, Michael!  That does make it clearer!

From: "bugzilla-daemon-590EEB7GvNiWaY/[email protected]"
<bugzilla-daemon-590EEB7GvNiWaY/[email protected]>
To: jrw32982-/[email protected]
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 <[email protected]> changed:

          What    |Removed                    |Added
----------------------------------------------------------------------------
            Status|NEW                        |RESOLVED
                CC|                            |[email protected] Resolution|---                        |CODE_FIX

--- Comment #1 from Michael Kerrisk <[email protected]> --- 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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