PERL Miscellaneous

Author

kill 0

Brandon Metcalf

2005-07-26, 5:02 pm

I'm seeing a behavior with using "kill 0,$pid" to determine if a
process is running that I didn't expect. It seems that only root can
correctly get the status on processes that root or another user that
is different from the one calling kill() owns. For example,

$ whoami
bmetcalf
$ ps -ef|grep gdm
root 965 1 0 Jul19 ? 00:00:00 /usr/bin/gdm
root 978 965 0 Jul19 ? 00:00:00 /usr/bin/gdm
root 979 978 0 Jul19 ? 00:37:20 /usr/X11R6/bin/X :0 -deferglyphs 16 -nolisten tcp -audit 0 -auth /var/lib/gdm/:0.Xauth -nolisten tcp vt7
bmetcalf 24443 23845 0 09:19 pts/5 00:00:00 grep gdm
$ perl -le '$a = kill 0,965;print $a'
0
$ su
Password:
# perl -le '$a = kill 0,965;print $a'
1

Is this the expected behavior? I'm sure it is since every platform
and version of Perl I've tried behave the same way.

--
Brandon

xhoster@gmail.com

2005-07-26, 5:02 pm

Brandon Metcalf <bmetcalf@nortel.com> wrote:
> I'm seeing a behavior with using "kill 0,$pid" to determine if a
> process is running that I didn't expect. It seems that only root can
> correctly get the status on processes that root or another user that
> is different from the one calling kill() owns. For example,
>
....
>
> Is this the expected behavior? I'm sure it is since every platform
> and version of Perl I've tried behave the same way.

Given the docs for kill, it is the behavior I would expect (** mine):

If SIGNAL is zero, no signal is sent to the process. This
is a useful way to check that the process is alive and
**hasn't changed its UID.** See perlport for notes on the
portability of this construct.

And by all means, do see perlport.

Xho

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Brandon Metcalf

2005-07-26, 5:02 pm

On 2005-07-26, xhoster@gmail.com <xhoster@gmail.com> wrote:
> Brandon Metcalf <bmetcalf@nortel.com> wrote:
> ...
>
> Given the docs for kill, it is the behavior I would expect (** mine):
>
> If SIGNAL is zero, no signal is sent to the process. This
> is a useful way to check that the process is alive and
> **hasn't changed its UID.** See perlport for notes on the
> portability of this construct.
>
> And by all means, do see perlport.

Sure, I looked at perlport but didn't see anything that explained what
I'm seeing. Also, I don't see anything in the documentation for
kill() what would explain this behavior. You highlighted "hasn't
changed its UID", but I don't understand what that has to do with the
problem.

--
Brandon

Jim Gibson

2005-07-26, 5:02 pm

In article <20050726113448.218$FQ@newsreader.com>, <xhoster@gmail.com>
wrote:

> Brandon Metcalf <bmetcalf@nortel.com> wrote:
> ...
>
> Given the docs for kill, it is the behavior I would expect (** mine):
>
> If SIGNAL is zero, no signal is sent to the process. This
> is a useful way to check that the process is alive and
> **hasn't changed its UID.** See perlport for notes on the
> portability of this construct.

And from the 'man kill' page:

"Only the super-user may send signals to other users' processes."

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Brandon Metcalf

2005-07-26, 5:02 pm

On 2005-07-26, Brandon Metcalf <bmetcalf@nortel.com> wrote:
> On 2005-07-26, xhoster@gmail.com <xhoster@gmail.com> wrote:
>
> Sure, I looked at perlport but didn't see anything that explained what
> I'm seeing. Also, I don't see anything in the documentation for
> kill() what would explain this behavior. You highlighted "hasn't
> changed its UID", but I don't understand what that has to do with the
> problem.

OK. This explains it:

$ ps -ef|grep gdm
root 965 1 0 Jul19 ? 00:00:00 /usr/bin/gdm
root 978 965 0 Jul19 ? 00:00:00 /usr/bin/gdm
root 979 978 0 Jul19 ? 00:40:01 /usr/X11R6/bin/X :0 -deferglyphs 16 -nolisten tcp -audit 0 -auth /var/lib/gdm/:0.Xauth -nolisten tcp vt7
bmetcalf 25710 25677 0 11:45 pts/13 00:00:00 grep gdm
$ perl -le 'kill 0,965 or print $!'
Operation not permitted

So, one has to have permission to actually send a signal 0.

--
Brandon

Brandon Metcalf

2005-07-26, 5:02 pm

On 2005-07-26, Jim Gibson <jgibson@mail.arc.nasa.gov> wrote:
> In article <20050726113448.218$FQ@newsreader.com>, <xhoster@gmail.com>
> wrote:
>
>
> And from the 'man kill' page:
>
> "Only the super-user may send signals to other users' processes."

Sure, I read that as well. But Perl's implementation of signal 0 is
special in that it doesn't actually send a signal to the process. It
simply checks to see if it's alive. It doesn't seem that special
permissions would be required to check the existence of a process.

But, per my last post, proper permissions are required.

--
Brandon

Villy Kruse

2005-07-27, 4:02 am

On Tue, 26 Jul 2005 15:17:03 +0000 (UTC),
Brandon Metcalf <bmetcalf@nortel.com> wrote:

> I'm seeing a behavior with using "kill 0,$pid" to determine if a
> process is running that I didn't expect. It seems that only root can
> correctly get the status on processes that root or another user that
> is different from the one calling kill() owns. For example,
>

On unix you have three posibilities when using kill(0, pid).

- kill returns 0 The program is running.
- kill returns -1 and errno = EPERM The program is running.
- kill returns -1 and errno = ESRCH The program is not running.

In your case you probably encountered the second posibility.

Villy

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