Re: [patch] Fix attaching to Linux stopped processes

[Jan Kratochvil <jan.kratochvil@redhat.com>]

On Mon, 18 Sep 2006 00:26:22 +0200, Daniel Jacobowitz wrote:
> On Mon, Sep 18, 2006 at 12:12:28AM +0200, Jan Kratochvil wrote:
> > Hi,
> > 
> > sleep 1h& pid=$!; kill -STOP $pid; gdb sleep $pid
> > 	->
> > Attaching to program: /bin/sleep, process 20768
> > ../../gdb/linux-nat.c:1057: internal-error: linux_nat_attach: Assertion `pid == GET_PID (inferior_ptid) && WIFSTOPPED (status) && WSTOPSIG (status) == SIGSTOP' failed.
> 
> > +  /* Do not check `WSTOPSIG (status) == SIGSTOP' as the status may be
> > +     arbitrary - depending on the signal that stopped the processes.
> > +     If the process was running we get SIGSTOP, if it was already stopped
> > +     by SIGSTOP we get 0.  The value gets used for `PTRACE_CONT'.  */
> 
> So I checked.  Sure enough, on my 2.6.18-rc4
> installation, this is _not_ what happens.  Instead, the traditional
> thing happens: wait hangs and no new event is returned.
> 
> Is this new in kernel.org?  Or is it a Red Hat kernel patch?  Where
> did it come from?

Thanks for checking it.
OK, the patch was originally for GDB on Red Hat "kernel-2.6.17-1.2647.fc6".

Still it is reproducible even on both linux-2.6.17.13 and linux-2.6.18-rc4, you
just need to "kill -CONT $pid" the inferior as gdb(1) will hang during
attaching to it.  Screenshot (sorry for graphics):
	http://www.jankratochvil.net/priv/gdb.png
gdb is built from the current clean CVS snapshot.

Also the patch looks right to me - after `PT_ATTACH' it is appropriate that
`WSTOPSIG (status)' will report the original signal that stopped process,
(WSTOPSIG (status) == SIGSTOP) <=> (process was in running/sleeping mode).


Regards,
Jan