svn commit: r270444 - in head/sys: kern sys

[John-Mark Gurney]

John Baldwin wrote this message on Tue, Aug 26, 2014 at 15:09 -0400:
> On Monday, August 25, 2014 6:30:34 pm John-Mark Gurney wrote:
> > John Baldwin wrote this message on Mon, Aug 25, 2014 at 13:35 -0400:
> > > On Monday, August 25, 2014 07:02:41 PM Mateusz Guzik wrote:
> > > > On Mon, Aug 25, 2014 at 10:23:19AM -0400, John Baldwin wrote:
> > > > > On Sunday, August 24, 2014 09:04:09 AM Mateusz Guzik wrote:
> > > > > > Author: mjg
> > > > > > Date: Sun Aug 24 09:04:09 2014
> > > > > > New Revision: 270444
> > > > > > URL: http://svnweb.freebsd.org/changeset/base/270444
> > > > > > 
> > > > > > Log:
> > > > > >   Fix getppid for traced processes.
> > > > > >   
> > > > > >   Traced processes always have the tracer set as the parent.
> > > > > >   Utilize proc_realparent to obtain the right process when needed.
> > > > > 
> > > > > Are you sure this won't break things?  I know of several applications that
> > > > > expect a debugger to be the parent when attached and change behavior as a
> > > > > result (e.g. inserting a breakpoint on an assertion failure rather than
> > > > > generating a core).
> > > > 
> > > > Well, this is what linux and solaris do.
> > > 
> > > Interesting.
> > > 
> > > > I don't feel strongly about this change. If you really want I'm happy to
> > > > revert.
> > > 
> > > In general I'd like to someday have the debugger-debuggee relationship not 
> > > override parent-child and this is a step in that direction.  However, this 
> > > will break existing applications, so this needs to be clearly documented in 
> > > the release notes.  In addition, we should probably advertise how a process 
> > > can correctly determine if it is being run under a debugger (right now you can 
> > > do 'getppid()' and use strcmp or strstr on the p_comm of that pid so you can 
> > > do different things for "gdb" vs "gcore", etc. so just checking P_TRACED from 
> > > kinfo_proc wouldn't be equivalent in functionality)
> > 
> > But what about when you attach gdb to a running process...  That
> > doesn't magicly make the now debugged process a child of gdb does it?
> 
> % cat hello.c
> #include <stdio.h>
> 
> int
> main()
> {
>         printf("hello world\n");
>         (void)getchar();
>         return (0);
> }
> % cc -g hello.c -o hello
> % ./hello 
> hello world
> load: 9.81  cmd: hello 42599 [ttyin] 1.67r 0.00u 0.00s 0% 1056k
> 
>  < different window >
> 
> % ps -O ppid -p `pgrep hello`
>   PID  PPID  TT  STAT      TIME COMMAND
> 42599  5340  16  I+     0:00.00 ./hello
> % gdb hello `pgrep hello`
> GNU gdb 6.1.1 [FreeBSD]
> ...
> (gdb) <hit Ctrl-Z>
> Suspended
> % ps -O ppid -p `pgrep hello`
>   PID  PPID  TT  STAT      TIME COMMAND
> 42599 45079  16  TX+    0:00.00 ./hello

Wow, learn something new every day...

But doesn't that break apps that use getppid to signal their parent
that forked them?

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."