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

[Bryan Drewery]

On 8/26/2014 4:23 PM, John Baldwin wrote:
> On Tuesday, August 26, 2014 3:32:10 pm John-Mark Gurney wrote:
>> 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?
> 
> Until mjg@'s commit, yes.  It's been that way in FreeBSD at least for
> as long as I can remember.  Certainly back to 4.x.
> 

Fun things can happen (local DoS) when you trace your own parent too.
Recently fixed by other commits. Easily thwarted by
security.bsd.unprivileged_proc_debug=0.

-- 
Regards,
Bryan Drewery

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