gdbserver for arm does not support multi-process debugging

[Luis Machado]

Hi,

 From reading the GDB NEWS file, proper support for gdbserver 
remote/extended-remote fork/vfork event reporting was included in GDB 
7.11, commit 19d9d4efd18bcc633e99cb6a3e39bd9b22ca70ce and others.

I'd give a more recent GDB a try, since GDB 7.6 is mora than 5 years old 
now.

On 10/29/19 7:42 AM, 刘康 wrote:
> Hi,
> My gdbserver andarm-linux-androideabi-gdbversion are both 7.6. and my 
> reproduction way is as follow:
> 
> 
> 
> and my gdbremotescript.gdb is
> 
> 
> the debug target "usb_upgrade_check" is the ELF of the source code 
> above(just ignore its strange name...it means nothing:-) Â  Â  )
> 
> my target and host is communicating via TCP/IP protocol and my target 
> gdbserver launch parameter is
> 
> 
> my target's kernel version is 3.10.86
> 
> When everything is ready I do "set detach-on-fork off" and "set 
> follow-fork-mode child" at host and execute the continue command.
> The problem comes: host always has only one inferior which trace parent 
> process, no new inferior will be created and the child process could not 
> be traced in any way.
> I have also tried version 7.9 and the performance is the same.
> 
> My kernel and libc works well for syscall like ptrace and waitpid , 
> because I have make sure that the linux_test_for_tracefork() has no 
> problem by adding gdbserver some log.
> 
> At 2019-10-28 20:26:33, "Luis Machado" <luis.machado@linaro.org> wrote:
>>Do let us know what you see. If it still doesn't work with the most 
>>recent versions, it may be a real bug somewhere (not necessarily in 
>>gdb/gdbserver).
>>
>>On 10/28/19 8:12 AM, 刘康 wrote:
>>> Hi,
>>> I see, maybe my gdbserver version is too old...I'll compile a latest 
>>> version and try again.
>>> 
>>> Thank you so much for your help!
>>> 
>>> 
>>> At 2019-10-24 18:38:46, "Luis Machado" <luis.machado@linaro.org> wrote:
>>>>Hi,
>>>>
>>>>I just ran a quick check (foll-fork.exp and follow-vfork.exp tests) on a 
>>>>arm machine and things worked as expected.
>>>>
>>>>What version of GDB/kernel are you using?
>>>>
>>>>On 10/24/19 2:32 AM, 刘康 wrote:
>>>>> Hi Luis,
>>>>> 
>>>>> Yes, I know that gdbserver willcatch PTRACE_EVENT_FORK in 
>>>>> gdb/gdbserver/linux-low.c:handle_extended_wait and get the child 
>>>>> process's pid via PTRACE_GETEVENTMSG.
>>>>> 
>>>>> I found the way of X86 version gdb to deal with PTRACE_EVENT_FORKwill 
>>>>> set waitstatus toTARGET_WAITKIND_FORKEDÂ and add a new inferior .What's 
>>>>> more ,it can trace child/parent process automatically as long as I set 
>>>>> "follow-fork-mode" to "child" or "parent".
>>>>> 
>>>>> But it seems like gdbserver will not tell PTRACE_EVENT_FORK to thehost 
>>>>> arm-XXX-gdb this event ,as a result , user cannot trace child process or 
>>>>> add a new inferior no matter what value of "detach-on-fork off" and 
>>>>> "follow-fork-mode" I have tried.
>>>>> 
>>>>> At 2019-10-18 20:27:19, "Luis Machado" <luis.machado@linaro.org> wrote:
>>>>>>Hi,
>>>>>>
>>>>>>On 10/17/19 3:19 AM, 刘康 wrote:
>>>>>>> Dear gdb@sourceware.org
>>>>>>> 
>>>>>>> 
>>>>>>> I am researching the gdbserver's source code for ARM, what confused me is that it does not support multi-process to debug the target's child process, cause it dose not interesting in PTRACE_EVENT_FORK, I don't know whether it's a bug or it's just a feature by design.
>>>>>>
>>>>>>The handling of PTRACE_EVENT_FORK, PTRACE_EVENT_VFORK and 
>>>>>>PTRACE_EVENT_CLONE is in generic linux code. See 
>>>>>>gdb/gdbserver/linux-low.c:handle_extended_wait.
>>>>>>
>>>>>>As long as the kernel supports reporting such events via ptrace and we 
>>>>>>are debugging using extended remote mode (not regular remote mode), it 
>>>>>>should work. If it doesn't, then we may have a problem that we should fix.
>>>>>>
>>>>>>Does that answer your question?
>>>>>>
>>>>>>Luis
>>>>> 
>>>>> 
>>>>> 
>>> 
>>> 
>>> 
> 
> 
>