debugging threaded apps running under a clone(CLONE_NEWPID)

[Serge E. Hallyn]

Quoting Cedric Le Goater (clg at fr.ibm.com):
> Hello !
> 
> to debug threaded apps, gdb uses a special libthread_db which seeks in the
> symbols of the exec the list of running threads. This list contains the glibc 
> 'struct pthread' descriptor with pids and tids which values are relative 
> to the namespace in which the threads were created.  
> 
> unless you run gdb in the same pid namespace, gdb will not see any thread
> in the debugged app. this is frustrating for some scenarios and some 
> support from the kernel would be needed to address this issue. Here
> are some ideas :
> 
>    . enter a pid namespace. hard.  
> 
>    . expose the pid numbers of a task in its pid namespaces, through some 
>      ways like /proc/self/stat or /proc/self/pids and modify gdb to make 
>      the conversion.
> 
> How would you do it ? 

perhaps a /proc/$$/vpid which prints out
	task_pid_nr_ns(tsk, current->nsproxy->pidns)
then each thread opens an fd for /proc/self/vpid and passes it
over an af_unix socket to the parent gdb, which then reads it
to get the pid in its own ns?

-serge