linux-kernel - Re: debugging threaded apps running under a clone(CLONE

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Message-ID: <20090223182153.GD13151@us.ibm.com>
Date:	Mon, 23 Feb 2009 12:21:53 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Cedric Le Goater <clg@...ibm.com>
Cc:	Linux Containers <containers@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: debugging threaded apps running under a clone(CLONE_NEWPID)

Quoting Cedric Le Goater (clg@...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
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