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Date: Mon, 28 Feb 2011 14:29:41 +0100
From: Tejun Heo <tj@kernel.org>
To: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Oleg Nesterov <oleg@redhat.com>, Roland McGrath <roland@redhat.com>,
        jan.kratochvil@redhat.com, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org, akpm@linux-foundation.org
Subject: Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after
 PTRACE_ATTACH
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On Mon, Feb 28, 2011 at 02:16:48PM +0100, Denys Vlasenko wrote:
> (gdb) print getpid()
> 
> gdb modifies IP, sets breakpoint on return address, and issues PTRACE_CONT(0).
> Kernel has to put the tracee into group-stop, right?
> Becuase if it doesn't, if it makes tracee run, then the kernel is
> still broken. For example,
> stracing a program and sending SIGSTOP on it won't work: the sequence
> of events will be
> got SIGSTOP because SIGSTOP was delivered
> PTRACE_SYSCALL(SIGSTOP) - "inject it"
> got SIGSTOP because tracee is in group-stop now
> PTRACE_SYSCALL(SIGSTOP) - equivalent to PTRACE_SYSCALL(0)
>   because we aren't in signal delivery ptrace-stop
> and tracee continues.
> 
> That's why I think gdb's "print getpid()" today depends on the bug.
> If we simply fix the bug (by making PTRACE_CONT/SYSCALL(0)
> re-enter group-stop), then "print getpid()" will stop working
> for stopped tracees.

There's no reason to make the tracee re-enter group stop after pulling
it out to execute 'print getpid()'.  The only thing necessary is a way
for the debugger to find out that group stop has been lifted.  The
debugger then can resume the tracee if it wishes so.  ie. group stop
becomes a trap point + a state which the debugger can monitor.  If the
debugger wants the tracee to follow the jctl behavior, it can do so by
resuming the tracee as it sees fit.

Thanks.

-- 
tejun
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