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SubjectRe: [PATCH] ptrace on stopped processes (2.4)
Hi All,
I'm working on adding a function

void debugbreak(void);

to glibc. I got the inspiration for this from Windows. Windows has a
DebugBreak() function, which when invoked from a process, spawns a
debugger, which then attaches to the process. I think this would be
invaluable to linux developers in situations where they currently have to
put an infinite loop and attach to a running process (say, to stop in a
library that has been LD_PRELOADed).

To this end, I've been experimenting and found out that gdb can't attach
to a process that has been stopped. I'd like to send a SIGSTOP as soon as
debugbreak() is invoked, so that all threads are stopped in a state close
to the one they were in, when the debugbreak() was invoked.

I spoke to Vic Zandy about this and he informed me that he had submitted a patch
that would allow ptrace to attach to stopped processes also (the thread of
discussion is pasted below). I believe the patch was not accepted at that time.
I was wondering what the official line on this is? If there are no serious
objections, will the community consider accepting the patch? It would go a long
way in helping me accomplish my goal.

Thank you,
Raj

PS - I am not subscribed to the kernel mailing list. Would appreciate your
cc-ing your reply to me.
-------------------------------------------------------------------------------
Vic Zandy wrote --

This is to respond to feedback for the ptrace patch I sent toward the
end of december. The original message is below.

From: Alan Cox <alan@lxorguk.ukuu.org.uk>
> > + if (signr == SIGSTOP && current->ptrace & PT_PTRACED)
> This does not I suspect do what you think - surely you want brackets ?

I agree the second term should be wrapped in parens (it is now in the
patch below); but isn't that logically equivalent to what I had?

From: Mike Coleman <mkc@mathdogs.com>:
> Also, is this something that used to work? Or would this be a change in the
> semantics of ptrace?

This is a change of semantics at least going back to 2.2.

>> Another bug is that it is not possible to use PTRACE_DETACH to leave a
>> process stopped, because ptrace ignores SIGSTOPs sent by the tracing
>> process.
>
> Unless I'm missing something (frequently the case), there are two cases here:
> (1) the tracer wants to leave the tracee stopped, and (2) the tracer wants the
> process to continue running in as natural a way as possible, meaning without
> sending it a SIGCONT (which can cause the SIGCONT signal handler to execute).
> As things currently stand, we have behavior (2), and (1) is not possible.
> With your change, we'd have behavior (1), and (2) would not be possible.

I agree that the ability to do (2) should be preserved, but I don't
see how this patch breaks it; do you have an example?

From: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>:
>> --- linux-2.4.16/kernel/ptrace.c Wed Nov 21 16:43:01 2001
>> +++ linux-2.4.16.1/kernel/ptrace.c Fri Dec 21 10:42:44 2001
>> @@ -89,8 +89,10 @@
>> SET_LINKS(task);
>> }
>> write_unlock_irq(&tasklist_lock);
>> -
>> - send_sig(SIGSTOP, task, 1);
>> + if (task->state != TASK_STOPPED)
>> + send_sig(SIGSTOP, task, 1);
>> + else
>> + task->exit_code = SIGSTOP;
>> return 0;
>>
>> bad:
>
> It seems that trace is started in the place different from
> usual. Then, I think PTRACE_KILL doesn't work.

I don't agree, it seems to work for me.

I'd still want to check uml and subterfuge, which I'll do after these
points are cleared up.

Thanks,
Vic

From: vic <zandy@cs.wisc.edu>
Subject: [PATCH] ptrace on stopped processes (2.4)
To: marcelo@conectiva.com.br
Cc: linux-kernel@vger.kernel.org, torvalds@transmeta.com,
alan@lxorguk.ukuu.org.uk
Date: Fri, 21 Dec 2001 13:53:32 -0600

This patch fixes a couple problems with ptrace's interaction with
stopped processes on Linux 2.4.

The most significant bug is that gdb cannot attach to a stopped
process. Specifically, the wait that follows the PTRACE_ATTACH will
block indefinitely.

Another bug is that it is not possible to use PTRACE_DETACH to leave a
process stopped, because ptrace ignores SIGSTOPs sent by the tracing
process.
This patch is against 2.4.16 on x86. I have tested gdb and strace.
After this patch is reviewed, I would be happy to submit an analogous
patch for the other platforms, although I cannot test it.

Vic Zandy

--- linux-2.4.16/arch/i386/kernel/signal.c Fri Sep 14 16:15:40 2001
+++ linux-2.4.16.1/arch/i386/kernel/signal.c Wed Jan 16 22:19:16 2002
@@ -620,9 +620,9 @@
continue;
current->exit_code = 0;

- /* The debugger continued. Ignore SIGSTOP. */
- if (signr == SIGSTOP)
- continue;
+ /* The debugger continued. */
+ if (signr == SIGSTOP && (current->ptrace & PT_PTRACED))
+ continue; /* ignore SIGSTOP */

/* Update the siginfo structure. Is this good? */
if (signr != info.si_signo) {
--- linux-2.4.16/kernel/ptrace.c Wed Nov 21 16:43:01 2001
+++ linux-2.4.16.1/kernel/ptrace.c Fri Dec 21 10:42:44 2001
@@ -89,8 +89,10 @@
SET_LINKS(task);
}
write_unlock_irq(&tasklist_lock);
-
- send_sig(SIGSTOP, task, 1);
+ if (task->state != TASK_STOPPED)
+ send_sig(SIGSTOP, task, 1);
+ else
+ task->exit_code = SIGSTOP;
return 0;

bad:
-
-
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