Re: ptrace single-stepping change breaks Wine

[Davide Libenzi]

On Wed, 29 Dec 2004, Linus Torvalds wrote:

> On Wed, 29 Dec 2004, Davide Libenzi wrote:
> > 
> > I think same. My test simply let the function processing to let thru and 
> > reach the fake signal sending point.
> 
> No, your test-case doesn't even send a signal at all, because your
> test-program just uses a PTRACE_SINGLESTEP without any "send signal" - so
> "exit_code" will be zero after the debuggee gets released from the
> "ptrace_notify()", and the suspect code will never be executed..
> 
> The problem I think I see (and which the comment alludes to) is that if
> the controlling process continues the debuggee with a signal, we'll be
> doing the wrong thing: the code in do_syscall_trace() will take that
> signal (in "current->exit_code") and send it as a real signal to the
> debuggee, but since it is debugged, it will be caught (again) by the
> controlling process, which apparently in the case of Wine gets very
> confused.
> 
> So I _think_ the problem happens for the following schenario:
>  - wine for some reason does a PTRACE_SINGLESTEP over a system call
>  - after the single-step, wine ends up trying to get the sub-process to 
>    enter a signal handler with ptrace( PTRACE_CONT, ... sig)
>  - the normal ptrace path (where we trap a signal - see kernel/signal.c 
>    and the "ptrace_stop()" logic) handles this correctly, but 
>    do_syscall_trace() will do a "send_sig()"
>  - we'll try to handle the signal when returning to the traced process
>  - now "get_signal_to_deliver()" will invoce the ptrace logic AGAIN, and 
>    call ptrace_stop() for this fake signal, and we'll report a new event 
>    to the controlling process.

Make sense to me. What about the one below? The problem is though, that we 
have two different events here. One is the single step trap and the other 
one is the signal. And according to the comment, strace want something 
different from SIGTRAP to continue with signal. So it wants the double 
event we are actually sending (ptrace_notify + send_sig). OTOH Wine does 
not seem to like the double event thingy. Hmm ...?



- Davide



--- ptrace.c.orig	2004-12-30 10:29:11.000000000 -0800
+++ ptrace.c	2004-12-30 11:11:23.000000000 -0800
@@ -573,18 +573,14 @@
 		return;
 	if (!(current->ptrace & PT_PTRACED))
 		return;
-	/* the 0x80 provides a way for the tracing parent to distinguish
-	   between a syscall stop and SIGTRAP delivery */
-	ptrace_notify(SIGTRAP | ((current->ptrace & PT_TRACESYSGOOD) &&
-				 !test_thread_flag(TIF_SINGLESTEP) ? 0x80 : 0));
 
-	/*
-	 * this isn't the same as continuing with a signal, but it will do
-	 * for normal use.  strace only continues with a signal if the
-	 * stopping signal is not SIGTRAP.  -brl
-	 */
 	if (current->exit_code) {
-		send_sig(current->exit_code, current, 1);
+		ptrace_notify(current->exit_code);
 		current->exit_code = 0;
+	} else {
+		/* the 0x80 provides a way for the tracing parent to distinguish
+		   between a syscall stop and SIGTRAP delivery */
+		ptrace_notify(SIGTRAP | ((current->ptrace & PT_TRACESYSGOOD) &&
+					 !test_thread_flag(TIF_SINGLESTEP) ? 0x80 : 0));
 	}
 }
-
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