Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE_ATTACH

[Denys Vlasenko <vda.linux@googlemail.com>]

On Sunday 20 February 2011 10:40, Jan Kratochvil wrote:
> Sure by default GDB does not do anything special, it will respawn (using
> PTRACE_CONT(SIGSTOP)) any SIGSTOP it sees due to the default setting of:
> 	(gdb) handle SIGSTOP
> 	Signal        Stop  Print Pass to program Description
> 	SIGSTOP       Yes Yes Yes   Stopped (signal)
> 
> Therefore there happens the double SIGSTOP reporting as discussed before:
> 	(gdb) run
> 	Starting program: /bin/sleep 1h
> 	# external kill -STOP <inferior pid>
> 	Program received signal SIGSTOP, Stopped (signal).
> 	# State:	t (tracing stop)
> 	(gdb) continue 
> 	Continuing.
> 	Program received signal SIGSTOP, Stopped (signal).
> 	# State:	t (tracing stop)
> 	(gdb) continue 
> 	Continuing.
> 	# State:	S (sleeping)
> 
> Your proposal is I expect:
> 	(gdb) run
> 	Starting program: /bin/sleep 1h
> 	# external kill -STOP <inferior pid>
> 	Program received signal SIGSTOP, Stopped (signal).
> 	# State:	t (tracing stop)
> 	(gdb) continue 
> 	Continuing.
> 	# State:	T (stopped)

Not exactly. Even after we fix kernel so that it properly preserves
group-stop across ptrace-stops, gdb will still see TWO 
waitpid:SIGSTOP events, not one.

First one says "the tracee has received SIGSTOP", and after PTRACE_CONT(SIGSTOP),
second one says "the tracee has stopped because of SIGSTOP".
Currently, neither strace nor gdb understands that second one
is different from first.

Here is how strace can be improved by querying PTRACE_GETSIGINFO:

+                               entered_stopped_state = 0;
+                               if (WSTOPSIG(status) == SIGSTOP ||
+                                   WSTOPSIG(status) == SIGTSTP) {
+                                       /*
+                                        * PTRACE_GETSIGINFO fails if this was
+                                        * genuine *stop* notification,
+                                        * not *signal* notification
+                                        */
+                                       if (ptrace(PTRACE_GETSIGINFO, pid,
+                                                   0, &si) != 0)
+                                               entered_stopped_state = 1;
+                               }
                                printleader(tcp);
-                               tprintf("--- %s (%s) @ %lx (%lx) ---",
+                               tprintf(entered_stopped_state
+                                       ? "--- stopped by %s ---"
+                                       : "--- %s (%s) @ %lx (%lx) ---",
                                        signame(WSTOPSIG(status)),
                                        strsignal(WSTOPSIG(status)), pc, addr);


Before patch strace shows confusing log:

--- SIGSTOP (Stopped (signal)) @ 0 (0) ---
--- SIGSTOP (Stopped (signal)) @ 0 (0) ---

After it is more understandable:

--- SIGSTOP (Stopped (signal)) @ 0 (0) ---
--- stopped by SIGSTOP ---


I think you can use similar trick in gdb, so that second message says
"Program stopped due to signal SIGSTOP, Stopped (signal)",
not "Program received signal SIGSTOP, Stopped (signal)".

-- 
vda