[PATCH] ptrace on stopped processes (2.4)

[vic <zandy@cs.wisc.edu>]

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	Fri Dec 21 11:05:45 2001
@@ -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: