Re: [PATCH] ptrace on stopped processes (2.4)

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

This is a repost of the ptrace patch to 2.4 kernels
we've discussed in recent months.

Since the last post, I have updated it to linux 2.4.18
(no changes) and tested it with subterfuge and uml. 

Subterfuge seems to be unaffected.

UML needs minor modifications; I've discussed them with
Jeff Dike and (I believe) he is happy.

I believe I have addressed everyone's concerns.

The patch fixes these two bugs:

    1. gdb and other tools cannot attach to a stopped
    process.  The wait that follows the PTRACE_ATTACH
    will block indefinitely.

    2. It is not possible to use PTRACE_DETACH to leave
    a process stopped, because ptrace ignores SIGSTOPs
    sent by the tracing process.

Vic

--- /home/vic/p/linux-2.4.18.orig/kernel/ptrace.c	Wed Mar 13 13:14:54 2002
+++ /home/vic/p/linux-2.4.18/kernel/ptrace.c	Mon Mar 18 21:58:11 2002
@@ -54,6 +54,7 @@
 
 int ptrace_attach(struct task_struct *task)
 {
+	int stopped;
 	task_lock(task);
 	if (task->pid <= 1)
 		goto bad;
@@ -90,7 +91,13 @@
 	}
 	write_unlock_irq(&tasklist_lock);
 
+	stopped = (task->state == TASK_STOPPED);
 	send_sig(SIGSTOP, task, 1);
+	/* If it was stopped when we got here,
+	   clear the pending SIGSTOP. */
+	if (stopped)
+		wake_up_process(task);
+
 	return 0;
 
 bad:
--- /home/vic/p/linux-2.4.18.orig/arch/i386/kernel/signal.c	Wed Mar 13 13:16:44 2002
+++ /home/vic/p/linux-2.4.18/arch/i386/kernel/signal.c	Wed Mar 13 16:31:38 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) {