[PATCH?] Crash in 2.4.17/ptrace

[Daniel Jacobowitz <dan@debian.org>]

I've been debugging frame buffer graphics lately, and encountering a
very annoying problem.  If the debugee has /dev/fb/0 mapped, and I try
to print out the contents of a pointer into that buffer, GDB crashes in
kernel/ptrace.c:access_process_vm.  The problem seems to be that
get_user_pages returns a NULL page.  Something as simple as this
prevents the crash:

--- 2.4.18-pre7/2.4.18-pre7/kernel/ptrace.c	Fri Dec 21 12:42:04 2001
+++ 2.4.17/kernel-source-2.4.17/kernel/ptrace.c	Mon Jan 28 15:30:39 2002
@@ -160,6 +160,18 @@ int access_process_vm(struct task_struct
 
 		flush_cache_page(vma, addr);
 
+#if 1
+		if (!page)
+		{
+			/* FIXME: Writes? */
+			if (!write) memset (buf, 0, bytes);
+			len -= bytes;
+			buf += bytes;
+			continue;
+		}
+#endif
+
+
 		maddr = kmap(page);
 		if (write) {
 			memcpy(maddr + offset, buf, bytes);


Of course, I would much rather be able to see the contents of the
framebuffer.  Any suggestions?

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer