x86: ptrace: set TS_COMPAT when 32-bit ptrace sets orig_eax>=0

The 32-bit ptrace syscall on a 64-bit kernel (32-bit debugger on
32-bit task) behaves differently than a native 32-bit kernel.  When
setting a register state of orig_eax>=0 and eax=-ERESTART* when the
debugged task is NOT on its way out of a 32-bit syscall, the task will
fail to do the syscall restart logic that it should do.

Test case available at http://sources.redhat.com/cgi-bin/cvsweb.cgi/~checkout~/tests/ptrace-tests/tests/erestartsys-trap.c?cvsroot=systemtap

This happens because the 32-bit ptrace syscall sets eax=0xffffffff
when it sets orig_eax>=0.  The resuming task will not sign-extend this
for the -ERESTART* check because TS_COMPAT is not set.  (So the task
thinks it is restarting after a 64-bit syscall, not a 32-bit one.)

The fix is to have 32-bit ptrace calls set TS_COMPAT when setting
orig_eax>=0.  This ensures that the 32-bit syscall restart logic
will apply when the child resumes.
Signed-off-by: Roland McGrath <roland@redhat.com>

arch/x86/kernel/ptrace.c

@@ -1111,10 +1111,22 @@ static int putreg32(struct task_struct *child, unsigned regno, u32 value)
 	R32(esi, si);
 	R32(ebp, bp);
 	R32(eax, ax);
-	R32(orig_eax, orig_ax);
 	R32(eip, ip);
 	R32(esp, sp);
 
+	case offsetof(struct user32, regs.orig_eax):
+		/*
+		 * A 32-bit debugger setting orig_eax means to restore
+		 * the state of the task restarting a 32-bit syscall.
+		 * Make sure we interpret the -ERESTART* codes correctly
+		 * in case the task is not actually still sitting at the
+		 * exit from a 32-bit syscall with TS_COMPAT still set.
+		 */
+		regs->orig_ax = value;
+		if (syscall_get_nr(child, regs) >= 0)
+			task_thread_info(child)->status |= TS_COMPAT;
+		break;
+
 	case offsetof(struct user32, regs.eflags):
 		return set_flags(child, value);