Fix atomicity of TIF update in flush_thread() for powerpc

Fix atomicity of TIF update in flush_thread() for powerpc

Fixes it correctly with *_ti_thread_flag.

Race :

parent process executing :
sys_ptrace()
(lock_kernel())
(ptrace_get_task_struct(pid))
arch_ptrace()
ptrace_detach()

     ptrace_disable(child);
       clear_singlestep(child);
         clear_tsk_thread_flag(child, TIF_SINGLESTEP);
         (which clears the TIF_SINGLESTEP flag atomically from a different
          process)

(put_task_struct(child))
(unlock_kernel())

And at the same time, in the child process :
sys_execve()
do_execve()
search_binary_handler()
load_elf_binary()
flush_old_exec()
flush_thread()
doing a non-atomic thread flag update

Applies on 2.6.20.

@@ -476,8 +476,13 @@ void flush_thread(void)
 #ifdef CONFIG_PPC64
 	struct thread_info *t = current_thread_info();
 
-	if (t->flags & _TIF_ABI_PENDING)
-		t->flags ^= (_TIF_ABI_PENDING | _TIF_32BIT);
+	if (test_ti_thread_flag(t, TIF_ABI_PENDING)) {
+		clear_ti_thread_flag(t, TIF_ABI_PENDING);
+		if (test_ti_thread_flag(t, TIF_32BIT))
+			clear_ti_thread_flag(t, TIF_32BIT);
+		else
+			set_ti_thread_flag(t, TIF_32BIT);
+	}
 #endif
 
 	discard_lazy_cpu_state();

Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68