linux-kernel - [PATCH] Fix atomicity of TIF update in flush

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Message-ID: <20070309024525.GB9462@Krystal>
Date:	Thu, 8 Mar 2007 21:45:25 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	akpm@...ux-foundation.org, mbligh@...gle.com,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	paulus@...ba.org
Subject: [PATCH] Fix atomicity of TIF update in flush_thread() for powerpc

Fix atomicity of TIF update in flush_thread() for powerpc

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.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>

--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -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_tsk_thread_flag(tsk, TIF_ABI_PENDING)) {
+		clear_tsk_thread_flag(tsk, TIF_ABI_PENDING);
+		if (test_tsk_thread_flag(tsk, TIF_32BIT))
+			clear_tsk_thread_flag(tsk, TIF_32BIT);
+		else
+			set_tsk_thread_flag(tsk, 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
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