x86: clear DF before calling signal handler

The Linux kernel currently does not clear the direction flag before
calling a signal handler, whereas the x86/x86-64 ABI requires that.

Linux had this behavior/bug forever, but this becomes a real problem
with gcc version 4.3, which assumes that the direction flag is
correctly cleared at the entry of a function.

This patches changes the setup_frame() functions to clear the
direction before entering the signal handler.
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/ia32/ia32_signal.c

@@ -500,7 +500,7 @@ int ia32_setup_frame(int sig, struct k_sigaction *ka,
 	regs->ss = __USER32_DS;
 
 	set_fs(USER_DS);
-	regs->flags &= ~X86_EFLAGS_TF;
+	regs->flags &= ~(X86_EFLAGS_TF | X86_EFLAGS_DF);
 	if (test_thread_flag(TIF_SINGLESTEP))
 		ptrace_notify(SIGTRAP);
 
@@ -600,7 +600,7 @@ int ia32_setup_rt_frame(int sig, struct k_sigaction *ka, siginfo_t *info,
 	regs->ss = __USER32_DS;
 
 	set_fs(USER_DS);
-	regs->flags &= ~X86_EFLAGS_TF;
+	regs->flags &= ~(X86_EFLAGS_TF | X86_EFLAGS_DF);
 	if (test_thread_flag(TIF_SINGLESTEP))
 		ptrace_notify(SIGTRAP);
 

arch/x86/kernel/signal_32.c

@@ -407,7 +407,7 @@ static int setup_frame(int sig, struct k_sigaction *ka,
 	 * The tracer may want to single-step inside the
 	 * handler too.
 	 */
-	regs->flags &= ~TF_MASK;
+	regs->flags &= ~(TF_MASK | X86_EFLAGS_DF);
 	if (test_thread_flag(TIF_SINGLESTEP))
 		ptrace_notify(SIGTRAP);
 
@@ -500,7 +500,7 @@ static int setup_rt_frame(int sig, struct k_sigaction *ka, siginfo_t *info,
 	 * The tracer may want to single-step inside the
 	 * handler too.
 	 */
-	regs->flags &= ~TF_MASK;
+	regs->flags &= ~(TF_MASK | X86_EFLAGS_DF);
 	if (test_thread_flag(TIF_SINGLESTEP))
 		ptrace_notify(SIGTRAP);
 

arch/x86/kernel/signal_64.c

@@ -295,7 +295,7 @@ static int setup_rt_frame(int sig, struct k_sigaction *ka, siginfo_t *info,
 	   see include/asm-x86_64/uaccess.h for details. */
 	set_fs(USER_DS);
 
-	regs->flags &= ~X86_EFLAGS_TF;
+	regs->flags &= ~(X86_EFLAGS_TF | X86_EFLAGS_DF);
 	if (test_thread_flag(TIF_SINGLESTEP))
 		ptrace_notify(SIGTRAP);
 #ifdef DEBUG_SIG