sh: initial stack protector support.

This implements basic -fstack-protector support, based on the early ARM
version in c743f38013. The SMP case is
limited to the initial canary value, while the UP case handles per-task
granularity (limited to 32-bit sh until a new enough sh64 compiler
manifests itself).

Signed-off-by: Filippo Arcidiacono <filippo.arcidiacono@st.com>
Reviewed-by: Carmelo Amoroso <carmelo.amoroso@st.com>
Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/include/asm/stackprotector.h

@@ -0,0 +1,27 @@
+#ifndef __ASM_SH_STACKPROTECTOR_H
+#define __ASM_SH_STACKPROTECTOR_H
+
+#include <linux/random.h>
+#include <linux/version.h>
+
+extern unsigned long __stack_chk_guard;
+
+/*
+ * Initialize the stackprotector canary value.
+ *
+ * NOTE: this must only be called from functions that never return,
+ * and it must always be inlined.
+ */
+static __always_inline void boot_init_stack_canary(void)
+{
+	unsigned long canary;
+
+	/* Try to get a semi random initial value. */
+	get_random_bytes(&canary, sizeof(canary));
+	canary ^= LINUX_VERSION_CODE;
+
+	current->stack_canary = canary;
+	__stack_chk_guard = current->stack_canary;
+}
+
+#endif /* __ASM_SH_STACKPROTECTOR_H */