powerpc: Randomise PIEs

Randomise ELF_ET_DYN_BASE, which is used when loading position independent
executables.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/elf.h

@@ -179,7 +179,8 @@ typedef elf_fpreg_t elf_vsrreghalf_t32[ELF_NVSRHALFREG];
    the loader.  We need to make sure that it is out of the way of the program
    that it will "exec", and that there is sufficient room for the brk.  */
 
-#define ELF_ET_DYN_BASE         (0x20000000)
+extern unsigned long randomize_et_dyn(unsigned long base);
+#define ELF_ET_DYN_BASE		(randomize_et_dyn(0x20000000))
 
 /*
  * Our registers are always unsigned longs, whether we're a 32 bit

arch/powerpc/kernel/process.c

@@ -1170,3 +1170,13 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
 
 	return ret;
 }
+
+unsigned long randomize_et_dyn(unsigned long base)
+{
+	unsigned long ret = PAGE_ALIGN(base + brk_rnd());
+
+	if (ret < base)
+		return base;
+
+	return ret;
+}