MIPS: Don't clobber personality high bits.

The high bits of current->personality carry settings that we don't want to
clobber on each exec.  Only clobber them if the lower bits that indicate
either PER_LINUX or PER_LINUX32 are invalid.

The clobbering prevents us from using useful bits like ADDR_NO_RANDOMIZE.
Reported-by: NCamm Maguire <camm@maguirefamily.org>
Signed-off-by: NDavid Daney <ddaney@caviumnetworks.com>
Cc: Camm Maguire <camm@maguirefamily.org>
Patchwork: https://patchwork.linux-mips.org/patch/1750/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/elf.h

@@ -249,7 +249,8 @@ extern struct mips_abi mips_abi_n32;
 
 #define SET_PERSONALITY(ex)						\
 do {									\
-	set_personality(PER_LINUX);					\
+	if (personality(current->personality) != PER_LINUX)		\
+		set_personality(PER_LINUX);				\
 									\
 	current->thread.abi = &mips_abi;				\
 } while (0)
@@ -296,6 +297,8 @@ do {									\
 
 #define SET_PERSONALITY(ex)						\
 do {									\
+	unsigned int p;							\
+									\
 	clear_thread_flag(TIF_32BIT_REGS);				\
 	clear_thread_flag(TIF_32BIT_ADDR);				\
 									\
@@ -304,7 +307,8 @@ do {									\
 	else								\
 		current->thread.abi = &mips_abi;			\
 									\
-	if (current->personality != PER_LINUX32)			\
+	p = personality(current->personality);				\
+	if (p != PER_LINUX32 && p != PER_LINUX)				\
 		set_personality(PER_LINUX);				\
 } while (0)