[PATCH 4.5 097/124] ARM: 8550/1: protect idiv patching against undefined gcc behavior

From: Greg Kroah-Hartman
Date: Sun Apr 17 2016 - 23:10:53 EST

4.5-stable review patch. If anyone has any objections, please let me know.


From: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>

commit 208fae5c3b9431013ad7bcea07cbcee114e7d163 upstream.

It was reported that a kernel with CONFIG_ARM_PATCH_IDIV=y stopped
booting when compiled with the upcoming gcc 6. Turns out that turning
a function address into a writable array is undefined and gcc 6 decided
it was OK to omit the store to the first word of the function while
still preserving the store to the second word.

Even though gcc 6 is now fixed to behave more coherently, it is a
mystery that gcc 4 and gcc 5 actually produce wanted code in the kernel.
And in fact the reduced test case to illustrate the issue does indeed
break with gcc < 6 as well.

In any case, let's guard the kernel against undefined compiler behavior
by hiding the nature of the array location as suggested by gcc

Reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70128

Signed-off-by: Nicolas Pitre <nico@xxxxxxxxxx>
Reported-by: Marcin Juszkiewicz <mjuszkiewicz@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

arch/arm/kernel/setup.c | 2 ++
1 file changed, 2 insertions(+)

--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -430,11 +430,13 @@ static void __init patch_aeabi_idiv(void
pr_info("CPU: div instructions available: patching division code\n");

fn_addr = ((uintptr_t)&__aeabi_uidiv) & ~1;
+ asm ("" : "+g" (fn_addr));
((u32 *)fn_addr)[0] = udiv_instruction();
((u32 *)fn_addr)[1] = bx_lr_instruction();
flush_icache_range(fn_addr, fn_addr + 8);

fn_addr = ((uintptr_t)&__aeabi_idiv) & ~1;
+ asm ("" : "+g" (fn_addr));
((u32 *)fn_addr)[0] = sdiv_instruction();
((u32 *)fn_addr)[1] = bx_lr_instruction();
flush_icache_range(fn_addr, fn_addr + 8);