4.5-stable review patch. If anyone has any objections, please let me know.
------------------ From: Nicolas Pitre <[email protected]> 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 developers. Reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70128 Signed-off-by: Nicolas Pitre <[email protected]> Reported-by: Marcin Juszkiewicz <[email protected]> Cc: Arnd Bergmann <[email protected]> Signed-off-by: Russell King <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]> --- 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);
