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ARM: 8550/1: protect idiv patching against undefined gcc behavior

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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 <nico@linaro.org>
Reported-by: Marcin Juszkiewicz <mjuszkiewicz@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: stable@vger.kernel.org # v4.5
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Nicolas Pitre 2016-03-14 02:55:45 +01:00 committed by Russell King
parent f2335a2a0a
commit 208fae5c3b
1 changed files with 2 additions and 0 deletions
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2
arch/arm/kernel/setup.c
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@ -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);