[PATCH v2] arm64: Fix bit-shifting UB in the MIDR_CPU_MODEL() macro

[D Scott Phillips]

CONFIG_UBSAN_SHIFT with gcc-5 complains that the shifting of
ARM_CPU_IMP_AMPERE (0xC0) into bits [31:24] by MIDR_CPU_MODEL() is
undefined behavior. Well, sort of, it actually spells the error as:

 arch/arm64/kernel/proton-pack.c: In function 'spectre_bhb_loop_affected':
 arch/arm64/include/asm/cputype.h:44:2: error: initializer element is not constant
   (((imp)   << MIDR_IMPLEMENTOR_SHIFT) | \
   ^

This isn't an issue for other Implementor codes, as all the other codes
have zero in the top bit and so are representable as a signed int.

Cast the implementor code to unsigned in MIDR_CPU_MODEL to remove the
undefined behavior.

Fixes: 0e5d5ae837c8 ("arm64: Add AMPERE1 to the Spectre-BHB affected list")
Reported-by: Geert Uytterhoeven <geert at linux-m68k.org>
Signed-off-by: D Scott Phillips <scott at os.amperecomputing.com>
---
Changes since v1 (https://lore.kernel.org/all/20221017210119.3581-1-scott@os.amperecomputing.com/):
- cast in MIDR_CPU_MODEL() instead of changing ARM_CPU_IMP_AMPERE

 arch/arm64/include/asm/cputype.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cputype.h b/arch/arm64/include/asm/cputype.h
index abc418650fec..65e53ef5a396 100644
--- a/arch/arm64/include/asm/cputype.h
+++ b/arch/arm64/include/asm/cputype.h
@@ -41,7 +41,7 @@
 	(((midr) & MIDR_IMPLEMENTOR_MASK) >> MIDR_IMPLEMENTOR_SHIFT)
 
 #define MIDR_CPU_MODEL(imp, partnum) \
-	(((imp)			<< MIDR_IMPLEMENTOR_SHIFT) | \
+	((_AT(u32, imp)		<< MIDR_IMPLEMENTOR_SHIFT) | \
 	(0xf			<< MIDR_ARCHITECTURE_SHIFT) | \
 	((partnum)		<< MIDR_PARTNUM_SHIFT))
 
-- 
2.37.3