[PATCH 16/18] KVM: arm64: Add build-time sanity checks for flags

[Marc Zyngier]

Flags are great, but flags can also be dangerous: it is easy
to encode a flag that is bigger than its container (unless the
container is a u64), and it is easy to construct a flag value
that doesn't fit in the mask that is associated with it.

Add a couple of build-time sanity checks that ensure we catch
these two cases.

Signed-off-by: Marc Zyngier <maz at kernel.org>
---
 arch/arm64/include/asm/kvm_host.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 4073a33af17c..70931231f0cb 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -420,8 +420,20 @@ struct kvm_vcpu_arch {
 	} steal;
 };
 
+#define __build_check_flag(v, flagset, f, m)			\
+	do {							\
+		typeof(v->arch.flagset) *_fset;			\
+								\
+		/* Check that the flags fit in the mask */	\
+		BUILD_BUG_ON(HWEIGHT(m) != HWEIGHT((f) | (m)));	\
+		/* Check that the flags fit in the type */	\
+		BUILD_BUG_ON((sizeof(*_fset) * 8) <= __fls(m));	\
+	} while (0)
+
 #define __vcpu_get_flag(v, flagset, f, m)			\
 	({							\
+		__build_check_flag(v, flagset, f, m);		\
+								\
 		v->arch.flagset & (m);				\
 	})
 
@@ -429,6 +441,8 @@ struct kvm_vcpu_arch {
 	do {							\
 		typeof(v->arch.flagset) *fset;			\
 								\
+		__build_check_flag(v, flagset, f, m);		\
+								\
 		fset = &v->arch.flagset;			\
 		if (HWEIGHT(m) > 1)				\
 			*fset &= ~(m);				\
@@ -439,6 +453,8 @@ struct kvm_vcpu_arch {
 	do {							\
 		typeof(v->arch.flagset) *fset;			\
 								\
+		__build_check_flag(v, flagset, f, m);		\
+								\
 		fset = &v->arch.flagset;			\
 		*fset &= ~(m);					\
 	} while (0)
-- 
2.34.1