KVM: arm64: Add build-time check for duplicate DECLARE_REG use

The DECLARE_REG() macro provides a convenient way to create a local
variable initialized from a cpu context in the hyp trap handlers.
However, a common error is to use the macro multiple times in the same
scope with the same register index, but for different logical purposes.

This results in valid C code that compiles without error, but introduces
subtle bugs where a developer expects two different variables to hold
values from two different registers, when in fact they are both sourced
from the same one.

To prevent this entire class of bugs, modify the DECLARE_REG() macro
to declare a dummy variable whose name is derived from the register
index. If the macro is used again with the same index in the same
scope, the compiler will fail with a "redeclaration of variable"
error, turning a subtle runtime bug into an obvious build-time failure.

Signed-off-by: Fuad Tabba <tabba@google.com>
Tested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>

diff --git a/arch/arm64/kvm/hyp/include/nvhe/trap_handler.h b/arch/arm64/kvm/hyp/include/nvhe/trap_handler.h
index 1e6d995968a1fbd1c93cc80bfe3693da917a4adb..ba5382c127870b62e44bf74f0655d13a07d339f7 100644 (file)
--- a/arch/arm64/kvm/hyp/include/nvhe/trap_handler.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/trap_handler.h
@@ -12,7 +12,8 @@
 #include <asm/kvm_host.h>
 
 #define cpu_reg(ctxt, r)       (ctxt)->regs.regs[r]
-#define DECLARE_REG(type, name, ctxt, reg)     \
+#define DECLARE_REG(type, name, ctxt, reg)                                     \
+                               __always_unused int ___check_reg_ ## reg;       \
                                type name = (type)cpu_reg(ctxt, (reg))
 
 #endif /* __ARM64_KVM_NVHE_TRAP_HANDLER_H__ */