If you build QEMU with the clang sanitizer enabled, you can see it
fire when running the arm-cpu-features test:

$ QTEST_QEMU_BINARY=./build/arm-clang/qemu-system-aarch64 ./build/arm-clang/tests/qtest/arm-cpu-features
[...]
../../target/arm/cpu64.c:125:19: runtime error: shift exponent 64 is too large for 64-bit type 'unsigned long long'
[...]

This happens because the user can specify some incorrect SVE
properties that result in our calculating a max_vq of 0.  We catch
this and error out, but before we do that we calculate

 vq_mask = MAKE_64BIT_MASK(0, max_vq);$

and the MAKE_64BIT_MASK() call is only valid for lengths that are
greater than zero, so we hit the undefined behaviour.

Change the logic so that if max_vq is 0 we specifically set vq_mask
to 0 without going via MAKE_64BIT_MASK().  This lets us drop the
max_vq check from the error-exit logic, because if max_vq is 0 then
vq_map must now be 0.

The UB only happens in the case where the user passed us an incorrect
set of SVE properties, so it's not a big problem in practice.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230704154332.3014896-1-peter.maydell@linaro.org