Re: [PATCH] target/arm: Avoid over-length shift in arm_cpu_sve_finalize() error case

[Alex Bennée]

Peter Maydell <peter.maydell@linaro.org> writes:

> 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.

Hmm that does make me worry we could have more land mines waiting to be
found. Would converting MAKE_64BIT_MASK into an inline function and
asserting be a better solution?

>
> 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>

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro