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

[Richard Henderson <richard.henderson@linaro.org>]

On 7/4/23 17:43, Peter Maydell wrote:
> 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>
> ---
>   target/arm/cpu64.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~