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

[Peter Maydell]

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(-)

diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c
index 6eaf8e32cfa..6012e4ef549 100644
--- a/target/arm/cpu64.c
+++ b/target/arm/cpu64.c
@@ -122,10 +122,10 @@ void arm_cpu_sve_finalize(ARMCPU *cpu, Error **errp)
         vq = ctz32(tmp) + 1;
 
         max_vq = vq <= ARM_MAX_VQ ? vq - 1 : ARM_MAX_VQ;
-        vq_mask = MAKE_64BIT_MASK(0, max_vq);
+        vq_mask = max_vq > 0 ? MAKE_64BIT_MASK(0, max_vq) : 0;
         vq_map = vq_supported & ~vq_init & vq_mask;
 
-        if (max_vq == 0 || vq_map == 0) {
+        if (vq_map == 0) {
             error_setg(errp, "cannot disable sve%d", vq * 128);
             error_append_hint(errp, "Disabling sve%d results in all "
                               "vector lengths being disabled.\n",
-- 
2.34.1