chore(cuda): keccakf memcpy handling of padding (#2167)

Currently the invalid memory access is never used, but it's still better
to address it so `compute-analyzer` does not complain.

Author: jonathanpwang

extensions/keccak256/circuit/cuda/src/keccakf.cu

@@ -180,8 +180,15 @@ __global__ void keccakf_kernel(
         uint8_t *chunk = input + blk * KECCAK_RATE_BYTES;
         bool is_last_block = (blk == num_blocks - 1);
         for (int round = 0; round < NUM_ABSORB_ROUNDS; round++) {
-            uint8_t i_bytes[8];
-            memcpy(&i_bytes, chunk + round * 8, sizeof(uint64_t));
+            uint8_t i_bytes[8] = {};
+            int round_base = round * sizeof(uint64_t);
+            // For the last block, zero-initialize and only copy available bytes
+            if (is_last_block && round_base < last_block_len) {
+                size_t bytes_to_copy = min(sizeof(uint64_t), last_block_len - round_base);
+                memcpy(i_bytes, chunk + round_base, bytes_to_copy);
+            } else if (!is_last_block) {
+                memcpy(i_bytes, chunk + round_base, sizeof(uint64_t));
+            }
 
             // Handle Keccak spec padding
             if (is_last_block) {
@@ -244,4 +251,4 @@ extern "C" int _keccakf_kernel(
         bitwise_num_bits
     );
     return CHECK_KERNEL();
-}
+}