问linux内核密码API，在AES-GCM算法中，如何设置aad的长度是零位？

EN

int aes_gcm_decrypt(struct crypto_aead *tfm, u8 *j_0, u8 *aad,
              u8 *data, size_t data_len, u8 *mic)
{
    struct scatterlist sg[3];
    struct aead_request *aead_req;
    int reqsize = sizeof(*aead_req) + crypto_aead_reqsize(tfm);
    int err;

    if (data_len == 0)
        return -EINVAL;

    aead_req = kzalloc(reqsize + GCM_AAD_LEN, GFP_ATOMIC);
    if (!aead_req)
        return -ENOMEM;

    sg_init_table(sg, 3);
    sg_set_buf(&sg[0], aad,0);
    sg_set_buf(&sg[1], data, data_len);
    sg_set_buf(&sg[2], mic, 16);

    aead_request_set_tfm(aead_req, tfm);
    aead_request_set_crypt(aead_req, sg, sg,
               data_len + 16, j_0);
    aead_request_set_ad(aead_req, sg[0].length);

    err = crypto_aead_decrypt(aead_req);
    kzfree(aead_req);

    return err;
}

/**
 * aead_request_set_crypt - set data buffers
 * @req: request handle
 * @src: source scatter / gather list
 * @dst: destination scatter / gather list
 * @cryptlen: number of bytes to process from @src
 * @iv: IV for the cipher operation which must comply with the IV size defined
 *      by crypto_aead_ivsize()
 *
 * Setting the source data and destination data scatter / gather lists which
 * hold the associated data concatenated with the plaintext or ciphertext. See
 * below for the authentication tag.
 *
 * For encryption, the source is treated as the plaintext and the
 * destination is the ciphertext. For a decryption operation, the use is
 * reversed - the source is the ciphertext and the destination is the plaintext.
 *
 * The memory structure for cipher operation has the following structure:
 *
 * - AEAD encryption input:  assoc data || plaintext
 * - AEAD encryption output: assoc data || cipherntext || auth tag
 * - AEAD decryption input:  assoc data || ciphertext || auth tag
 * - AEAD decryption output: assoc data || plaintext
 *
 * Albeit the kernel requires the presence of the AAD buffer, however,
 * the kernel does not fill the AAD buffer in the output case. If the
 * caller wants to have that data buffer filled, the caller must either
 * use an in-place cipher operation (i.e. same memory location for
 * input/output memory location).
 */