SElinux avc 打印及关闭

Android SELinux的avc：denied log 是哪里打印的及关闭?
某些场景下并不需要打开SELinux,也就是SELinux设为Permissive，但如果程序设计不符合SELinux sepolicy，日志会频繁打印如下类似avc：denied 的 log。
如下：

[   24.018396] type=1400 audit(1622165470.867:666): avc: denied { read } for pid=2358 comm="testagent" name="ApkLife.data" dev="tmpfs" ino=28549 scontext=u:r:stbdetector:s0 tcontext=u:object_r:system_app_tmpfs:s0 tclass=file permissive=1
[   24.024144] type=1400 audit(1622165470.867:667): avc: denied { open } for pid=2358 comm="testagent" path="/data/local/vixtel/temp/ApkLife.data" dev="tmpfs" ino=28549 scontext=u:r:stbdetector:s0 tcontext=u:object_r:system_app_tmpfs:s0 tclass=file permissive=1

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1. avc：denied log是哪里打印出来的？

Linux version 4.14：
在Linux kernel代码中，security/lsm_audit.c 中有common_lsm_audit函数，定义如下：

/**
* common_lsm_audit - generic LSM auditing function
* @a:  auxiliary audit data
* @pre_audit: lsm-specific pre-audit callback
* @post_audit: lsm-specific post-audit callback
*
* setup the audit buffer for common security information
* uses callback to print LSM specific information
*/
void common_lsm_audit(struct common_audit_data *a,
    void (*pre_audit)(struct audit_buffer *, void *),
    void (*post_audit)(struct audit_buffer *, void *))
{
    struct audit_buffer *ab;

    if (a == NULL)
        return;
    /* we use GFP_ATOMIC so we won't sleep */
    ab = audit_log_start(current->audit_context, GFP_ATOMIC | __GFP_NOWARN,
                 AUDIT_AVC);

    if (ab == NULL)
        return;

    if (pre_audit)
        pre_audit(ab, a);

    dump_common_audit_data(ab, a);

    if (post_audit)
        post_audit(ab, a);

    audit_log_end(ab);
}

在这个函数中依次调用了 pre_audit ， dump_common_audit_data ，post_audit 三个函数来打印相关信息。
其中 pre_audit 和 post_audit定义如下：

// common_lsm_audit 的调用

common_lsm_audit(a, avc_audit_pre_callback, avc_audit_post_callback);

// 函数定义
static void avc_audit_pre_callback(struct audit_buffer *ab, void *a)
{
    struct common_audit_data *ad = a;
    audit_log_format(ab, "avc:  %s ",
             ad->selinux_audit_data->denied ? "denied" : "granted");
    avc_dump_av(ab, ad->selinux_audit_data->tclass,
            ad->selinux_audit_data->audited);
    audit_log_format(ab, " for ");
}

/**
* avc_audit_post_callback - SELinux specific information
* will be called by generic audit code
* @ab: the audit buffer
* @a: audit_data
*/
static void avc_audit_post_callback(struct audit_buffer *ab, void *a)
{
    struct common_audit_data *ad = a;
    audit_log_format(ab, " ");
    avc_dump_query(ab, ad->selinux_audit_data->state,
               ad->selinux_audit_data->ssid,
               ad->selinux_audit_data->tsid,
               ad->selinux_audit_data->tclass);
    if (ad->selinux_audit_data->denied) {
        audit_log_format(ab, " permissive=%u",
                 ad->selinux_audit_data->result ? 0 : 1);
    }
}

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2. avc：denied log在不需要时如何关闭？

简单的方法就是把打印log的部分直接注释掉，如下：

void common_lsm_audit(struct common_audit_data *a,
    void (*pre_audit)(struct audit_buffer *, void *),
    void (*post_audit)(struct audit_buffer *, void *))
{
    struct audit_buffer *ab;

    if (a == NULL)
        return;
+#if 0 // 屏蔽代码
    /* we use GFP_ATOMIC so we won't sleep */
    ab = audit_log_start(current->audit_context, GFP_ATOMIC | __GFP_NOWARN,
                 AUDIT_AVC);


    if (ab == NULL)
        return;

    if (pre_audit)
        pre_audit(ab, a);

    dump_common_audit_data(ab, a);

    if (post_audit)
        post_audit(ab, a);

    audit_log_end(ab);
+#endif // 屏蔽代码
}