ubus接口调用解析时遇到的一点问题

本文主要描述在openwrt环境下，使用blobmsg_parse接口解析双层BLOBMSG_TYPE_TABLE出现的Segmentation fault问题。带解析的数据来源于ubus call system info返回的值。

问题描述

前置条件：ubus call system info接口的返回值内容如下：

$ubus call system info
{
    "localtime": 1628263880,
    "uptime": 77629,
    "load": [
        0,
        704,
        576
    ],
    "memory": {
        "total": 233041920,
        "free": 51875840,
        "shared": 188416,
        "buffered": 0,
        "available": 96661504,
        "cached": 8239104
    },
    "swap": {
        "total": 0,
        "free": 0
    }
}

我的目的是获取memory中的available字段的值，从而得到系统中可以的内存大小。但是在解析第二层memory,BLOBMSG_TYPE_TABLE却出现了Segmentation fault。

下面是出错的部分代码：

// 定义第一层system info结构
enum
{
    DEF_SYS_LOCAL_TIME,
    DEF_SYS_UPTIME,
    DEF_SYS_LOAD,
    DEF_SYS_MEM,
    __SYS_INFO_MAXMUM
};

static const struct blobmsg_policy sys_info_policy[] = {
    [DEF_SYS_LOCAL_TIME] = {.name = "localtime", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_UPTIME] = {.name = "uptime", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_LOAD] = {.name = "load", .type = BLOBMSG_TYPE_ARRAY},
    [DEF_SYS_MEM] = {.name = "memory", .type = BLOBMSG_TYPE_TABLE},
};

/* 解析BLOBMSG_TYPE_ARRAY */
enum
{
    DEF_SYS_LOAD_ITEM1,
    DEF_SYS_LOAD_ITEM2,
    DEF_SYS_LOAD_ITEM3,
    __SYS_LOAD_ITEM_MAXMUM
};

static const struct blobmsg_policy sys_load_policy[] = {
    [DEF_SYS_LOAD_ITEM1] = {.type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_LOAD_ITEM2] = {.type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_LOAD_ITEM3] = {.type = BLOBMSG_TYPE_INT32},
};

// 定义第二层memory的结构
enum
{
    DEF_SYS_MEM_TOTAL,
    DEF_SYS_MEM_FREE,
    DEF_SYS_MEM_SHARED,
    DEF_SYS_MEM_BUFFERED,
    DEF_SYS_MEM_AVAILABLE,
    DEF_SYS_MEM_CACHED,
    __SYS_MEM_INFO_MAXMUM
};

static const struct blobmsg_policy sys_mem_info_policy[] = {
    [DEF_SYS_MEM_TOTAL] = {.name = "total", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_MEM_FREE] = {.name = "free", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_MEM_SHARED] = {.name = "shared", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_MEM_BUFFERED] = {.name = "buffered", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_MEM_AVAILABLE] = {.name = "available", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_MEM_CACHED] = {.name = "cached", .type = BLOBMSG_TYPE_INT32},
};

static void handle_sys_mem_cb(struct ubus_request *req, int type, struct blob_attr *msg)
{
    if (msg != NULL)
    {
        struct blob_attr *tb[__SYS_INFO_MAXMUM];
        // 解析最外层
        blobmsg_parse(sys_info_policy, __SYS_INFO_MAXMUM, tb, blob_data(msg), blob_len(msg));
        // 打印localtime
        if (tb[DEF_SYS_LOCAL_TIME])
        {
            printf("%d---%u\n", __LINE__, blobmsg_get_u32(tb[DEF_SYS_LOCAL_TIME]));
        }
        for (int i = 0; i < __SYS_INFO_MAXMUM; i++)
        {
            printf("%d--%s\n", __LINE__, blobmsg_name(tb[i]));
            printf("%d--%d\n", __LINE__, blobmsg_type(tb[i]));
            printf("%d--%d\n", __LINE__, blobmsg_data_len(tb[i]));
            printf("%d--%d\n", __LINE__, blobmsg_len(tb[i]));
        }
        // 解析load
        if (tb[DEF_SYS_LOAD])
        {
            struct blob_attr *sub_tb[__SYS_LOAD_ITEM_MAXMUM];
            blobmsg_parse_array(sys_load_policy, ARRAY_SIZE(sys_load_policy), sub_tb, blobmsg_data(tb[DEF_SYS_LOAD]), blobmsg_data_len(tb[DEF_SYS_LOAD]));
            for (int i = 0; i < __SYS_LOAD_ITEM_MAXMUM; i++)
            {
                printf("%d--%u\n", __LINE__, blobmsg_get_u32(sub_tb[i]));
                printf("%d--%d\n", __LINE__, blobmsg_data_len(sub_tb[i]));
                printf("%d--%d\n", __LINE__, blobmsg_len(sub_tb[i]));
            }
        }
        // 解析memory
        if (tb[DEF_SYS_MEM])
        {
            printf("%d--%d\n", __LINE__, blobmsg_type(tb[DEF_SYS_MEM]) == BLOBMSG_TYPE_TABLE);
            struct blob_attr *mem_attr[__SYS_MEM_INFO_MAXMUM];
            int iRet = blobmsg_parse(sys_mem_info_policy, ARRAY_SIZE(sys_mem_info_policy), mem_attr, blobmsg_data(tb[DEF_SYS_MEM]), blobmsg_data_len(tb[DEF_SYS_MEM]));
            printf("%d--%d\n", __LINE__, iRet);
            for (int i = 0; i < __SYS_MEM_INFO_MAXMUM; i++)
            {
                printf("%d--%d\n", __LINE__, i);
                printf("%d--%s\n", __LINE__, blobmsg_name(mem_attr[i])); //Segmentation fault
                printf("%d--%ld\n", __LINE__, blobmsg_get_u32(mem_attr[i]));
                printf("%d--%d\n", __LINE__, blobmsg_type(mem_attr[i]));
                printf("%d--%d\n", __LINE__, blobmsg_data_len(mem_attr[i]));
                printf("%d--%d\n", __LINE__, blobmsg_len(mem_attr[i]));
            }
        }
    }
}

当代码调试到98行时出现段错误。我查看了u32_t的大小范围是0~4294976295，很明显memory下的变量的值都在这个范围内，看起来是没有问题的，真是百思不得其解。
最后我想到一个办法，那就是将tb[DEF_SYS_MEM]这个变量写入二进制文件中，查看一下具体内容。

排查步骤

将tb[DEF_SYS_MEM]写入二进制文件，采用下面的代码：

FILE *fp = fopen("./mem.txt", "wb");
if (fp != NULL)
{
    fwrite(blobmsg_data(tb[DEF_SYS_MEM]), blobmsg_data_len(tb[DEF_SYS_MEM]), 1, fp);
    fclose(fp);
}

使用WinHex工具插件文件的十六进制内容如下图：
根据位置对应关系可以看到每一条数据的开头是0x8400；然后接下来的2字节是blob_attr的长度为0x0014；再接下来的2字节是标题长度0x0005，也就是total的长度；再接下来的5字节就是total，还有1字节是total字符串的结尾’\0’；这一条数据的最后8个字节就是我们想要获取的数据。
通过计算可以知道2+2+2+5+1+8=20=0x14。从这儿可以看出memory下的数据是采用8字节进行存储的，所以应该使用BLOBMSG_TYPE_INT64，而非之前使用的BLOBMSG_TYPE_INT32。

确定原因

通过上面的步骤逐步排查，可以确定是因为之前的代码中采用了BLOBMSG_TYPE_INT32来解析memory导致的问题。

// 定义第二层memory的结构
enum
{
    DEF_SYS_MEM_TOTAL,
    DEF_SYS_MEM_FREE,
    DEF_SYS_MEM_SHARED,
    DEF_SYS_MEM_BUFFERED,
    DEF_SYS_MEM_AVAILABLE,
    DEF_SYS_MEM_CACHED,
    __SYS_MEM_INFO_MAXMUM
};

static const struct blobmsg_policy sys_mem_info_policy[] = {
    [DEF_SYS_MEM_TOTAL] = {.name = "total", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_MEM_FREE] = {.name = "free", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_MEM_SHARED] = {.name = "shared", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_MEM_BUFFERED] = {.name = "buffered", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_MEM_AVAILABLE] = {.name = "available", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_MEM_CACHED] = {.name = "cached", .type = BLOBMSG_TYPE_INT32},
};
// 需要修改如下
enum
{
    DEF_SYS_MEM_TOTAL,
    DEF_SYS_MEM_FREE,
    DEF_SYS_MEM_SHARED,
    DEF_SYS_MEM_BUFFERED,
    DEF_SYS_MEM_AVAILABLE,
    DEF_SYS_MEM_CACHED,
    __SYS_MEM_INFO_MAXMUM
};

static const struct blobmsg_policy sys_mem_info_policy[] = {
        [DEF_SYS_MEM_TOTAL] = {.name = "total", .type = BLOBMSG_TYPE_INT64},
        [DEF_SYS_MEM_FREE] = {.name = "free", .type = BLOBMSG_TYPE_INT64},
        [DEF_SYS_MEM_SHARED] = {.name = "shared", .type = BLOBMSG_TYPE_INT64},
        [DEF_SYS_MEM_BUFFERED] = {.name = "buffered", .type = BLOBMSG_TYPE_INT64},
    [DEF_SYS_MEM_AVAILABLE] = {.name = "available", .type = BLOBMSG_TYPE_INT64},
    [DEF_SYS_MEM_CACHED] = {.name = "cached", .type = BLOBMSG_TYPE_INT64},
};

// 同时取值时应该使用blobmsg_get_u64
printf("%d--%ld\n",__LINE__, blobmsg_get_u64(mem_attr[i]));

解决方法

将memory的所有字段都改为BLOBMSG_TYPE_INT64，同时采用blobmsg_get_u64获取值即可。

下面是修改过后的所有代码：

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include "libubox/uloop.h"
#include "libubus.h"

/*
{
        "localtime": 1628263880,
        "uptime": 77629,
        "load": [
                0,
                704,
                576
        ],
        "memory": {
                "total": 233041920,
                "free": 51875840,
                "shared": 188416,
                "buffered": 0,
                "available": 96661504,
                "cached": 8239104*40
        },
        "swap": {
                "total": 0,
                "free": 0
        }
}
*/

enum
{
    DEF_SYS_LOCAL_TIME,
    DEF_SYS_UPTIME,
    DEF_SYS_LOAD,
    DEF_SYS_MEM,
    __SYS_INFO_MAXMUM
};

static const struct blobmsg_policy sys_info_policy[] = {
    [DEF_SYS_LOCAL_TIME] = {.name = "localtime", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_UPTIME] = {.name = "uptime", .type = BLOBMSG_TYPE_INT32},
    [DEF_SYS_LOAD] = {.name = "load", .type = BLOBMSG_TYPE_ARRAY},
    [DEF_SYS_MEM] = {.name = "memory", .type = BLOBMSG_TYPE_TABLE},
};

/* 解析BLOBMSG_TYPE_ARRAY */
enum{
    DEF_SYS_LOAD_ITEM1,
    DEF_SYS_LOAD_ITEM2,
    DEF_SYS_LOAD_ITEM3,
    __SYS_LOAD_ITEM_MAXMUM
};

static const struct blobmsg_policy sys_load_policy[]={
    [DEF_SYS_LOAD_ITEM1]={.type=BLOBMSG_TYPE_INT32},
    [DEF_SYS_LOAD_ITEM2]={.type=BLOBMSG_TYPE_INT32},
    [DEF_SYS_LOAD_ITEM3]={.type=BLOBMSG_TYPE_INT32},
};


enum
{
    DEF_SYS_MEM_TOTAL,
    DEF_SYS_MEM_FREE,
    DEF_SYS_MEM_SHARED,
    DEF_SYS_MEM_BUFFERED,
    DEF_SYS_MEM_AVAILABLE,
    DEF_SYS_MEM_CACHED,
    __SYS_MEM_INFO_MAXMUM
};

static const struct blobmsg_policy sys_mem_info_policy[] = {
        [DEF_SYS_MEM_TOTAL] = {.name = "total", .type = BLOBMSG_TYPE_INT64},
        [DEF_SYS_MEM_FREE] = {.name = "free", .type = BLOBMSG_TYPE_INT64},
        [DEF_SYS_MEM_SHARED] = {.name = "shared", .type = BLOBMSG_TYPE_INT64},
        [DEF_SYS_MEM_BUFFERED] = {.name = "buffered", .type = BLOBMSG_TYPE_INT64},
    [DEF_SYS_MEM_AVAILABLE] = {.name = "available", .type = BLOBMSG_TYPE_INT64},
    [DEF_SYS_MEM_CACHED] = {.name = "cached", .type = BLOBMSG_TYPE_INT64},
};

static void handle_sys_mem_cb(struct ubus_request* req, int type, struct blob_attr* msg)
{
    if (msg != NULL)
    {
        struct blob_attr* tb[__SYS_INFO_MAXMUM];
        int localtime = 0;
        blobmsg_parse(sys_info_policy, __SYS_INFO_MAXMUM, tb, blob_data(msg), blob_len(msg));
        if(tb[DEF_SYS_LOCAL_TIME]){
            printf("%d---%u\n",__LINE__, blobmsg_get_u32(tb[DEF_SYS_LOCAL_TIME]));
        }
        for(int i=0;i<__SYS_INFO_MAXMUM;i++){
            printf("%d--%s\n",__LINE__, blobmsg_name(tb[i]));
            printf("%d--%d\n",__LINE__, blobmsg_type(tb[i]));
            printf("%d--%d\n",__LINE__, blobmsg_data_len(tb[i]));
            printf("%d--%d\n",__LINE__, blobmsg_len(tb[i]));
        }
        if(tb[DEF_SYS_LOAD]){
            struct blob_attr* sub_tb[__SYS_LOAD_ITEM_MAXMUM];
            blobmsg_parse_array(sys_load_policy, ARRAY_SIZE(sys_load_policy), sub_tb, blobmsg_data(tb[DEF_SYS_LOAD]), blobmsg_data_len(tb[DEF_SYS_LOAD]));
            for(int i=0; i<__SYS_LOAD_ITEM_MAXMUM;i++){
                printf("%d--%u\n",__LINE__, blobmsg_get_u32(sub_tb[i]));
                printf("%d--%d\n",__LINE__, blobmsg_data_len(sub_tb[i]));
                printf("%d--%d\n",__LINE__, blobmsg_len(sub_tb[i]));
            }
        }
        if(tb[DEF_SYS_MEM]){
            printf("%d--%d\n", __LINE__, blobmsg_type(tb[DEF_SYS_MEM])==BLOBMSG_TYPE_TABLE);
            FILE* fp=fopen("./mem.txt", "wb");
            if(fp!=NULL){
                fwrite(blobmsg_data(tb[DEF_SYS_MEM]), blobmsg_data_len(tb[DEF_SYS_MEM]), 1, fp);
                fclose(fp);
            }
            struct blob_attr* mem_attr[__SYS_MEM_INFO_MAXMUM];
            int iRet=blobmsg_parse(sys_mem_info_policy, ARRAY_SIZE(sys_mem_info_policy), mem_attr, blobmsg_data(tb[DEF_SYS_MEM]), blobmsg_data_len(tb[DEF_SYS_MEM]));
            printf("%d--%d\n",__LINE__, iRet);
            for(int i=0;i<__SYS_MEM_INFO_MAXMUM;i++){
                printf("%d--%d\n",__LINE__, i);
                printf("%d--%s\n",__LINE__, blobmsg_name(mem_attr[i]));
                printf("%d--%ld\n",__LINE__, blobmsg_get_u64(mem_attr[i]));
                printf("%d--%d\n",__LINE__, blobmsg_type(mem_attr[i]));
                printf("%d--%d\n",__LINE__, blobmsg_data_len(mem_attr[i]));
                printf("%d--%d\n",__LINE__, blobmsg_len(mem_attr[i]));
            }
            // if(mem_attr[DEF_SYS_MEM_TOTAL]){
            //     printf("buffered:%d---%u\n",__LINE__, blobmsg_get_u32(mem_attr[DEF_SYS_MEM_AVAILABLE]));
            // }
            // if(mem_attr[DEF_SYS_MEM_AVAILABLE]){
            //     printf("available:%d---%u\n",__LINE__, blobmsg_get_u32(mem_attr[DEF_SYS_MEM_AVAILABLE]));
            // }

        }

    }
}

/*****************
FUN:获取系统可用内存
ARG1: signal_strength，字符串指针，用来保存返回值
ARG2: len，signal_strength的长度
RETURN:成功0、失败-1
*****************/
int get_sys_mem()
{
    int iRet = -1;
    struct ubus_context* ubus_ctx = ubus_connect(NULL);
    uint32_t sys_mem_id;
    if (ubus_ctx != NULL)
    {
        if (ubus_lookup_id(ubus_ctx, "system", &sys_mem_id) != UBUS_STATUS_OK)
        {
            return iRet;
        }
        ubus_invoke(ubus_ctx, sys_mem_id, "info", NULL, handle_sys_mem_cb, NULL, 5000);
        ubus_free(ubus_ctx);
        iRet = 0;
    }
    return iRet;
}

int main(){
    get_sys_mem();
}

总结

针对这种二进制中含有字符串的数据，如果不知道其中的结构，可以使用fwrite将其写入文件然后使用WinHex工具查看其中的值。
那么问题来了，如何提前判断该接口的返回值是64为还是32位呢？总不能每次调用接口都查看二进制数据吧！我觉得在调用接口之前接口文档中必须明确说明返回字段的类型。

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目录

ubus接口调用解析时遇到的一点问题

问题描述

排查步骤

确定原因

解决方法

总结

See Also

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