C++ Ebpf内核代码：permission denied：invalid access to map value

我正在写一个ebpf程序来读取tc上的数据包.下面的代码被简化，以更好地理解问题.假设我有一个ebpf map，其值是如下定义的 struct :

struct value {
    int index;
    char flags[MAX_FLAGS_LEN];
};

struct {
    __uint(type, BPF_MAP_TYPE_LRU_HASH);
    __uint(max_entries, 32);
    __type(key, __u8); 
    __type(value, struct value); 
} output_map SEC(".maps");

如果我的ebpf程序在主要部分全部写出来，它会工作得很好，如下所示.

SEC("classifier")
int tc_ingress(struct __sk_buff *skb)
{
    __u8 key = 1;
    struct value *value;
    value = bpf_map_lookup_elem(&output_map, &key);

    if (value == NULL){
        struct value initial_value = {
            .index = 1,
            .flags = ","
        };
        bpf_map_update_elem(&output_map, &key, &initial_value, BPF_NOEXIST);
    } else {
        if (value->index >= sizeof(value->flags)){
            goto out;
        }

    }

 
    if (value->index < sizeof(value->flags)){
        value->flags[value->index] = ',';
        value->index++;
    }

    if (value->index < sizeof(value->flags)){
        value->flags[value->index] = ',';
        value->index++;
    }

    bpf_printk("Flags: %d", value->flags);

out: 
    return TC_ACT_OK;
}

但是，当我重构代码时，如下所示:

static __always_inline void store_flags(struct value *value){
    if (value->index < sizeof(value->flags)){
        value->flags[value->index] = ',';
        value->index++;
    }

    if (value->index < sizeof(value->flags)){
        value->flags[value->index] = ',';
        value->index++;
    }
}

SEC("classifier")
int tc_ingress(struct __sk_buff *skb)
{
    __u8 key = 1;
    struct value *value;
    value = bpf_map_lookup_elem(&output_map, &key);

    if (value == NULL){
        struct value initial_value = {
            .index = 1,
            .flags = ","
        };
        bpf_map_update_elem(&output_map, &key, &initial_value, BPF_NOEXIST);
    } else {
        if (value->index >= sizeof(value->flags)){
            goto out;
        }

    }

    store_flags(value);

    bpf_printk("Flags: %d", value->flags);

out: 
    return TC_ACT_OK;
}

我会得到这个错误:

permission denied:invalid access to map value，value_size = 20 off = 20 size = 1:R4 max value超出允许的内存范围(省略了52行)

有谁知道解决办法吗？

Edit: Dylan's answer contains detailed reasoning why it happened. Modifying the code to this fixed it.

static __always_inline void store_flags(struct value *value){
    int index = value->index;

    if (index < sizeof(value->flags)){
        value->flags[index] = ',';
        index++;
    }

    if (index < sizeof(value->flags)){
        value->flags[index] = ',';
        index++;
    }

    value->index=index;
}

SEC("classifier")
int tc_ingress(struct __sk_buff *skb)
{
    __u8 key = 1;
    struct value *value;
    value = bpf_map_lookup_elem(&output_map, &key);

    if (value == NULL){
        struct value initial_value = {
            .index = 1,
            .flags = ","
        };
        bpf_map_update_elem(&output_map, &key, &initial_value, BPF_NOEXIST);
    } else {
        store_flags(value);
    }

    bpf_printk("Flags: %d", value->flags);

    return TC_ACT_OK;
}