IP/TCP/UDP checsum

　　今天调试bug时， 忘了将原始的check_sum值reset，导致发包-抓包后发现。check-sum 错误。

来看一看check-sum：简单讲就是对要计算的数据，以16bit为单元进行累加，然后取反

checksum在收包和发包时意义不一样

TCP收包时：

/*
 *    @csum: Checksum (must include start/offset pair)
 *    @csum_start: Offset from skb->head where checksumming should start
 *    @csum_offset: Offset from csum_start where checksum should be stored
 *    @ip_summed: Driver fed us an IP checksum
 */
struct sk_buff {
    union {
        __wsum        csum;
        struct {
            __u16    csum_start;
            __u16    csum_offset;
        };
    };

    __u8    ip_summed:2,
            

skb->ip_summed一般的取值如下 ；skb->csum:存放硬件或者软件计算的payload的checksum不包括伪头

/* Don't change this without changing skb_csum_unnecessary! */
#define CHECKSUM_NONE        0
#define CHECKSUM_UNNECESSARY    1
#define CHECKSUM_COMPLETE    2
#define CHECKSUM_PARTIAL    3 

• CHECKSUM_UNNECESSARY

　　　　CHECKSUM_UNNECESSARY表示底层硬件已经计算了CSUM；所以TCP层在收到包后，发现skb->ip_summed为CHECKSUM_UNNECESSARY就不会再检查checksum:

• CHECKSUM_NONE

　　　　csum中的校验和无效，可能有以下几种原因：设备不支持硬件校验和计算；设备计算了硬件校验和，但发现该数据帧已经损坏。部分驱动不会丢弃，而是将ip_summed设置为CHECKSUM_NONE，然后交给上层协议栈重新计算并处理这种错误。

• CHECKSUM_COMPLETE

　　网卡已经计算了L4层报头和payload的校验和，并且skb->csum已经被赋值，此时L4层的接收者只需要加伪头并验证校验结果。

TCP发包时：

skb->ip_summed用于L4校验和的状态，以通知底层网卡是否还需要处理校验和；此时ip_summed可以被设置的值有下面两种

•  CHECKSUM_NONE

　　CHECKSUM_NONE表示协议栈已经计算了校验和，设备不需要做任何事情

•  CHECKSUM_PARTIAL

　　　　 CHECKSUM_PARTIAL表示使用硬件checksum ，协议栈已经计算L4层的伪头的校验和

skb->csum表示为csum_start和csum_offset，它表示硬件网卡存放将要计算的校验值的地址，和最后填充的便宜。这个域在输出包时使用，只在校验值在硬件计算的情况下才对于网卡真正有意义。硬件checksum功能只能用于非分片报文

1. TCP校验和覆盖TCP首部和TCP数据，而IP首部中的校验和只覆盖IP的首部，不覆盖IP数据报中的任何数据。
2. TCP的校验和是必需的，而UDP的校验和是可选的。
3. TCP和UDP计算校验和时，都要加上一个12字节的伪首部。

　　伪首部共有12字节，包含如下信息：源IP地址、目的IP地址、保留字节(置0)、传输层协议号(TCP是6)、TCP报文长度(报头+数据)。伪首部是为了增加TCP校验和的检错能力：如检查TCP报文是否收错了(目的IP地址)、传输层协议是否选对了(传输层协议号)等。

RFC 793的TCP校验和定义

　　The checksum field is the 16 bit one's complement of the one's complement sum of all 16-bit words in the header and text. If a segment contains an odd number of header and text octets to be checksummed, the last octet is padded on the right with zeros to form a 16-bit word for checksum purposes. The pad is not transmitted as part of the segment. While computingthe checksum, the checksum field itself is replaced with zeros.

　　把伪首部、TCP报头、TCP数据分为16位的字，如果总长度为奇数个字节，则在最后增添一个位都为0的字节。把TCP报头中的校验和字段置为0；

　　校验和的计算与顺序无关, 可以从数据块开始计算, 也可以从未尾开始向前计算

RFC 1071的IP校验和定义

1. Adjacent octets to be checksummed are paired to form 16-bit integers, and the 1's complement sum of these 16-bit integers is formed.

2. To generate a checksum, the checksum field itself is cleared, the 16-bit 1's complement sum is computed over the octets concerned, and the 1's complement of this sum is placed in the checksum field.

3. To check a checksum, the 1's complement sum is computed over the same set of octets, including the checksum field. If the result is all 1 bits (-0 in 1's complement arithmetic), the check succeeds.

内核协议栈中：

为了提高计算效率, TCP包的校验和并不一次算出,而是采用32位部分累加和(sk->csum)进行增量计算.
csum_partial()用来计算数据块的32位部分累加和, 累加和可以用csum_fold()折叠为16位校验和.csum_partial_copy_nocheck()可在拷贝用户数据的同时计算出它的部分累加和.
为了加快执行速度, csum_partial()将8个32位字分为一组用分立的指令进行32位累加,这样可加长循环体中指令长度, 提高CPU指令流水线的效率.

TCP包接收校验的初始化
static __sum16 tcp_v4_checksum_init(struct sk_buff *skb)
{
    const struct iphdr *iph = ip_hdr(skb);
    //如果TCP包本身的校验已经完成
    if (skb->ip_summed == CHECKSUM_COMPLETE) {
        if (!tcp_v4_check(skb->len, iph->saddr, iph->daddr, skb->csum)) { //附加伪头进行校验
            skb->ip_summed = CHECKSUM_UNNECESSARY;
            return 0;
        }
    }
    //生成包含伪头的累加和
    skb->csum = csum_tcpudp_nofold(iph->saddr, iph->daddr, skb->len, IPPROTO_TCP, 0);
    if (skb->len <= 76) {
        return __skb_checksum_complete(skb); //计算数据部分校验和
    }
    return 0;
}
    附加伪头进行校验
static inline __sum16 tcp_v4_check(int len, __be32 saddr, __be32 daddr, __wsum base)
{
    return csum_tcpudp_magic(saddr, daddr, len, IPPROTO_TCP, base);
}
static inline __sum16 csum_tcpudp_magic(__be32 saddr, __be32 daddr, unsigned short len, unsigned short proto, __wsum sum)
{
    return csum_fold(csum_tcpudp_nofold(saddr, daddr, len, proto, sum));
}
    生成包含伪头的累加和(源，目的，长度，协议号)
static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr, unsigned short len, unsigned short proto, __wsum sum)
{
    __asm__(
            "addl %1, %0    ;
"          //addl 加法
            "adcl %2, %0    ;
"          //adcl 带进位的加法
            "adcl %3, %0    ;
"
            "adcl $0, %0    ;
"          //如果有进位，进行累加
            : "=r" (sum)
            : "g" (daddr), "g"(saddr), "g"((len + proto) << 8), "0"(sum)
           );

    return sum;
}
    将32位累加和折叠成16位校验和
static inline __sum16 csum_fold(__wsum sum)
{
    __asm__(
            "addl %1, %0            ;
"
            "adcl $0xffff, %0       ;
"
            : "=r" (sum)
            : "r" ((__force u32)sum << 16), "0" ((__force u32)sum & 0xffff0000)
           );
    return (__force __sum16)(~(__force u32)sum >> 16);
}
    基于伪头累加和，完成全包校验
static __inline__ int tcp_checksum_complete(struct sk_buff *skb)
{
    return skb->ip_summed != CHECKSUM_UNNECESSARY && __tcp_checksum_complete(skb);
}
__sum16 __skb_checksum_complete(struct sk_buff *skb)
{
    return __skb_checksum_complete_head(skb, skb->len);
}
__sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len)
{
    __sum16 sum;

    sum = csum_fold(skb_checksum(skb, 0, len, skb->csum));
    if (likely(!sum)) {
        if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE))
            netdev_rx_csum_fault(skb->dev);
        skb->ip_summed = CHECKSUM_UNNECESSARY;
    }
    return sum;
}
__wsum skb_checksum(const struct sk_buff *skb, int offset, int len, __wsum csum)
{
    int start = skb_headlen(skb);
    int i, copy = start - offset;
    int pos = 0;

    /* Checksum header. */
    if (copy > 0) {
        if (copy > len)
            copy = len;

        csum = csum_partial(skb->data + offset, copy, csum);
        if ((len -= copy) == 0)
            return csum;

        offset += copy;
        pos     = copy;
    }
    ......
}
    计算32位中间累加和
unsigned int csum_partial(const unsigned char * buff, int len, unsigned int sum)
{
    //arch/x86/lib/checksum_32.S 汇编文件
}
    基于TCP用户数据的中间累加和, 生成TCP包校验码
void tcp_v4_send_check(struct sock *sk, int len, struct sk_buff *skb)
{
    struct inet_sock *inet = inet_sk(sk);
    struct tcphdr *th = tcp_hdr(skb);

    if (skb->ip_summed == CHECKSUM_PARTIAL) {
        th->check = ~tcp_v4_check(len, inet->saddr, inet->daddr, 0); //附加伪头进行校验
        skb->csum_start = skb_transport_header(skb) - skb->head;
        skb->csum_offset = offsetof(struct tcphdr, check);
    } else {
        //完整的tcp校验和计算方法
        th->check = tcp_v4_check(len, inet->saddr, inet->daddr, csum_partial((char *)th, th->doff << 2, skb->csum));
    }
}
    在拷贝用户数据时同时计算累加和
unsigned int csum_partial_copy_nocheck(const char *src, char *dst, int len, int sum)
{
    return csum_partial_copy_generic(src, dst, len, sum, NULL, NULL); // arch/x86/lib/checksum_32.S
}
    ip头校验和计算
static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
{
    unsigned int sum;

    __asm__ __volatile__(
            "movl (%1), %0      ;
"
            "subl $4, %2         ;
"
            "jbe 2f              ;
"
            "addl 4(%1), %0     ;
"  //sum = sum + *(iph+4)
            "adcl 8(%1), %0     ;
"  //sum = sum + *(iph+8) + carry
            "adcl 12(%1), %0    ;
"  //sum = sum + *(iph+12) + carry
            "1:                adcl 16(%1), %0     ;
" //sum = sum + *(iph+16) + carry
            "lea 4(%1), %1      ;
"  //iph = iph + 4
            "decl %2            ;
"
            "jne 1b             ;
"
            "adcl $0, %0        ;
"
            "movl %0, %2       ;
"
            "shrl $16, %0       ;
"
            "addw %w2, %w0   ;
"
            "adcl $0, %0        ;
"
            "notl %0            ;
"
            "2:                                    ;
"
            /* Since the input registers which are loaded with iph and ihl are modified, we must also specify them as outputs,
               or gcc will assume they contain their original values. */
            : "=r" (sum), "=r" (iph), "=r" (ihl)
            : "1" (iph), "2" (ihl)
                   : "memory"
                       );
    return (__force __sum16)sum;
}
    递减ip->ttl，更新校验和
static inline int ip_decrease_ttl(struct iphdr *iph)
{
    u32 check = (__force u32)iph->check;
    check += (__force u32)htons(0x0100);
    iph->check = (__force __sum16)(check + (check>=0xFFFF));
    return --iph->ttl;
}
static inline __wsum csum_add(__wsum csum, __wsum addend)
{
    u32 res = (__force u32)csum;
    res += (__force u32)addend;
    return (__force __wsum)(res + (res < (__force u32)addend));
}

static inline __wsum csum_sub(__wsum csum, __wsum addend)
{
    return csum_add(csum, ~addend);
}

static inline __wsum csum_block_add(__wsum csum, __wsum csum2, int offset)
{
    u32 sum = (__force u32)csum2;
    if (offset & 1)
        sum = ((sum & 0xFF00FF)<<8) + ((sum>>8) & 0xFF00FF);
    return csum_add(csum, (__force __wsum)sum);
}
static inline __wsum csum_block_sub(__wsum csum, __wsum csum2, int offset)
{
    u32 sum = (__force u32)csum2;
    if (offset & 1)
        sum = ((sum & 0xFF00FF)<<8) + ((sum>>8) & 0xFF00FF);
    return csum_sub(csum, (__force __wsum)sum);
}
[/函数实现]
转载：https://www.cnblogs.com/super-king/p/3284884.html

 https://hustcat.github.io/checksum-in-kernel/

https://www.kernel.org/doc/Documentation/networking/checksum-offloads.txt

https://w180112.pixnet.net/blog/post/200083785

http://blog.chinaunix.net/uid-25518484-id-5709671.html