1. BIO简介

BIO chain

BIO可以组成一条链，即使是单个BIO，实质也是有一个节点的链。

一个链通常由1个source/sink BIO、1个或多个filter BIO组成。

数据从第一个BIO写入或读出，并传递到最后一个节点（通常是source/sink BIO）。

BIO数据结构

源码位置：\crypto\bio\bio_local.h

struct bio_st {// BIO_new初始化需要提供libctx和method参数// BIO *BIO_new(const BIO_METHOD *method)// {//     return BIO_new_ex(NULL, method);// }OSSL_LIB_CTX *libctx;	// NULL: default contextzconst BIO_METHOD *method;/* bio, mode, argp, argi, argl, ret */
#ifndef OPENSSL_NO_DEPRECATED_3_0BIO_callback_fn callback;
#endifBIO_callback_fn_ex callback_ex;char *cb_arg;               /* first argument for the callback */// 这里用int来作标志应该有点浪费// 用bool或bit更好些int init;		// 初始化标志int shutdown;int flags;                  /* extra storage */int retry_reason;int num;void *ptr;		// BIO_set_data() // bio链本质是双向链表struct bio_st *next_bio;    /* used by filter BIOs */struct bio_st *prev_bio;    /* used by filter BIOs */CRYPTO_REF_COUNT references;uint64_t num_read;uint64_t num_write;CRYPTO_EX_DATA ex_data;CRYPTO_RWLOCK *lock;	// 线程读写锁
};

BIO_METHOD数据结构

源码路径：\include\internal\bio.h

struct bio_method_st {int type;char *name;int (*bwrite) (BIO *, const char *, size_t, size_t *);int (*bwrite_old) (BIO *, const char *, int);int (*bread) (BIO *, char *, size_t, size_t *);int (*bread_old) (BIO *, char *, int);int (*bputs) (BIO *, const char *);int (*bgets) (BIO *, char *, int);long (*ctrl) (BIO *, int, long, void *);int (*create) (BIO *);int (*destroy) (BIO *);long (*callback_ctrl) (BIO *, int, BIO_info_cb *);
};

该结构除了类型和名称，其余均是函数指针。

2. Base64示例分析

借用官网Base64示例：https://www.openssl.org/docs/man3.0/man3/BIO_f_base64.html，它将"hello world \n"的base64输出到stdout：

#include <iostream>
#include <openssl/bio.h>
#include <openssl/evp.h>
int main()
{BIO* bio, * b64;char message[] = "Hello World \n";b64 = BIO_new(BIO_f_base64());bio = BIO_new_fp(stdout, BIO_NOCLOSE);BIO_push(b64, bio);BIO_write(b64, message, strlen(message));BIO_flush(b64);// SGVsbG8gV29ybGQgCg==BIO_free_all(b64);getchar();return 0;
}

用vscode在源码目录搜索“base64”，并没有找到实现源码，于是全局搜索编码表，定位到源码路径：

// \crypto\evp\encode.c
static const unsigned char data_bin2ascii[65] ="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

把encode.c拖到vs中，在引用编码表的地方下断点，调试示例程序，成功断下：

请添加图片描述

BIO_flush调用栈如下：

libcrypto-3.dll!evp_encodeblock_int(evp_Encode_Ctx_st * ctx, unsigned char * t, const unsigned char * f, int dlen) Line 238	C
libcrypto-3.dll!EVP_EncodeFinal(evp_Encode_Ctx_st * ctx, unsigned char * out, int * outl) Line 222	C
libcrypto-3.dll!b64_ctrl(bio_st * b, int cmd, long num, void * ptr) Line 506	C
libcrypto-3.dll!BIO_ctrl(bio_st * b, int cmd, long larg, void * parg) Line 579	C
TestOpenSSL.exe!main() Line 13	C++

EVP是OpenSSL的算法实现接口，结合evp文档（/html/man7/evp.html），这里正是base64编解码逻辑：

The EVP_EncodeXXX and EVP_DecodeXXX functions implement base 64 encoding and decoding.

初始化

b64 = BIO_new(BIO_f_base64());
bio = BIO_new_fp(stdout, BIO_NOCLOSE);

BIO_new_fp()，内部其实是调用了BIO_new(BIO_s_file())初始化一个source/sink BIO。所以只分析第一个base64 filter BIO。

BIO_f_base64()返回一个base64的静态BIO_METHOD：

// \crypto\evp\bio_b64.c
static const BIO_METHOD methods_b64 = {BIO_TYPE_BASE64,"base64 encoding",bwrite_conv,b64_write,bread_conv,b64_read,b64_puts,NULL,                       /* b64_gets, */b64_ctrl,b64_new,b64_free,b64_callback_ctrl,
};const BIO_METHOD *BIO_f_base64(void)
{return &methods_b64;
}

通过BIO_TYPE_BASE64这个宏type，可以在\include\openssl\bio.h定位到其它method type：

/*在源码层面，BIO其实是有3种。
*/
/* There are the classes of BIOs */
# define BIO_TYPE_DESCRIPTOR     0x0100 /* socket, fd, connect or accept */
# define BIO_TYPE_FILTER         0x0200
# define BIO_TYPE_SOURCE_SINK    0x0400/* These are the 'types' of BIOs */
# define BIO_TYPE_NONE             0
# define BIO_TYPE_MEM            ( 1|BIO_TYPE_SOURCE_SINK)
# define BIO_TYPE_FILE           ( 2|BIO_TYPE_SOURCE_SINK)# define BIO_TYPE_FD             ( 4|BIO_TYPE_SOURCE_SINK|BIO_TYPE_DESCRIPTOR)
// ...
# define BIO_TYPE_BASE64         (11|BIO_TYPE_FILTER)

BIO_new则根据base 64 method初始化filter BIO：

请添加图片描述
method->create为b64_new()，用于初始化bio->ptr==b64_ctx

// \crypto\evp\bio_b64.c
typedef struct b64_struct {/** BIO *bio; moved to the BIO structure*/int buf_len;int buf_off;int tmp_len;                /* used to find the start when decoding */int tmp_nl;                 /* If true, scan until '\n' */int encode;int start;                  /* have we started decoding yet? */int cont;                   /* <= 0 when finished */EVP_ENCODE_CTX *base64;		char buf[EVP_ENCODE_LENGTH(B64_BLOCK_SIZE) + 10];char tmp[B64_BLOCK_SIZE];
} BIO_B64_CTX;

函数栈如下：

>	libcrypto-3.dll!b64_new(bio_st * bi) Line 71	CBIO_B64_CTX *ctx;//...BIO_set_data(bi, ctx); // set bi->ptrBIO_set_init(bi, 1);	// initialized libcrypto-3.dll!BIO_new_ex(ossl_lib_ctx_st * libctx, const bio_method_st * method) Line 104	Cif (method->create != NULL && !method->create(bio)) {// ...goto err;}if (method->create == NULL)bio->init = 1;	// 若没有单独的create初始化函数，则直接设置init标志libcrypto-3.dll!BIO_new(const bio_method_st * method) Line 122	Creturn BIO_new_ex(NULL, method);TestOpenSSL.exe!main() Line 9	C++

构造BIO链

源码位置：\crypto\bio\bio_lib.c

// BIO_push(b64, bio);
BIO *BIO_push(BIO *b, BIO *bio)
{BIO *lb;if (b == NULL)return bio;lb = b;while (lb->next_bio != NULL)lb = lb->next_bio;lb->next_bio = bio;if (bio != NULL)bio->prev_bio = lb;/* called to do internal processing */BIO_ctrl(b, BIO_CTRL_PUSH, 0, lb);return b;
}

在BIO链表中，stdout source/sink BIO是在 base64 filter BIO之后的。

写数据

BIO_write(b64, message, strlen(message));
BIO_flush(b64);

再看一下methods_b64：

static const BIO_METHOD methods_b64 = {BIO_TYPE_BASE64,"base64 encoding",bwrite_conv,	// int (*bwrite) (BIO *, const char *, size_t, size_t *);b64_write,		// int (*bwrite_old) (BIO *, const char *, int);//...
}

调用BIO_write，其实是调用base64 method的bwrite和bwrite_old函数，形成如下函数栈：

libcrypto-3.dll!EVP_EncodeInit(evp_Encode_Ctx_st * ctx) Line 156	C
libcrypto-3.dll!b64_write(bio_st * b, const char * in, int inl) Line 346	C
libcrypto-3.dll!bwrite_conv(bio_st * bio, const char * data, unsigned int datal, unsigned int * written) Line 77	Cret = bio->method->bwrite_old(bio, data, (int)datal);
libcrypto-3.dll!bio_write_intern(bio_st * b, const void * data, unsigned int dlen, unsigned int * written) Line 362	Cret = b->method->bwrite(b, data, dlen, &local_written);
libcrypto-3.dll!BIO_write(bio_st * b, const void * data, int dlen) Line 384	C
TestOpenSSL.exe!main() Line 12	C++

但经过调试，执行BIO_write后标准输出（就是屏幕）并没有回显，在evp_encodeblock_int的断点也没有断下。而根据最开始贴出的BIO_flush调用栈，它在执行缓冲区刷新时才开始执行evp_encodeblock_int，文档如下：

https://www.openssl.org/docs/man3.0/man3/BIO_flush.html
BIO_flush() normally writes out any internally buffered data, in some cases it is used to signal EOF and that no more data will be written.

该刷新函数底层是调用BIO_ctrl()实现的，该类控制函数通常并不直接使用，而是通过BIO_flush等定义于\include\openssl\bio.h中的宏函数来调用的。

执行evp_encodeblock_int后，b64_ctrl()会将base64写入bio链下一节点，并执行下一节点的ctrl函数：

libcrypto-3.dll!b64_write(bio_st * b, const char * in, int inl) Line 354	Cnext = BIO_next(b);// ...while (n > 0) {i = BIO_write(next, &(ctx->buf[ctx->buf_off]), n);	// 写入stdout bio  这时屏幕会出现base64值// ...}
libcrypto-3.dll!b64_ctrl(bio_st * b, int cmd, long num, void * ptr) Line 490	Ccase BIO_CTRL_FLUSH:/* do a final write */while (ctx->buf_len != ctx->buf_off) {i = b64_write(b, NULL, 0);	// <--// ...}// .../* Finally flush the underlying BIO */ret = BIO_ctrl(next, cmd, num, ptr);	// stdout bio的ctrl函数为file_ctrl()
libcrypto-3.dll!BIO_ctrl(bio_st * b, int cmd, long larg, void * parg) Line 579	Cret = b->method->ctrl(b, cmd, larg, parg);
TestOpenSSL.exe!main() Line 13	C++

free

BIO_free_all(b64);

这个函数的作用，猜也能猜的到，遍历链表逐个释放空间。

// \crypto\bio\bio_lib.c
void BIO_free_all(BIO *bio)
{BIO *b;int ref;while (bio != NULL) {b = bio;	// 从第一个节点开始释放ref = b->references;bio = bio->next_bio;BIO_free(b);/* Since ref count > 1, don't free anyone else.意思是别人还在用 别删*/if (ref > 1)break;}
}// 需要释放的东西还是很多的
int BIO_free(BIO *a)
{int ret;if (a == NULL)return 0;// 引用数减1if (CRYPTO_DOWN_REF(&a->references, &ret, a->lock) <= 0)return 0;REF_PRINT_COUNT("BIO", a);if (ret > 0)return 1;REF_ASSERT_ISNT(ret < 0);if (HAS_CALLBACK(a)) {ret = (int)bio_call_callback(a, BIO_CB_FREE, NULL, 0, 0, 0L, 1L, NULL);if (ret <= 0)return 0;}if ((a->method != NULL) && (a->method->destroy != NULL))a->method->destroy(a);	// b64_free()CRYPTO_free_ex_data(CRYPTO_EX_INDEX_BIO, a, &a->ex_data);CRYPTO_THREAD_lock_free(a->lock);OPENSSL_free(a);return 1;
}