如何减少OpenSSL 3的动态内存分配，尤其是RAND_bytes的调用？

测试代码（MRE） #

#include <openssl/rand.h>

int main() {
        unsigned char buf[16];
        return RAND_bytes(buf, 16);
}

OpenSSL 1编译后的内存分配情况 #

使用OpenSSL 1编译链接后，通过valgrind检测到的malloc调用情况如下：

$ valgrind ./a.out
==4904== Memcheck, a memory error detector
==4904== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==4904== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==4904== Command: ./a.out
==4904== 
==4904== 
==4904== HEAP SUMMARY:
==4904==     in use at exit: 0 bytes in 0 blocks
==4904==   total heap usage: 32 allocs, 32 frees, 21,412 bytes allocated
==4904== 
==4904== All heap blocks were freed -- no leaks are possible
==4904== 
==4904== For counts of detected and suppressed errors, rerun with: -v
==4904== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

OpenSSL 3编译后的内存分配情况 #

使用OpenSSL 3编译链接后，valgrind检测到malloc调用次数大幅增加：

$ valgrind --tool=memcheck ./a.out
==18334== Memcheck, a memory error detector
==18334== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==18334== Using Valgrind-3.20.0 and LibVEX; rerun with -h for copyright info
==18334== Command: ./a.out
==18334== 
==18334== 
==18334== HEAP SUMMARY:
==18334==     in use at exit: 0 bytes in 0 blocks
==18334==   total heap usage: 6,934 allocs, 6,934 frees, 626,370 bytes allocated
==18334== 
==18334== All heap blocks were freed -- no leaks are possible
==18334== 
==18334== For lists of detected and suppressed errors, rerun with: -s
==18334== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

问题背景 #

推测该差异源于OpenSSL 3的动态provider接口，即使尝试加载OpenSSL的"null" provider，malloc调用次数仍有2500次，现需找到将OpenSSL 3内存使用降至接近OpenSSL 1水平的方案。

可行优化方案 #

• 静态链接+手动加载必要provider：OpenSSL 3默认会加载多个动态provider（如default、legacy等），静态链接时可手动初始化仅需的provider（比如满足随机数需求的base provider），避免冗余初始化。修改代码并编译：

  #include <openssl/rand.h>
  #include <openssl/provider.h>
  
  int main() {
      unsigned char buf[16];
      // 仅加载base provider
      OSSL_PROVIDER *base = OSSL_PROVIDER_load(NULL, "base");
      if (!base) return 1;
      
      int ret = RAND_bytes(buf, 16);
      
      OSSL_PROVIDER_unload(base);
      return ret;
  }
  

  编译命令：gcc -o a.out test.c -lcrypto -static

• 编译OpenSSL 3时禁用动态provider：自行编译OpenSSL 3时，添加--no-dynamic-providers配置选项，强制使用静态编译的基础provider，彻底关闭动态加载机制，大幅减少初始化阶段的内存分配：

  ./configure --no-dynamic-providers --prefix=/usr/local/openssl3
  make && make install
  

  使用该版本编译测试代码，malloc调用次数会接近OpenSSL 1的水平。

• 复用初始化上下文：如果是长期运行的服务，可在程序启动时一次性完成OpenSSL的初始化工作，复用全局上下文，避免每次调用都重复初始化相关组件，降低整体内存分配频次。

• 直接调用低级随机数接口：若仅需随机数生成，可跳过高层的RAND_bytes，直接调用base provider提供的低级核心API，减少抽象层带来的初始化开销，但这种方式对OpenSSL 3核心API的熟悉度要求较高，代码耦合性较强。

内容的提问来源于stack exchange，提问作者Greg