Build on arm7vl (RPi 3B+) fails due to invalid HMAC detection

[mprather]

Setup:

• vcpkg is the package manager
• device: RPi 3B+
• OS: Raspian/Debian 12
• arch: arm7v

Repro:
Attempt to build the libwebsockets library

vcpkg install libwebsockets

Result:
Compilation fails.

Log:

In file included from /home/azure/agent/_work/55/no-cache/b/libwebsockets/src/v4.4.1-86cef274ae.clean/include/libwebsockets.h:778,
                 from /home/azure/agent/_work/55/no-cache/b/libwebsockets/src/v4.4.1-86cef274ae.clean/lib/core/./private-lib-core.h:150,
                 from /home/azure/agent/_work/55/no-cache/b/libwebsockets/src/v4.4.1-86cef274ae.clean/lib/plat/unix/unix-init.c:28:
/home/azure/agent/_work/55/no-cache/b/libwebsockets/src/v4.4.1-86cef274ae.clean/include/libwebsockets/lws-genhash.h:93:18: error: field ‘ctx’ has incomplete type
   93 |         HMAC_CTX ctx;
      |                  ^~~

Investigation:
Investigation found that on this device the config call that sets up the LWS_HAVE_HMAC_CTX_new variable in tls/CMakeLists.txt always returns false on the system.

~line 349:

CHECK_FUNCTION_EXISTS(${VARIA}HMAC_CTX_new LWS_HAVE_HMAC_CTX_new PARENT_SCOPE)

OpenSSL is not installed on the system but it is built by vcpkg (as one of lws dependencies). I've verified that OpenSSL was properly built as a dependency. I've also confirmed that the vcpkg folders for include and libraries are listed in CMAKE_REQUIRED_INCLUDES and CMAKE_REQUIRED_LIBRARIES variables, respectively, before calling check_function_exists. Everything is properly setup but the check failed nonetheless.

The reason is because OpenSSL’s ARMv7 build depends on 64‑bit GCC atomics in libatomic. Without including that library as a required library, the check function would never succeed.

I wrote a simple test app to determine why the check was failing - was it really forcing use of the older api or something else?

App:

include <openssl/hmac.h>

int main() { 
  HMAC_CTX *ctx = HMAC_CTX_new(); 
  return ctx == NULL; 
}

Result of Compilation without atomic
Failure with warnings and lots of linker errors

gcc test.cpp -I/home/azure/agent/_work/69/vcpkg/installed/arm-linux/include -L/home/azure/agent/_work/69/vcpkg/installed/arm-linux/lib -lcrypto -lssl -o test
test.cpp: In function ‘int main()’:
test.cpp:3:42: warning: ‘HMAC_CTX* HMAC_CTX_new()’ is deprecated: Since OpenSSL 3.0 [-Wdeprecated-declarations]
    3 | int main() { HMAC_CTX *ctx = HMAC_CTX_new(); return ctx == NULL; }
      |                              ~~~~~~~~~~~~^~
In file included from test.cpp:1:
/home/azure/agent/_work/69/vcpkg/installed/arm-linux/include/openssl/hmac.h:33:33: note: declared here
   33 | OSSL_DEPRECATEDIN_3_0 HMAC_CTX *HMAC_CTX_new(void);
      |                                 ^~~~~~~~~~~~
/usr/bin/ld: /home/azure/agent/_work/69/vcpkg/installed/arm-linux/lib/libcrypto.a(libcrypto-lib-threads_pthread.o): in function `ossl_rcu_read_lock':
threads_pthread.c:(.text+0x304): undefined reference to `__atomic_fetch_sub_8'
/usr/bin/ld: threads_pthread.c:(.text+0x320): undefined reference to `__atomic_fetch_add_8'
/usr/bin/ld: /home/azure/agent/_work/69/vcpkg/installed/arm-linux/lib/libcrypto.a(libcrypto-lib-threads_pthread.o): in function `ossl_rcu_read_unlock':

Result of Compilation with atomic
Success with warnings:

gcc test.cpp -I/home/azure/agent/_work/69/vcpkg/installed/arm-linux/include -L/home/azure/agent/_work/69/vcpkg/installed/arm-linux/lib -lcrypto -lssl -latomic -o test
test.cpp: In function ‘int main()’:
test.cpp:3:42: warning: ‘HMAC_CTX* HMAC_CTX_new()’ is deprecated: Since OpenSSL 3.0 [-Wdeprecated-declarations]
    3 | int main() { HMAC_CTX *ctx = HMAC_CTX_new(); return ctx == NULL; }
      |                              ~~~~~~~~~~~~^~
In file included from test.cpp:1:
/home/azure/agent/_work/69/vcpkg/installed/arm-linux/include/openssl/hmac.h:33:33: note: declared here
   33 | OSSL_DEPRECATEDIN_3_0 HMAC_CTX *HMAC_CTX_new(void);
      |                                 ^~~~~~~~~~~~

Result: the program compiles and links. Only warnings stating that a deprecated api is in use. This provided 2 key pieces of info - the atomic library needed to be referenced, and the built version of OpenSSL was usable for the LWS_HAVE_HMAC_CTX_new compilation branches.

I will push a PR shortly that contains a recommended fix.

Reference issue:
microsoft/vcpkg#48725