Enable overriding of OpenSSL version (#27208)

Also change the order of attempts to load the libssl.so so that
the version 1.0.2 is tried first to make it less probable that some
of our other dependencies end up loading conflicting version of libssl
on Debian 8 that has bumped the libssl soname to 1.0.2.

Author: janvorli

src/Native/Unix/System.Security.Cryptography.Native/opensslshim.cpp

@@ -13,24 +13,48 @@
 FOR_ALL_OPENSSL_FUNCTIONS
 #undef PER_FUNCTION_BLOCK
 
+// x.x.x, considering the max number of decimal digits for each component
+static const int MaxVersionStringLength = 32;
+#define SONAME_BASE "libssl.so."
+
 static void* libssl = nullptr;
 
 bool OpenLibrary()
 {
-    // First try the default versioned so naming as described in the OpenSSL doc
-    libssl = dlopen("libssl.so.1.0.0", RTLD_LAZY);
-    if (libssl == nullptr)
+    // If there is an override of the version specified using the CLR_OPENSSL_VERSION_OVERRIDE
+    // env variable, try to load that first.
+    // The format of the value in the env variable is expected to be the version numbers,
+    // like 1.0.0, 1.0.2 etc.
+    char* versionOverride = getenv("CLR_OPENSSL_VERSION_OVERRIDE");
+
+    if ((versionOverride != nullptr) && strnlen(versionOverride, MaxVersionStringLength + 1) <= MaxVersionStringLength)
     {
-        // Fedora derived distros use different naming for the version 1.0.0
-        libssl = dlopen("libssl.so.10", RTLD_LAZY);
+        char soName[sizeof(SONAME_BASE) + MaxVersionStringLength] = SONAME_BASE;
+
+        strcat(soName, versionOverride);
+        libssl = dlopen(soName, RTLD_LAZY);
     }
 
     if (libssl == nullptr)
     {
-        // Debian 9 has dropped support for SSLv3 and so they have bumped their soname
+        // Debian 9 has dropped support for SSLv3 and so they have bumped their soname. Let's try it
+        // before trying the version 1.0.0 to make it less probable that some of our other dependencies 
+        // end up loading conflicting version of libssl.
         libssl = dlopen("libssl.so.1.0.2", RTLD_LAZY);
     }
 
+    if (libssl == nullptr)
+    {
+        // Now try the default versioned so naming as described in the OpenSSL doc
+        libssl = dlopen("libssl.so.1.0.0", RTLD_LAZY);
+    }
+
+    if (libssl == nullptr)
+    {
+        // Fedora derived distros use different naming for the version 1.0.0
+        libssl = dlopen("libssl.so.10", RTLD_LAZY);
+    }
+
     return libssl != nullptr;
 }