ndpiReader: install custom memory allocators before any nDPI API usage (#3164)

ndpiReader was setting custom memory allocation hooks only inside
test_lib(), which is executed after several code paths that already
use nDPI APIs (e.g., parseOptions, help/extcap paths, host checks,
DoH initialization, etc.).

This could lead to inconsistent allocator usage where memory is
allocated using the default libc allocator (when hooks are unset)
and later freed using custom allocators, resulting in undefined
behavior with non-trivial allocators.

This partially addresses #1280.

Author: fabiodepin

Makefile.am

@@ -43,7 +43,8 @@ EXTRA_DIST = README.md README.fuzzer.md CHANGELOG.md CONTRIBUTING.md \
 		lists/README.md \
 		lists/protocols/README.md \
 	sonar-project.properties .github .ci-ignore \
-	src/lib/plugins/README.md
+	src/lib/plugins/README.md \
+	example/README.md
 
 doc:
 	make -C doc html

doc/library_initialization.rst

@@ -82,3 +82,32 @@ A more complex example, with global context and a shared Oookla LRU cache (all t
         
         ndpi_global_deinit(g_ctx);
 
+
+
+Another example, with a custom memory allocator
+
+.. code:: c
+        
+        struct ndpi_detection_module_struct *ndpi_struct;
+        ndpi_cfg_error rc;
+        int ret;
+
+        /* If you want to set a custom allocator for all memory allocations performed by
+           the library, you must call `ndpi_set_memory_alloction_functions() ONCE,
+           BEFORE ANY nDPI functions */
+
+        ndpi_set_memory_alloction_functions(malloc_wrapper,
+                                            free_wrapper,
+                                            calloc_wrapper,
+                                            realloc_wrapper,
+                                            aligned_malloc_wrapper,
+                                            aligned_free_wrapper,
+                                            flow_malloc_wrapper,
+                                            flow_free_wrapper);
+
+        ndpi_struct = ndpi_init_detection_module(NULL);
+        if(!ndpi_struct) {
+        	ERROR;
+        }
+        
+        /* Continue as the previous examples */

example/README.md

@@ -0,0 +1,10 @@
+# nDPI example tools
+
+This directory contains sample programs that link against libnDPI, mainly **ndpiReader** (CLI over pcaps and live capture) and **ndpiSimpleIntegration**. DPDK-specific notes live in [README.DPDK](README.DPDK).
+
+## Custom memory allocators
+
+nDPI can use application-provided allocators via `ndpi_set_memory_alloction_functions()` (see `ndpi_api.h` / `ndpi_memory.c`). **Every pointer obtained through `ndpi_malloc()`, `ndpi_calloc()`, `ndpi_strdup()`, APIs that allocate internally (e.g. `ndpi_init_bin()`), or memory owned by the library must be released with the matching `ndpi_free()` path that uses the same allocator configuration as at allocation time.**
+From a practical point of view, that means that the application code must installs its hooks once, via `ndpi_set_memory_alloction_functions()` **before ANY library functions**.
+
+**IF** your own application wants to use `ndpi_malloc()` and similar functions also for its own allocations, `ndpi_set_memory_alloction_functions()` **must** be called before **ANY** allocations, from the library and from the application, both. All these memory must be freed via `ndpi_free()`.

example/ndpiReader.c

@@ -5574,15 +5574,6 @@ void test_lib() {
 #endif
   struct ndpi_global_context *g_ctx;
 
-  ndpi_set_memory_alloction_functions(malloc_wrapper,
-                                      free_wrapper,
-                                      calloc_wrapper,
-                                      realloc_wrapper,
-                                      aligned_malloc_wrapper,
-                                      aligned_free_wrapper,
-                                      flow_malloc_wrapper,
-                                      flow_free_wrapper);
-
 #ifndef USE_GLOBAL_CONTEXT
   /* ndpiReader works even if libnDPI has been compiled without global context support,
      but you can't configure any cache with global scope */
@@ -5771,6 +5762,20 @@ int main(int argc, char **argv) {
     return(-1);
   }
 
+  /* Set a custom allocator for the library.
+     **IF** you want to use `ndpi_malloc()` and similar functions to allocate memory ALSO
+     from your application code, you must be sure to call `ndpi_set_memory_alloction_functions()`
+     BEFORE ANY allocations (from the library and from the application, both)
+  */
+  ndpi_set_memory_alloction_functions(malloc_wrapper,
+                                      free_wrapper,
+                                      calloc_wrapper,
+                                      realloc_wrapper,
+                                      aligned_malloc_wrapper,
+                                      aligned_free_wrapper,
+                                      flow_malloc_wrapper,
+                                      flow_free_wrapper);
+
   gettimeofday(&startup_time, NULL);
   memset(ndpi_thread_info, 0, sizeof(ndpi_thread_info));