Convert mbedtls_free and mbedtls_calloc into functions

When MBEDTLS_PLATFORM_MEMORY is defined but MBEDTLS_PLATFORM_FREE_MACRO or
MBEDTLS_PLATFORM_CALLOC_MACRO are not defined then the actual functions
used to allocate and free memory are stored in function pointers.
These pointers are exposed to the caller, and it means that the caller
and the library have to share a data section.

In TF-A, we execute in a very constrained environment, where some images
are executed from ROM and other images are executed from SRAM. The
images that are executed from ROM cannot be modified. The SRAM size
is very small and we are moving libraries to the ROM that can be shared
between the different SRAM images.  These SRAM images could import all the
symbols used in mbedtls, but it would create an undesirable hard binary
dependency between the different images. For this reason, all the library
functions in ROM are accesed using a jump table whose base address is
known, allowing the images to execute with different versions of the ROM.

This commit changes the function pointers to actual functions,
so that the SRAM images only have to use the new exported symbols
(mbedtls_calloc and mbedtls_free) using the jump table. In
our scenario, mbedtls_platform_set_calloc_free is called from
mbedtls_memory_buffer_alloc_init which initializes the function pointers
to the internal buffer_alloc_calloc and buffer_alloc_free functions.

No functional changes to mbedtls_memory_buffer_alloc_init.

Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>

include/mbedtls/platform.h

@@ -121,8 +121,8 @@ extern "C" {
 #else
 /* For size_t */
 #include <stddef.h>
-extern void * (*mbedtls_calloc)( size_t n, size_t size );
-extern void (*mbedtls_free)( void *ptr );
+extern void *mbedtls_calloc( size_t n, size_t size );
+extern void mbedtls_free( void *ptr );
 
 /**
  * \brief               This function dynamically sets the memory-management

library/platform.c

@@ -51,14 +51,24 @@ static void platform_free_uninit( void *ptr )
 #define MBEDTLS_PLATFORM_STD_FREE     platform_free_uninit
 #endif /* !MBEDTLS_PLATFORM_STD_FREE */
 
-void * (*mbedtls_calloc)( size_t, size_t ) = MBEDTLS_PLATFORM_STD_CALLOC;
-void (*mbedtls_free)( void * )     = MBEDTLS_PLATFORM_STD_FREE;
+static void * (*mbedtls_calloc_func)( size_t, size_t ) = MBEDTLS_PLATFORM_STD_CALLOC;
+static void (*mbedtls_free_func)( void * ) = MBEDTLS_PLATFORM_STD_FREE;
+
+void * mbedtls_calloc( size_t nmemb, size_t size )
+{
+    return (*mbedtls_calloc_func)( nmemb, size );
+}
+
+void mbedtls_free( void * ptr )
+{
+    (*mbedtls_free_func)( ptr );
+}
 
 int mbedtls_platform_set_calloc_free( void * (*calloc_func)( size_t, size_t ),
                               void (*free_func)( void * ) )
 {
-    mbedtls_calloc = calloc_func;
-    mbedtls_free = free_func;
+    mbedtls_calloc_func = calloc_func;
+    mbedtls_free_func = free_func;
     return( 0 );
 }
 #endif /* MBEDTLS_PLATFORM_MEMORY */