appimagetool: calculate size via _runtime_binary_start and _runtime_b…

…inary_end. (#288)

On my Ubuntu yakkety x86_64 sytem, the _runtime_binary_size
symbol is unreliable. For non-ASLR, it points somewhere after
0x555555554000 (where appimagetool is loaded). And with ASLR,
it's... random.

So, instead of relying on the address of _runtime_binary_size
being the size of the runtime blob, calculate the actual size
by using _runtime_binary_start and _runtime_binary_end.

appimagetool.c

@@ -53,7 +53,7 @@
 #include "getsection.h"
 
 extern int _binary_runtime_start;
-extern int _binary_runtime_size;
+extern int _binary_runtime_end;
 
 
 static gint repeats = 2;
@@ -481,7 +481,7 @@ main (int argc, char *argv[])
         fprintf (stderr, "Generating squashfs...\n");
         /* runtime is embedded into this executable
         * http://stupefydeveloper.blogspot.de/2008/08/cc-embed-binary-data-into-elf.html */
-        int size = (int)&_binary_runtime_size;
+        int size = (int)((void *)&_binary_runtime_end - (void *)&_binary_runtime_start);
         char *data = (char *)&_binary_runtime_start;
         if (verbose)
             printf("Size of the embedded runtime: %d bytes\n", size);