support.c: ensure 256-bits alignment of the stack

This ensure floating point memory accesses work properly on x86. In the
past this was working only in certain cases depending on the number of
entries in the auxiliary vector, arguments and environment variables.

support.c

@@ -122,8 +122,11 @@ static void *prepare_stack(void *stack, int argc, char **argv) {
 
   arge = argp;
 
+  // Push the environment variables
+  unsigned env_count = 0;
   while (*--argp != NULL) {
     MOVE(stack, strlen(*argp) + 1);
+    env_count++;
   }
 
   // Push the arguments
@@ -137,9 +140,26 @@ static void *prepare_stack(void *stack, int argc, char **argv) {
   PUSH_STR(stack, "4 I used a dice");
   random_address = (target_reg) stack;
 
-  // Force 16 bytes alignment
-  stack = (void *) ((uintptr_t) stack & ~15);
+  // Compute the value of stack pointer once we'll be done
 
+  // WARNING: keep this number in sync with the number of auxiliary entries
+  const unsigned aux_count = 17;
+  unsigned entries_count = aux_count * 2 + 1 + env_count + 1 + argc + 1;
+  uintptr_t final_stack = ((uintptr_t) stack
+                           - entries_count * sizeof(target_reg));
+
+  // Force 256 bits alignment of the final stack value
+  const unsigned alignment = 256 / 8 - 1;
+  uintptr_t alignment_offset = (final_stack
+                                - (final_stack & ~((uintptr_t) alignment)));
+  stack -= alignment_offset;
+  final_stack -= alignment_offset;
+  assert((final_stack & alignment) == 0);
+
+  // Push the auxiliary vector
+
+  // WARNING: if you add something here, update aux_count
+  uintptr_t aux_start = (uintptr_t) stack;
   PUSH_AUX(stack, AT_NULL, 0);
   PUSH_AUX(stack, AT_PHDR, phdr_address);
   PUSH_AUX(stack, AT_PHENT, e_phentsize);
@@ -157,7 +177,11 @@ static void *prepare_stack(void *stack, int argc, char **argv) {
   PUSH_AUX(stack, AT_CLKTCK, sysconf(_SC_CLK_TCK));
   PUSH_AUX(stack, AT_RANDOM, random_address);
   PUSH_AUX(stack, AT_PLATFORM, platform_address);
+  // WARNING: if you add something here, update aux_count
 
+  assert(aux_start - aux_count * 2 * sizeof(target_reg) == (uintptr_t) stack);
+
+  // Push a separator
   PUSH_REG(stack, 0);
 
   // Copy arguments and environment variables, and store their address
@@ -182,6 +206,9 @@ static void *prepare_stack(void *stack, int argc, char **argv) {
 
   PUSH_REG(stack, argc);
 
+  assert((((uintptr_t) stack) & alignment) == 0);
+  assert((uintptr_t) stack == final_stack);
+
 #undef PUSH_AUX
 #undef PUSH_REG
 #undef PUSH