src: use __builtin_align_down, if available

In _test_malloc, we manually align a pointer to MALLOC_ALIGNMENT
using `& ~(MALLOC_ALIGNMENT - 1)`. However, this has two problems:

1. We're casting the pointer to `size_t`, which isn't guaranteed
   to hold a pointer. Instead, we should cast to `uintptr_t`.
2. Modifying a pointer as a integer is undefined behavior, and on
   some platforms (e.g. CHERI), this does not work.

C++11 has std::align that does this for us, but unfortunately, there
isn't a way to do this in ISO C that is guaranteed to work, except for
in Clang v10+, which has a builtin extension called
__builtin_align_down that can align pointers safely for us.

See:
https://clang.llvm.org/docs/LanguageExtensions.html#alignment-builtins
on-behalf-of: @nqminds <info@nqminds.com>

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

2 files changed, 15 insertions, 4 deletions

diff --git a/ConfigureChecks.cmake b/ConfigureChecks.cmake
index 028774f..f010e54 100644
--- a/ConfigureChecks.cmake
+++ b/ConfigureChecks.cmake
@@ -1,5 +1,6 @@
 include(CheckIncludeFile)
 include(CheckSymbolExists)
+include(CheckCSourceCompiles)
 include(CheckFunctionExists)
 include(CheckLibraryExists)
 include(CheckTypeSize)
@@ -89,6 +90,8 @@ check_function_exists(signal HAVE_SIGNAL)
 check_function_exists(strsignal HAVE_STRSIGNAL)
 check_function_exists(strcmp HAVE_STRCMP)
 check_function_exists(clock_gettime HAVE_CLOCK_GETTIME)
+# Supported by Clang v10+
+check_c_source_compiles("int main(void) { int a = 1; return __builtin_align_down(&a, 8);}" HAVE_BUILTIN_ALIGN_DOWN)
 
 if (WIN32)
     check_function_exists(_vsnprintf_s HAVE__VSNPRINTF_S)
diff --git a/src/cmocka.c b/src/cmocka.c
index bc8f335..f23b6bf 100644
--- a/src/cmocka.c
+++ b/src/cmocka.c
@@ -2327,9 +2327,16 @@ static void vcm_free_error(char *err_msg)
     libc_free(err_msg);
 }
 
+/* Rounds the given pointer down to a multiple of the given alignment. */
+#ifdef HAVE_BUILTIN_ALIGN_DOWN
+#define ALIGN_DOWN(x, a) (__builtin_align_down((x), (a)))
+#else
+#define ALIGN_DOWN(x, a) ((uintptr_t)(x) & ~((a)-1))
+#endif
+
 /* Use the real malloc in this function. */
 #undef malloc
-void* _test_malloc(const size_t size, const char* file, const int line) {
+void* _test_malloc(const size_t size, const char *file, const int line) {
     char *ptr = NULL;
     MallocBlockInfo block_info;
     ListNode * const block_list = get_allocated_blocks_list();
@@ -2344,9 +2351,10 @@ void* _test_malloc(const size_t size, const char* file, const int line) {
     assert_non_null(block);
 
     /* Calculate the returned address. */
-    ptr = (char*)(((size_t)block + MALLOC_GUARD_SIZE +
-                  sizeof(struct MallocBlockInfoData) +
-                  MALLOC_ALIGNMENT) & ~(MALLOC_ALIGNMENT - 1));
+    ptr = (char *)(ALIGN_DOWN((block + MALLOC_GUARD_SIZE +
+                               sizeof(struct MallocBlockInfoData) +
+                               MALLOC_ALIGNMENT),
+                              MALLOC_ALIGNMENT));
 
     /* Initialize the guard blocks. */
     memset(ptr - MALLOC_GUARD_SIZE, MALLOC_GUARD_PATTERN, MALLOC_GUARD_SIZE);