restrict max aligment boundary to prevent bug with segment determination (found by Matthew Parkinson).

include/mimalloc-types.h

@@ -158,7 +158,10 @@ typedef int32_t  mi_ssize_t;
 #define MI_BIN_HUGE  (73U)
 
 #if (MI_LARGE_OBJ_WSIZE_MAX >= 655360)
-#error "define more bins"
+#error "mimalloc internal: define more bins"
+#endif
+#if (MI_ALIGNED_MAX > MI_SEGMENT_SIZE/2)
+#error "mimalloc internal: the max aligned boundary is too large for the segment size"
 #endif
 
 // Used as a special value to encode block sizes in 32 bits.

include/mimalloc.h

@@ -166,6 +166,7 @@ mi_decl_export void mi_process_info(size_t* elapsed_msecs, size_t* user_msecs, s
 // Note that `alignment` always follows `size` for consistency with unaligned
 // allocation, but unfortunately this differs from `posix_memalign` and `aligned_alloc`.
 // -------------------------------------------------------------------------------------
+#define MI_ALIGNED_MAX   (1024*1024UL)    // maximum supported alignment is 1MiB
 
 mi_decl_nodiscard mi_decl_export mi_decl_restrict void* mi_malloc_aligned(size_t size, size_t alignment) mi_attr_noexcept mi_attr_malloc mi_attr_alloc_size(1) mi_attr_alloc_align(2);
 mi_decl_nodiscard mi_decl_export mi_decl_restrict void* mi_malloc_aligned_at(size_t size, size_t alignment, size_t offset) mi_attr_noexcept mi_attr_malloc mi_attr_alloc_size(1);

src/alloc-aligned.c

@@ -20,6 +20,7 @@ static void* mi_heap_malloc_zero_aligned_at(mi_heap_t* const heap, const size_t
   mi_assert(alignment > 0);
   if (mi_unlikely(size > PTRDIFF_MAX)) return NULL;   // we don't allocate more than PTRDIFF_MAX (see <https://sourceware.org/ml/libc-announce/2019/msg00001.html>)
   if (mi_unlikely(alignment==0 || !_mi_is_power_of_two(alignment))) return NULL; // require power-of-two (see <https://en.cppreference.com/w/c/memory/aligned_alloc>)
+  if (mi_unlikely(alignment>MI_ALIGNED_MAX)) return NULL; // we cannot align at a boundary larger than this (or otherwise we cannot find segment headers)
   const uintptr_t align_mask = alignment-1;  // for any x, `(x & align_mask) == (x % alignment)`
   
   // try if there is a small block available with just the right alignment