zlib: fix pointer alignment

The function AllocForBrotli prefixes the allocated memory with its
size, and returns a pointer to the region after it. This pointer can
however no longer be suitably aligned. Correct this by allocating
the maximum of the the size of the size_t and the max alignment.

On Arm 32bits the size_t is 4 bytes long, but the alignment is 8 for
some NEON instructions. When Brotli is compiled with optimizations
enabled newer GCC versions will use the NEON instructions and trigger
a bus error killing node.

see google/brotli#1159

PR-URL: #57727
Reviewed-By: Shelley Vohr <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Daniel Lemire <[email protected]>
Reviewed-By: Gerhard Stöbich <[email protected]>

Author: jhofstee

src/node_zlib.cc

@@ -608,7 +608,8 @@ class CompressionStream : public AsyncWrap, public ThreadPoolWork {
   }
 
   static void* AllocForBrotli(void* data, size_t size) {
-    size += sizeof(size_t);
+    constexpr size_t offset = std::max(sizeof(size_t), alignof(max_align_t));
+    size += offset;
     CompressionStream* ctx = static_cast<CompressionStream*>(data);
     char* memory = UncheckedMalloc(size);
     if (memory == nullptr) [[unlikely]] {
@@ -617,15 +618,16 @@ class CompressionStream : public AsyncWrap, public ThreadPoolWork {
     *reinterpret_cast<size_t*>(memory) = size;
     ctx->unreported_allocations_.fetch_add(size,
                                            std::memory_order_relaxed);
-    return memory + sizeof(size_t);
+    return memory + offset;
   }
 
   static void FreeForZlib(void* data, void* pointer) {
     if (pointer == nullptr) [[unlikely]] {
       return;
     }
     CompressionStream* ctx = static_cast<CompressionStream*>(data);
-    char* real_pointer = static_cast<char*>(pointer) - sizeof(size_t);
+    constexpr size_t offset = std::max(sizeof(size_t), alignof(max_align_t));
+    char* real_pointer = static_cast<char*>(pointer) - offset;
     size_t real_size = *reinterpret_cast<size_t*>(real_pointer);
     ctx->unreported_allocations_.fetch_sub(real_size,
                                            std::memory_order_relaxed);