mem: Fix resizing not accounting for alignment padding between Chunk header and data buffer

Fixes #32.

1 files changed, 11 insertions, 5 deletions

diff --git a/src/lib/mem.zig b/src/lib/mem.zig
index a9addf6..9e77add 100644
--- a/src/lib/mem.zig
+++ b/src/lib/mem.zig
@@ -72,6 +72,8 @@ pub fn ChunkAllocator(comptime config: ChunkAllocatorConfig) type {
         pub fn alloc(ctx: *anyopaque, len: usize, log2_ptr_align: u8, ret_addr: usize) ?[*]u8 {
             _ = ret_addr;
 
+            if (len == 0) return null;
+
             const self: *Self = @ptrCast(@alignCast(ctx));
 
             const ptr_align = @as(usize, 1) << @as(std.mem.Allocator.Log2Align, @intCast(log2_ptr_align));
@@ -110,18 +112,22 @@ pub fn ChunkAllocator(comptime config: ChunkAllocatorConfig) type {
         pub fn resize(ctx: *anyopaque, buf: []u8, log2_buf_align: u8, new_len: usize, ret_addr: usize) bool {
             _ = ret_addr;
 
+            if (new_len == 0) return false;
+
             const self: *Self = @ptrCast(@alignCast(ctx));
 
             const ptr_align = @as(usize, 1) << @as(std.mem.Allocator.Log2Align, @intCast(log2_buf_align));
 
+            const adjust_off = std.mem.alignPointerOffset(buf.ptr, ptr_align) orelse return false;
+            const aligned_new_len = new_len + adjust_off;
+
+            const len_ptr: [*]const u8 = @ptrFromInt(buf.len);
+            const padding_len = std.mem.alignPointerOffset(len_ptr, ptr_align) orelse return false;
+
             const head = self.head orelse return false;
             const bound = @intFromPtr(head) + (self.pages * paging.page_size);
 
-            // fixme: Doesn't account for alignment between header and data.
-            const chunk = @as(*align(1) Chunk, @ptrCast(buf.ptr - @sizeOf(Chunk)));
-
-            const adjust_off = std.mem.alignPointerOffset(buf.ptr, ptr_align) orelse return false;
-            const aligned_new_len = new_len + adjust_off;
+            const chunk = @as(*align(1) Chunk, @ptrCast(buf.ptr - padding_len - @sizeOf(Chunk)));
 
             if (aligned_new_len < chunk.len) {
                 const regained = chunk.len - aligned_new_len;