Fix buffer zeroing with offset in map_buffer (#2916)

Co-authored-by: Connor Fitzgerald <connorwadefitzgerald@gmail.com>

CHANGELOG.md

@@ -93,6 +93,7 @@ the same every time it is rendered, we now warn if it is missing.
 - Fix panics that occur when using `as_hal` functions when the hal generic type does not match the hub being looked up in by @i509VCB [#2871](https://github.com/gfx-rs/wgpu/pull/2871)
 - Add some validation in map_async by @nical in [#2876](https://github.com/gfx-rs/wgpu/pull/2876)
 - Fix bugs when mapping/unmapping zero-sized buffers and ranges by @nical in [#2877](https://github.com/gfx-rs/wgpu/pull/2877)
+- Fix out-of-bound write in `map_buffer` with non-zero offset by @nical in [#2916](https://github.com/gfx-rs/wgpu/pull/2916)
 - Validate the number of color attachments in `create_render_pipeline` by @nical in [#2913](https://github.com/gfx-rs/wgpu/pull/2913)
 
 #### DX12

wgpu-core/src/device/mod.rs

@@ -190,24 +190,17 @@ fn map_buffer<A: hal::Api>(
     //
     // If this is a write mapping zeroing out the memory here is the only reasonable way as all data is pushed to GPU anyways.
     let zero_init_needs_flush_now = mapping.is_coherent && buffer.sync_mapped_writes.is_none(); // No need to flush if it is flushed later anyways.
-    for uninitialized_range in buffer.initialization_status.drain(offset..(size + offset)) {
-        let num_bytes = uninitialized_range.end - uninitialized_range.start;
-        unsafe {
-            ptr::write_bytes(
-                mapping
-                    .ptr
-                    .as_ptr()
-                    .offset(uninitialized_range.start as isize),
-                0,
-                num_bytes as usize,
-            )
-        };
+    let mapped = unsafe { std::slice::from_raw_parts_mut(mapping.ptr.as_ptr(), size as usize) };
+
+    for uninitialized in buffer.initialization_status.drain(offset..(size + offset)) {
+        // The mapping's pointer is already offset, however we track the uninitialized range relative to the buffer's start.
+        let fill_range =
+            (uninitialized.start - offset) as usize..(uninitialized.end - offset) as usize;
+        mapped[fill_range].fill(0);
+
         if zero_init_needs_flush_now {
             unsafe {
-                raw.flush_mapped_ranges(
-                    buffer.raw.as_ref().unwrap(),
-                    iter::once(uninitialized_range.start..uninitialized_range.start + num_bytes),
-                )
+                raw.flush_mapped_ranges(buffer.raw.as_ref().unwrap(), iter::once(uninitialized))
             };
         }
     }