From 4a1e9c2e4d313999a082fc80c26e2b8bae9a832d Mon Sep 17 00:00:00 2001
From: Nicolas Silva <nical@fastmail.com>
Date: Thu, 24 Nov 2022 15:39:16 +0100
Subject: [PATCH] Fix incorrect offset in get_mapped_range.

---
 wgpu-core/src/device/mod.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/wgpu-core/src/device/mod.rs b/wgpu-core/src/device/mod.rs
index 494fa41b7a9..fc5d4afde52 100644
--- a/wgpu-core/src/device/mod.rs
+++ b/wgpu-core/src/device/mod.rs
@@ -5688,7 +5688,10 @@ impl<G: GlobalIdentityHandlerFactory> Global<G> {
                         max: range.end,
                     });
                 }
-                unsafe { Ok((ptr.as_ptr().offset(offset as isize), range_size)) }
+                // ptr points to the beginning of the range we mapped in map_async
+                // rather thant the beginning of the buffer.
+                let relative_offset = (offset - range.start) as isize;
+                unsafe { Ok((ptr.as_ptr().offset(relative_offset), range_size)) }
             }
             resource::BufferMapState::Idle | resource::BufferMapState::Waiting(_) => {
                 Err(BufferAccessError::NotMapped)