From ef15a8182ba0b6b7b5d6abb9d00eeb93f7cf0247 Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 2 Dec 2023 14:24:17 +0100
Subject: codegen, miri: fix computing the offset of an unsized field in a
 packed struct

---
 .../pass/issues/issue-3200-packed-field-offset.rs  | 35 ++++++++++++++++++++
 .../pass/issues/issue-3200-packed2-field-offset.rs | 38 ++++++++++++++++++++++
 2 files changed, 73 insertions(+)
 create mode 100644 src/tools/miri/tests/pass/issues/issue-3200-packed-field-offset.rs
 create mode 100644 src/tools/miri/tests/pass/issues/issue-3200-packed2-field-offset.rs

(limited to 'src')

diff --git a/src/tools/miri/tests/pass/issues/issue-3200-packed-field-offset.rs b/src/tools/miri/tests/pass/issues/issue-3200-packed-field-offset.rs
new file mode 100644
index 00000000000..e58fe60b5f6
--- /dev/null
+++ b/src/tools/miri/tests/pass/issues/issue-3200-packed-field-offset.rs
@@ -0,0 +1,35 @@
+#![feature(layout_for_ptr)]
+use std::mem;
+
+#[repr(packed, C)]
+struct PackedSized {
+    f: u8,
+    d: [u32; 4],
+}
+
+#[repr(packed, C)]
+struct PackedUnsized {
+    f: u8,
+    d: [u32],
+}
+
+impl PackedSized {
+    fn unsize(&self) -> &PackedUnsized {
+        // We can't unsize via a generic type since then we get the error
+        // that packed structs with unsized tail don't work if the tail
+        // might need dropping.
+        let len = 4usize;
+        unsafe { mem::transmute((self, len)) }
+    }
+}
+
+fn main() { unsafe {
+    let p = PackedSized { f: 0, d: [1, 2, 3, 4] };
+    let p = p.unsize() as *const PackedUnsized;
+    // Make sure the size computation does *not* think there is
+    // any padding in front of the `d` field.
+    assert_eq!(mem::size_of_val_raw(p), 1 + 4*4);
+    // And likewise for the offset computation.
+    let d = std::ptr::addr_of!((*p).d);
+    assert_eq!(d.cast::<u32>().read_unaligned(), 1);
+} }
diff --git a/src/tools/miri/tests/pass/issues/issue-3200-packed2-field-offset.rs b/src/tools/miri/tests/pass/issues/issue-3200-packed2-field-offset.rs
new file mode 100644
index 00000000000..910b5d94320
--- /dev/null
+++ b/src/tools/miri/tests/pass/issues/issue-3200-packed2-field-offset.rs
@@ -0,0 +1,38 @@
+#![feature(layout_for_ptr)]
+use std::mem;
+
+#[repr(packed(4))]
+struct Slice([u32]);
+
+#[repr(packed(2), C)]
+struct PackedSized {
+    f: u8,
+    d: [u32; 4],
+}
+
+#[repr(packed(2), C)]
+struct PackedUnsized {
+    f: u8,
+    d: Slice,
+}
+
+impl PackedSized {
+    fn unsize(&self) -> &PackedUnsized {
+        // We can't unsize via a generic type since then we get the error
+        // that packed structs with unsized tail don't work if the tail
+        // might need dropping.
+        let len = 4usize;
+        unsafe { mem::transmute((self, len)) }
+    }
+}
+
+fn main() { unsafe {
+    let p = PackedSized { f: 0, d: [1, 2, 3, 4] };
+    let p = p.unsize() as *const PackedUnsized;
+    // Make sure the size computation correctly adds exact 1 byte of padding
+    // in front of the `d` field.
+    assert_eq!(mem::size_of_val_raw(p), 1 + 1 + 4*4);
+    // And likewise for the offset computation.
+    let d = std::ptr::addr_of!((*p).d);
+    assert_eq!(d.cast::<u32>().read_unaligned(), 1);
+} }
-- 
cgit 1.4.1-3-g733a5