add some interesting tests for alignment corner cases

2 files changed, 44 insertions, 0 deletions

diff --git a/src/tools/miri/tests/fail/unaligned_pointers/field_requires_parent_struct_alignment.rs b/src/tools/miri/tests/fail/unaligned_pointers/field_requires_parent_struct_alignment.rs
new file mode 100644
index 00000000000..fa1812adc29
--- /dev/null
+++ b/src/tools/miri/tests/fail/unaligned_pointers/field_requires_parent_struct_alignment.rs
@@ -0,0 +1,24 @@
+/// This tests that when a field sits at offset 0 in a 4-aligned struct, accessing the field
+/// requires alignment 4 even if the field type has lower alignment requirements.
+
+#[repr(C)]
+pub struct S {
+    x: u8,
+    y: u32,
+}
+
+unsafe fn foo(x: *const S) -> u8 {
+    unsafe { (*x).x } //~ERROR: accessing memory with alignment 1, but alignment 4 is required
+}
+
+fn main() {
+    unsafe {
+        let mem = [0u64; 16];
+        let odd_ptr = std::ptr::addr_of!(mem).cast::<u8>().add(1);
+        // `odd_ptr` is now not aligned enough for `S`.
+        // If accessing field `x` can exploit that it is at offset 0
+        // in a 4-aligned struct, that field access requires alignment 4,
+        // thus making this UB.
+        foo(odd_ptr.cast());
+    }
+}
diff --git a/src/tools/miri/tests/fail/unaligned_pointers/field_requires_parent_struct_alignment.stderr b/src/tools/miri/tests/fail/unaligned_pointers/field_requires_parent_struct_alignment.stderr
new file mode 100644
index 00000000000..0f030a6e27c
--- /dev/null
+++ b/src/tools/miri/tests/fail/unaligned_pointers/field_requires_parent_struct_alignment.stderr
@@ -0,0 +1,20 @@
+error: Undefined Behavior: accessing memory with alignment ALIGN, but alignment ALIGN is required
+  --> $DIR/field_requires_parent_struct_alignment.rs:LL:CC
+   |
+LL |     unsafe { (*x).x }
+   |              ^^^^^^ accessing memory with alignment ALIGN, but alignment ALIGN is required
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `foo` at $DIR/field_requires_parent_struct_alignment.rs:LL:CC
+note: inside `main`
+  --> $DIR/field_requires_parent_struct_alignment.rs:LL:CC
+   |
+LL |         foo(odd_ptr.cast());
+   |         ^^^^^^^^^^^^^^^^^^^
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+