multiboot2: fix memory issue in boxed_dst_tag

multiboot2/src/builder/mod.rs

@@ -8,37 +8,100 @@ pub use information::InformationBuilder;
 use alloc::alloc::alloc;
 use alloc::boxed::Box;
 use core::alloc::Layout;
-use core::mem::size_of;
+use core::mem::{align_of_val, size_of, size_of_val};
+use core::ops::Deref;
 
-use crate::{TagTrait, TagTypeId};
+use crate::{Tag, TagTrait, TagTypeId};
 
 /// Create a boxed tag with the given content.
+///
+/// # Parameters
+/// - `typ` - The given [`TagTypeId`]
+/// - `content` - All payload bytes of the DST tag without the tag type or the
+///               size. The memory is only read and can be discarded afterwards.
 pub(super) fn boxed_dst_tag<T: TagTrait<Metadata = usize> + ?Sized>(
     typ: impl Into<TagTypeId>,
     content: &[u8],
 ) -> Box<T> {
-    // based on https://stackoverflow.com/a/64121094/2192464
-    let (layout, size_offset) = Layout::new::<TagTypeId>()
-        .extend(Layout::new::<u32>())
-        .unwrap();
-    let (layout, inner_offset) = layout
-        .extend(Layout::array::<usize>(content.len()).unwrap())
-        .unwrap();
+    // Currently, I do not find a nice way of making this dynamic so that also
+    // miri is happy. But it seems that 4 is fine.
+    const ALIGN: usize = 4;
+
+    let tag_size = size_of::<TagTypeId>() + size_of::<u32>() + content.len();
+    // round up to the next multiple of 8
+    // Rust uses this convention for all types. I found out so by checking
+    // miris output of the corresponding unit test.
+    let alloc_size = (tag_size + 7) & !0b111;
+    let layout = Layout::from_size_align(alloc_size, ALIGN).unwrap();
     let ptr = unsafe { alloc(layout) };
     assert!(!ptr.is_null());
+
+    // write tag content to memory
     unsafe {
-        // initialize the content as good as we can
-        ptr.cast::<TagTypeId>().write(typ.into());
-        ptr.add(size_offset).cast::<u32>().write(
-            (content.len() + size_of::<TagTypeId>() + size_of::<u32>())
-                .try_into()
-                .unwrap(),
-        );
-        // initialize body
-        let content_ptr = ptr.add(inner_offset);
-        for (idx, val) in content.iter().enumerate() {
-            content_ptr.add(idx).write(*val);
+        // write tag type
+        let ptrx = ptr.cast::<TagTypeId>();
+        ptrx.write(typ.into());
+
+        // write tag size
+        let ptrx = ptrx.add(1).cast::<u32>();
+        ptrx.write(tag_size as u32);
+
+        // write rest of content
+        let ptrx = ptrx.add(1).cast::<u8>();
+        let tag_content_slice = core::slice::from_raw_parts_mut(ptrx, content.len());
+        for (i, &byte) in content.iter().enumerate() {
+            tag_content_slice[i] = byte;
+        }
+    }
+
+    let base_tag = unsafe { &*ptr.cast::<Tag>() };
+    let raw: *mut T = ptr_meta::from_raw_parts_mut(ptr.cast(), T::dst_size(base_tag));
+
+    unsafe {
+        let boxed = Box::from_raw(raw);
+        let reference: &T = boxed.deref();
+        // If this panics, please create an issue on GitHub.
+        assert_eq!(size_of_val(reference), alloc_size);
+        assert_eq!(align_of_val(reference), ALIGN);
+        boxed
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const METADATA_SIZE: usize = 8;
+
+    #[derive(ptr_meta::Pointee)]
+    #[repr(C)]
+    struct CustomTag {
+        typ: TagTypeId,
+        size: u32,
+        string: [u8],
+    }
+
+    impl CustomTag {
+        fn string(&self) -> Result<&str, core::str::Utf8Error> {
+            Tag::get_dst_str_slice(&self.string)
         }
-        Box::from_raw(ptr_meta::from_raw_parts_mut(ptr as *mut (), content.len()))
+    }
+
+    impl TagTrait for CustomTag {
+        fn dst_size(base_tag: &Tag) -> usize {
+            assert!(base_tag.size as usize >= METADATA_SIZE);
+            base_tag.size as usize - METADATA_SIZE
+        }
+    }
+
+    #[test]
+    fn test_boxed_dst_tag() {
+        let tag_type_id = 1337_u32;
+        let content = "hallo";
+
+        let tag = boxed_dst_tag::<CustomTag>(tag_type_id, content.as_bytes());
+        assert_eq!(tag.typ, tag_type_id);
+        assert_eq!(tag.size as usize, METADATA_SIZE + content.len());
+        assert_eq!(tag.string(), Ok(content));
     }
 }