libbpf-cargo: Wrap generated "unsafe" type in MaybeUninit

Certain Rust types that we use in our generated skeleton are not valid
for all bit patterns. Bools, for example, are defined to be one byte in
size, but their representation allows only for 0 and 1 values to be used
in said byte -- everything else is undefined behavior. Enums have
similar problems, in that not the entire space of the backing type is
necessarily used by variants.
The result is an unsoundness issue in our generated skeletons, because C
code could accidentally set an enum to an invalid value and Rust code
would exhibit undefined behavior.
This change fixes this problem by wrapping the corresponding attributes
in MaybeUninit. In so doing users have to be explicit when accessing
them and make sure that the current representation is indeed valid.

Closes: #589
Signed-off-by: Daniel Müller <[email protected]>

Author: d-e-s-o

libbpf-cargo/src/gen/btf.rs

@@ -24,6 +24,20 @@ use libbpf_rs::ReferencesType;
 
 const ANON_PREFIX: &str = "__anon_";
 
+/// Check whether the provided type is "unsafe" to use.
+///
+/// A type is considered unsafe by this function if it is not valid for
+/// any bit pattern.
+fn is_unsafe(ty: BtfType<'_>) -> bool {
+    let ty = ty.skip_mods_and_typedefs();
+
+    btf_type_match!(match ty {
+        BtfKind::Int(t) => matches!(t.encoding, types::IntEncoding::Bool),
+        BtfKind::Enum | BtfKind::Enum64 => true,
+        _ => false,
+    })
+}
+
 pub struct GenBtf<'s> {
     btf: Btf<'s>,
     // We use refcell here because the design of this type unfortunately causes a lot of borrowing
@@ -394,7 +408,13 @@ impl<'s> GenBtf<'s> {
             let field_name = if let Some(name) = member.name {
                 name.to_string_lossy()
             } else {
-                field_ty_str.as_str().into()
+                Cow::Borrowed(field_ty_str.as_str())
+            };
+
+            let field_ty_str = if is_unsafe(field_ty) {
+                Cow::Owned(format!("std::mem::MaybeUninit<{field_ty_str}>"))
+            } else {
+                Cow::Borrowed(field_ty_str.as_str())
             };
 
             agg_content.push(format!(r#"    pub {field_name}: {field_ty_str},"#));