[PATCH v3 2/2] rust: pin-init: fix incorrect accessor reference lifetime

Gary Guo
2026-04-27T15:43:15Z

When a field has been initialized, `init!`/`pin_init!` create a reference
or pinned reference to the field so it can be accessed later during the
initialization of other fields. However, the reference it created is
incorrectly `&'static` rather than just the scope of the initializer.

This means that you can do

init!(Foo {
a: 1,
_: {
let b: &'static u32 = a;
}
})

which is unsound.

This is caused by `&mut (*#slot).#ident`, which actually allows arbitrary
lifetime, so this is effectively `'static`. Somewhat ironically, the safety
justification of creating the accessor is.. "SAFETY: TODO".

Fix it by adding `let_binding` method on `DropGuard` to shorten lifetime.
This results exactly what we want for these accessors. The safety and
invariant comments of `DropGuard` have been reworked; instead of reasoning
about what caller can do with the guard, express it in a way that the
ownership is transferred to the guard and `forget` takes it back, so the
unsafe operations within the `DropGuard` can be more easily justified.

Fixes: 42415d163e5d ("rust: pin-init: add references to previously initialized fields")
Cc: stable@vger.kernel.org
Signed-off-by: Gary Guo <gary@garyguo.net>
---
rust/pin-init/internal/src/init.rs | 106 ++++++++++++++++---------------------
rust/pin-init/src/__internal.rs | 28 ++++++----
2 files changed, 66 insertions(+), 68 deletions(-)

diff --git a/rust/pin-init/internal/src/init.rs b/rust/pin-init/internal/src/init.rs
index 0a6600e8156c..487ee0013faf 100644
--- a/rust/pin-init/internal/src/init.rs
+++ b/rust/pin-init/internal/src/init.rs
@@ -249,18 +249,6 @@ fn init_fields(
});
// Again span for better diagnostics
let write = quote_spanned!(ident.span()=> ::core::ptr::write);
- let accessor = if pinned {
- let project_ident = format_ident!("__project_{ident}");
- quote! {
- // SAFETY: TODO
- unsafe { #data.#project_ident(&mut (*#slot).#ident) }
- }
- } else {
- quote! {
- // SAFETY: TODO
- unsafe { &mut (*#slot).#ident }
- }
- };
quote! {
#(#attrs)*
{
@@ -268,51 +256,31 @@ fn init_fields(
// SAFETY: TODO
unsafe { #write(&raw mut (*#slot).#ident, #value_ident) };
}
- #(#cfgs)*
- #[allow(unused_variables)]
- let #ident = #accessor;
}
}
InitializerKind::Init { ident, value, .. } => {
// Again span for better diagnostics
let init = format_ident!("init", span = value.span());
- // NOTE: the field accessor ensures that the initialized field is properly aligned.
- // Unaligned fields will cause the compiler to emit E0793. We do not support
- // unaligned fields since `Init::__init` requires an aligned pointer; the call to
- // `ptr::write` below has the same requirement.
- let (value_init, accessor) = if pinned {
- let project_ident = format_ident!("__project_{ident}");
- (
- quote! {
- // SAFETY:
- // - `slot` is valid, because we are inside of an initializer closure, we
- // return when an error/panic occurs.
- // - We also use `#data` to require the correct trait (`Init` or `PinInit`)
- // for `#ident`.
- unsafe { #data.#ident(&raw mut (*#slot).#ident, #init)? };
- },
- quote! {
- // SAFETY: TODO
- unsafe { #data.#project_ident(&mut (*#slot).#ident) }
- },
- )
+ let value_init = if pinned {
+ quote! {
+ // SAFETY:
+ // - `slot` is valid, because we are inside of an initializer closure, we
+ // return when an error/panic occurs.
+ // - We also use `#data` to require the correct trait (`Init` or `PinInit`)
+ // for `#ident`.
+ unsafe { #data.#ident(&raw mut (*#slot).#ident, #init)? };
+ }
} else {
- (
- quote! {
- // SAFETY: `slot` is valid, because we are inside of an initializer
- // closure, we return when an error/panic occurs.
- unsafe {
- ::pin_init::Init::__init(
- #init,
- &raw mut (*#slot).#ident,
- )?
- };
- },
- quote! {
- // SAFETY: TODO
- unsafe { &mut (*#slot).#ident }
- },
- )
+ quote! {
+ // SAFETY: `slot` is valid, because we are inside of an initializer
+ // closure, we return when an error/panic occurs.
+ unsafe {
+ ::pin_init::Init::__init(
+ #init,
+ &raw mut (*#slot).#ident,
+ )?
+ };
+ }
};
quote! {
#(#attrs)*
@@ -320,9 +288,6 @@ fn init_fields(
let #init = #value;
#value_init
}
- #(#cfgs)*
- #[allow(unused_variables)]
- let #ident = #accessor;
}
}
InitializerKind::Code { block: value, .. } => quote! {
@@ -335,18 +300,41 @@ fn init_fields(
if let Some(ident) = kind.ident() {
// `mixed_site` ensures that the guard is not accessible to the user-controlled code.
let guard = format_ident!("__{ident}_guard", span = Span::mixed_site());
+
+ // NOTE: The reference is derived from the guard so that it only lives as long as the
+ // guard does and cannot escape the scope. If it's created via `&mut (*#slot).#ident`
+ // like the unaligned field guard, it will become effectively `'static`.
+ let accessor = if pinned {
+ let project_ident = format_ident!("__project_{ident}");
+ quote! {
+ // SAFETY: the initialization is pinned.
+ unsafe { #data.#project_ident(#guard.let_binding()) }
+ }
+ } else {
+ quote! {
+ #guard.let_binding()
+ }
+ };
+
res.extend(quote! {
#(#cfgs)*
- // Create the drop guard:
+ // Create the drop guard.
//
- // We rely on macro hygiene to make it impossible for users to access this local
- // variable.
- // SAFETY: We forget the guard later when initialization has succeeded.
- let #guard = unsafe {
+ // SAFETY:
+ // - `&raw mut (*slot).#ident` is valid.
+ // - `make_field_check` checks that `&raw mut (*slot).#ident` is properly aligned.
+ // - `(*slot).#ident` has been initialized above.
+ // - We only need the ownership to the pointee back when initialization has
+ // succeeded, where we `forget` the guard.
+ let mut #guard = unsafe {
::pin_init::__internal::DropGuard::new(
&raw mut (*slot).#ident
)
};
+
+ #(#cfgs)*
+ #[allow(unused_variables)]
+ let #ident = #accessor;
});
guards.push(guard);
guard_attrs.push(cfgs);
diff --git a/rust/pin-init/src/__internal.rs b/rust/pin-init/src/__internal.rs
index 90adbdc1893b..5720a621aed7 100644
--- a/rust/pin-init/src/__internal.rs
+++ b/rust/pin-init/src/__internal.rs
@@ -238,32 +238,42 @@ struct Foo {
/// When a value of this type is dropped, it drops a `T`.
///
/// Can be forgotten to prevent the drop.
+///
+/// # Invariants
+///
+/// - `ptr` is valid and properly aligned.
+/// - `*ptr` is initialized and owned by this guard.
pub struct DropGuard<T: ?Sized> {
ptr: *mut T,
}

impl<T: ?Sized> DropGuard<T> {
- /// Creates a new [`DropGuard<T>`]. It will [`ptr::drop_in_place`] `ptr` when it gets dropped.
+ /// Creates a drop guard and transfer the ownership of the pointer content.
///
- /// # Safety
+ /// The ownership is only relinguished if the guard is forgotten via [`core::mem::forget`].
///
- /// `ptr` must be a valid pointer.
+ /// # Safety
///
- /// It is the callers responsibility that `self` will only get dropped if the pointee of `ptr`:
- /// - has not been dropped,
- /// - is not accessible by any other means,
- /// - will not be dropped by any other means.
+ /// - `ptr` is valid and properly aligned.
+ /// - `*ptr` is initialized, and the ownership is transferred to this guard.
#[inline]
pub unsafe fn new(ptr: *mut T) -> Self {
+ // INVARIANT: By safety requirement.
Self { ptr }
}
+
+ /// Create a let binding for accessor use.
+ #[inline]
+ pub fn let_binding(&mut self) -> &mut T {
+ // SAFETY: Per type invariant.
+ unsafe { &mut *self.ptr }
+ }
}

impl<T: ?Sized> Drop for DropGuard<T> {
#[inline]
fn drop(&mut self) {
- // SAFETY: A `DropGuard` can only be constructed using the unsafe `new` function
- // ensuring that this operation is safe.
+ // SAFETY: `self.ptr` is valid, properly aligned and `*self.ptr` is owned by this guard.
unsafe { ptr::drop_in_place(self.ptr) }
}
}

--
2.51.2