From: Alice Ryhl Date: Thu, 12 Jun 2025 11:17:33 +0000 (+0000) Subject: rust: revocable: document why &T is not used in RevocableGuard X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=d6763e0abb43d550791eb66d2b91e82cb29807f9;p=users%2Fjedix%2Flinux-maple.git rust: revocable: document why &T is not used in RevocableGuard When a reference appears in a function argument, the reference is assumed to be valid for the entire duration of that function call; this is called a stack protector [1]. Because of that, custom pointer types whose destructor may invalidate the pointee (i.e. they are more similar to Box than &T) cannot internally use a reference, and must instead use a raw pointer. This issue is something that is often missed during unsafe review. For examples, see [2] and [3]. To ensure that people don't try to simplify RevocableGuard by changing the raw pointer to a reference, add a comment to that effect. Link: https://perso.crans.org/vanille/treebor/protectors.html [1] Link: https://users.rust-lang.org/t/unsafe-code-review-semi-owning-weak-rwlock-t-guard/95706 [2] Link: https://lore.kernel.org/all/aEqdur4JTFa1V20U@google.com/ [3] Signed-off-by: Alice Ryhl Reviewed-by: Benno Lossin Link: https://lore.kernel.org/r/20250612-revocable-ptr-comment-v1-1-db36785877f6@google.com [ Adjusted title. - Miguel ] Signed-off-by: Miguel Ojeda --- diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs index 06a3cdfce344..1cd4511f0260 100644 --- a/rust/kernel/revocable.rs +++ b/rust/kernel/revocable.rs @@ -231,6 +231,10 @@ impl PinnedDrop for Revocable { /// /// The RCU read-side lock is held while the guard is alive. pub struct RevocableGuard<'a, T> { + // This can't use the `&'a T` type because references that appear in function arguments must + // not become dangling during the execution of the function, which can happen if the + // `RevocableGuard` is passed as a function argument and then dropped during execution of the + // function. data_ref: *const T, _rcu_guard: rcu::Guard, _p: PhantomData<&'a ()>,