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rust: revocable: document why &T is not used in RevocableGuard

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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<T> 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 <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250612-revocable-ptr-comment-v1-1-db36785877f6@google.com
[ Adjusted title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Alice Ryhl 2025-06-12 11:17:33 +00:00 committed by Miguel Ojeda
parent fbcd4b7bf5
commit d6763e0abb
1 changed files with 4 additions and 0 deletions
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4
rust/kernel/revocable.rs
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@ -231,6 +231,10 @@ impl<T> PinnedDrop for Revocable<T> {
///
/// 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 ()>,