rust: pin-init: internal: init: document load-bearing fact of field accessors

[ Upstream commit 580cc37 ]

The functions `[Pin]Init::__[pinned_]init` and `ptr::write` called from
the `init!` macro require the passed pointer to be aligned. This fact is
ensured by the creation of field accessors to previously initialized
fields.

Since we missed this very important fact from the beginning [1],
document it in the code.

Link: https://rust-for-linux.zulipchat.com/#narrow/channel/561532-pin-init/topic/initialized.20field.20accessor.20detection/with/576210658 [1]
Fixes: 90e53c5 ("rust: add pin-init API core")
Cc: <stable@vger.kernel.org> # 6.6.y, 6.12.y: 42415d1: rust: pin-init: add references to previously initialized fields
Cc: <stable@vger.kernel.org> # 6.6.y, 6.12.y, 6.18.y, 6.19.y
Signed-off-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260302140424.4097655-2-lossin@kernel.org
[ Updated Cc: stable@ tags as discussed. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
[ Moved changes to the declarative macro, because 6.19.y and earlier do not
  have `syn`. Also duplicated the comment for all field accessor creations.
  - Benno ]
Signed-off-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: BennoLossin

rust/pin-init/src/macros.rs

@@ -1312,6 +1312,10 @@ macro_rules! __init_internal {
         // return when an error/panic occurs.
         // We also use the `data` to require the correct trait (`Init` or `PinInit`) for `$field`.
         unsafe { $data.$field(::core::ptr::addr_of_mut!((*$slot).$field), init)? };
+        // 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.
         // SAFETY:
         // - the project function does the correct field projection,
         // - the field has been initialized,
@@ -1351,6 +1355,10 @@ macro_rules! __init_internal {
         // return when an error/panic occurs.
         unsafe { $crate::Init::__init(init, ::core::ptr::addr_of_mut!((*$slot).$field))? };
 
+        // 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.
         // SAFETY:
         // - the field is not structurally pinned, since the line above must compile,
         // - the field has been initialized,
@@ -1391,6 +1399,10 @@ macro_rules! __init_internal {
             unsafe { ::core::ptr::write(::core::ptr::addr_of_mut!((*$slot).$field), $field) };
         }
 
+        // 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.
         #[allow(unused_variables)]
         // SAFETY:
         // - the field is not structurally pinned, since no `use_data` was required to create this
@@ -1431,6 +1443,10 @@ macro_rules! __init_internal {
             // SAFETY: The memory at `slot` is uninitialized.
             unsafe { ::core::ptr::write(::core::ptr::addr_of_mut!((*$slot).$field), $field) };
         }
+        // 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.
         // SAFETY:
         // - the project function does the correct field projection,
         // - the field has been initialized,