Re: [PATCH v2 2/2] rust: pin-init: internal: init: document load-bearing fact of field accessors

[Gary Guo]

On Mon Mar 2, 2026 at 2:04 PM GMT, Benno Lossin wrote:
> 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: 90e53c5e70a6 ("rust: add pin-init API core")
> Cc: stable@xxxxxxxxxxxxxxx # 6.19.y and 6.18.y: patch should apply without issues
> Cc: stable@xxxxxxxxxxxxxxx # 6.12.y and 6.6.y: need prerequisite see below `---` for more info

Hmm, if this patch is applied as is, the --- below is going to be cut out and
this line wouldn't make sense.

Perhaps we should just put

    Cc: stable@xxxxxxxxxxxxxxx # 6.12.y and 6.6.y: need commit 42415d163e5d ("rust: pin-init: add references to previously initialized fields")

Or leave this cc out and ask for manual picking?

> Signed-off-by: Benno Lossin <lossin@xxxxxxxxxx>

Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>

Best,
Gary

> ---
> As already explained in the previous email, we discovered an unsoundness
> in pin-init that exists since the beginning, but was unknowingly fixed
> in commit 42415d163e5d ("rust: pin-init: add references to previously
> initialized fields").
> 
> We introduced pin-init in 90e53c5e70a6 ("rust: add pin-init API core"),
> which was included in 6.4. The affected stable trees that are still
> maintained are: 6.12 and 6.6. Note that 6.18 and 6.19 already contain
> 42415d163e5d, so they are unaffected.
> 
> We still should backport this piece of documentation explaining the need
> for the field accessors for soundness. For this reasons we also want to
> backport it to 6.18 and 6.19.
> 
> Note that this patch depends on 42415d163e5d; so the only versions this
> patch can go in directly are 6.18 and 6.19. I will send separate patch
> series' for the older versions. The series' will include a backport of
> 42415d163e5d as well as a modified version of this patch, since this
> patch depends on the `syn` rewrite, which is not present in older
> versions.
> ---
>  rust/pin-init/internal/src/init.rs | 8 ++++++++
>  1 file changed, 8 insertions(+)