[PATCH] rust: bindings: derive Zeroable for bindgen's opaque array wrappers

[PATCH] rust: bindings: derive Zeroable for bindgen's opaque array wrappers

[Sasha Levin]

bindgen 0.73 emits the blob of some opaque types, such as ones aligned
to more than 4 bytes, as a `__BindgenOpaqueArray` or
`__BindgenOpaqueArray<align>` newtype instead of a plain array. The
newtype only derives `PartialEq`, `Eq`, `Copy`, `Clone`, `Debug` and
`Hash`, and the `MaybeZeroable` custom derive does not reach it, so a
struct that embeds such an opaque type is no longer `Zeroable`.

`spinlock` is opaque (see rust/bindgen_parameters), and with lockdep its
blob becomes a `__BindgenOpaqueArray8<[u8; 72]>`. Commit 71ec3c7d6db8
("blk-mq: quiesce a queue that joins a quiesced tag set") added a
`spinlock_t` to `struct blk_mq_tag_set`, so an allmodconfig build with
bindgen 0.73 now fails:

  error[E0277]: the trait bound `bindings::__BindgenOpaqueArray8<[u8; 72]>: pin_init::Zeroable` is not satisfied
    --> rust/kernel/block/mq/tag_set.rs:41:49

Which wrappers exist depends on the bindgen version and on the opaque
types of the configuration, so they cannot be covered by unconditional
impls in rust/bindings/lib.rs like `__BindgenBitfieldUnit` is. Derive
`Zeroable` for each wrapper in the generated bindings instead. The
pattern allows for the spacing of bindgen's output when rustfmt is not
available.

Fixes: 71ec3c7d6db8 ("blk-mq: quiesce a queue that joins a quiesced tag set")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
Jens, Linus: the block pull brought this in. 71ec3c7d6db8 puts the first
spinlock_t into a struct that the Rust code zero-initializes, and since
then a Rust allmodconfig build of mainline fails with bindgen 0.73.
Sadly the commit wasn't in linux-next before the pull request went out,
so the build breakage wasn't caught there.

Build tested x86_64 and arm64 allmodconfig with bindgen 0.73.2, and the
Rust code with bindgen 0.72.1 (no change to the generated bindings) and
0.73.0, with and without formatted bindgen output.

 rust/Makefile | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/rust/Makefile b/rust/Makefile
index 0ac9ab28cc2b5..0165626decbe7 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -514,13 +514,20 @@ endif
 bindgen_c_flags_final = $(bindgen_c_flags_lto) -fno-builtin -D__BINDGEN__
 
 # `--rust-target` points to our minimum supported Rust version.
+#
+# `bindgen` 0.73 emits the blob of some opaque types as a
+# `__BindgenOpaqueArray[<align>]` newtype, which the custom derives do not
+# reach. Derive `Zeroable` for those wrappers, so that a struct embedding such
+# an opaque type (e.g. `spinlock_t` with lockdep) is still `Zeroable`. The
+# pattern also matches the unformatted output used when `rustfmt` is missing.
 quiet_cmd_bindgen = BINDGEN $@
       cmd_bindgen = \
 	$(BINDGEN) $< $(bindgen_target_flags) --rust-target 1.85 \
 		--use-core --with-derive-default --ctypes-prefix ffi --no-layout-tests \
 		--no-debug '.*' --enable-function-attribute-detection \
 		-o $@ -- $(bindgen_c_flags_final) -DMODULE \
-		$(bindgen_target_cflags) $(bindgen_target_extra)
+		$(bindgen_target_cflags) $(bindgen_target_extra); \
+	sed -Ei 's/pub struct __BindgenOpaqueArray[0-9]* *< *T *> *\( *pub T *\) *;/$(pound)[derive(pin_init::Zeroable)] &/g' $@
 
 $(obj)/bindings/bindings_generated.rs: private bindgen_target_flags = \
     $(shell grep -Ev '^#|^$$' $(src)/bindgen_parameters)

base-commit: 36aad21d50d1af3f6954bb0c988257a908202477
-- 
2.53.0

Re: [PATCH] rust: bindings: derive Zeroable for bindgen's opaque array wrappers

[Alice Ryhl]

On Sat, Oct 31, 2026 at 4:28 PM Sasha Levin <sashal@kernel.org> wrote:
>
> bindgen 0.73 emits the blob of some opaque types, such as ones aligned
> to more than 4 bytes, as a `__BindgenOpaqueArray` or
> `__BindgenOpaqueArray<align>` newtype instead of a plain array. The
> newtype only derives `PartialEq`, `Eq`, `Copy`, `Clone`, `Debug` and
> `Hash`, and the `MaybeZeroable` custom derive does not reach it, so a
> struct that embeds such an opaque type is no longer `Zeroable`.
>
> `spinlock` is opaque (see rust/bindgen_parameters), and with lockdep its
> blob becomes a `__BindgenOpaqueArray8<[u8; 72]>`. Commit 71ec3c7d6db8
> ("blk-mq: quiesce a queue that joins a quiesced tag set") added a
> `spinlock_t` to `struct blk_mq_tag_set`, so an allmodconfig build with
> bindgen 0.73 now fails:
>
>   error[E0277]: the trait bound `bindings::__BindgenOpaqueArray8<[u8; 72]>: pin_init::Zeroable` is not satisfied
>     --> rust/kernel/block/mq/tag_set.rs:41:49
>
> Which wrappers exist depends on the bindgen version and on the opaque
> types of the configuration, so they cannot be covered by unconditional
> impls in rust/bindings/lib.rs like `__BindgenBitfieldUnit` is. Derive
> `Zeroable` for each wrapper in the generated bindings instead. The
> pattern allows for the spacing of bindgen's output when rustfmt is not
> available.
>
> Fixes: 71ec3c7d6db8 ("blk-mq: quiesce a queue that joins a quiesced tag set")
> Signed-off-by: Sasha Levin <sashal@kernel.org>

Is it possible to instead fix this by adding the impl of Zeroable in
rust/bindings/lib.rs, similar to this piece of code that we already
have:

unsafe impl<Storage> Zeroable for __BindgenBitfieldUnit<Storage> where
Storage: Zeroable {}

Alice

Re: [PATCH] rust: bindings: derive Zeroable for bindgen's opaque array wrappers

[Sasha Levin]

On Sat, Oct 31, 2026 at 05:17:46PM +0100, Alice Ryhl wrote:
>Is it possible to instead fix this by adding the impl of Zeroable in
>rust/bindings/lib.rs, similar to this piece of code that we already
>have:
>
>unsafe impl<Storage> Zeroable for __BindgenBitfieldUnit<Storage> where
>Storage: Zeroable {}

I looked at doing it that way, but the wrapper types don't always exist,
so a fixed impl in lib.rs won't build everywhere:

 - bindgen 0.71 and 0.72 (we still support 0.71.1) don't generate them
   at all, the blob is a plain array there.

 - 0.73 only emits the alignments a config actually needs. On x86_64,
   allmodconfig gets __BindgenOpaqueArray, __BindgenOpaqueArray8,
   __BindgenOpaqueArray64 and __BindgenOpaqueArray4096, but defconfig
   with Rust enabled has no __BindgenOpaqueArray4096, so an impl for it
   would break the build there.

__BindgenBitfieldUnit works because it's always generated. We could do it
in lib.rs with a Kconfig symbol for the bindgen version plus some way to
know which alignments a config ends up with, but tagging the wrappers
right after bindgen runs seemed simpler.

--
Thanks,
Sasha

Re: [PATCH] rust: bindings: derive Zeroable for bindgen's opaque array wrappers

[Alice Ryhl]

On Sat, Oct 31, 2026 at 5:29 PM Sasha Levin <sashal@kernel.org> wrote:
>
> On Sat, Oct 31, 2026 at 05:17:46PM +0100, Alice Ryhl wrote:
> >Is it possible to instead fix this by adding the impl of Zeroable in
> >rust/bindings/lib.rs, similar to this piece of code that we already
> >have:
> >
> >unsafe impl<Storage> Zeroable for __BindgenBitfieldUnit<Storage> where
> >Storage: Zeroable {}
>
> I looked at doing it that way, but the wrapper types don't always exist,
> so a fixed impl in lib.rs won't build everywhere:
>
>  - bindgen 0.71 and 0.72 (we still support 0.71.1) don't generate them
>    at all, the blob is a plain array there.
>
>  - 0.73 only emits the alignments a config actually needs. On x86_64,
>    allmodconfig gets __BindgenOpaqueArray, __BindgenOpaqueArray8,
>    __BindgenOpaqueArray64 and __BindgenOpaqueArray4096, but defconfig
>    with Rust enabled has no __BindgenOpaqueArray4096, so an impl for it
>    would break the build there.
>
> __BindgenBitfieldUnit works because it's always generated. We could do it
> in lib.rs with a Kconfig symbol for the bindgen version plus some way to
> know which alignments a config ends up with, but tagging the wrappers
> right after bindgen runs seemed simpler.

Fair enough!

Reviewed-by: Alice Ryhl <aliceryhl@google.com>