Re: [PATCH v2 5/5] rust: enable `clippy::as_underscore` lint

[Benno Lossin <benno.lossin@proton.me>]

On Thu Mar 13, 2025 at 11:47 AM CET, Tamir Duberstein wrote:
> On Wed, Mar 12, 2025 at 6:38 PM Benno Lossin <benno.lossin@proton.me> wrote:
>>
>> On Wed Mar 12, 2025 at 11:24 PM CET, Tamir Duberstein wrote:
>> > On Wed, Mar 12, 2025 at 5:30 PM Benno Lossin <benno.lossin@proton.me> wrote:
>> >>
>> >> On Wed Mar 12, 2025 at 10:10 PM CET, Tamir Duberstein wrote:
>> >> > On Wed, Mar 12, 2025 at 5:04 PM Tamir Duberstein <tamird@gmail.com> wrote:
>> >> >>
>> >> >> On Wed, Mar 12, 2025 at 5:01 PM Benno Lossin <benno.lossin@proton.me> wrote:
>> >> >> > Always enable the features, we have `allow(stable_features)` for this
>> >> >> > reason (then you don't have to do this dance with checking if it's
>> >> >> > already stable or not :)
>> >> >>
>> >> >> It's not so simple. In rustc < 1.84.0 the lints *and* the strict
>> >> >> provenance APIs are behind `feature(strict_provenance)`. In rustc >=
>> >> >> 1.84.0 the lints are behind `feature(strict_provenance_lints)`. So you
>> >> >> need to read the config to learn that you need to enable
>> >> >> `feature(strict_provenance_lints)`.
>> >>
>> >> I see... And `strict_provenance_lints` doesn't exist in <1.84? That's a
>> >> bit of a bummer...
>> >>
>> >> But I guess we could have this config option (in `init/Kconfig`):
>> >>
>> >>     config RUSTC_HAS_STRICT_PROVENANCE
>> >>             def_bool RUSTC_VERSION >= 108400
>> >>
>> >> and then do this in `lib.rs`:
>> >>
>> >>     #![cfg_attr(CONFIG_RUSTC_HAS_STRICT_PROVENANCE, feature(strict_provenance_lints))]
>> >
>> > Yep! That's exactly what I did, but as I mentioned up-thread, the
>> > result is that we only cover the `kernel` crate.
>>
>> Ah I see, can't we just have the above line in the other crate roots?
>
> The most difficult case is doctests. You'd have to add this to every
> example AFAICT.
>
>> >> > Actually this isn't even the only problem. It seems that
>> >> > `-Astable_features` doesn't affect features enabled on the command
>> >> > line at all:
>> >> >
>> >> > error[E0725]: the feature `strict_provenance` is not in the list of
>> >> > allowed features
>> >> >  --> <crate attribute>:1:9
>> >> >   |
>> >> > 1 | feature(strict_provenance)
>> >> >   |         ^^^^^^^^^^^^^^^^^
>> >>
>> >> That's because you need to append the feature to `rust_allowed_features`
>> >> in `scripts/Makefile.build` (AFAIK).
>> >
>> > Thanks, that's a helpful pointer, and it solves some problems but not
>> > all. The root Makefile contains this bit:
>> >
>> >> KBUILD_HOSTRUSTFLAGS := $(rust_common_flags) -O -Cstrip=debuginfo \
>> >> -Zallow-features= $(HOSTRUSTFLAGS)
>> >
>> > which means we can't use the provenance lints against these host
>> > targets (including e.g. `rustdoc_test_gen`). We can't remove this
>> > -Zallow-features= either because then core fails to compile.
>> >
>> > I'm at the point where I think I need more involved help. Want to take
>> > a look at my attempt? It's here:
>> > https://github.com/tamird/linux/tree/b4/ptr-as-ptr.

With doing `allow(clippy::incompatible_msrv)`, I meant doing that
globally, not having a module to re-export the functions :)

>> I'll take a look tomorrow, you're testing my knowledge of the build
>> system a lot here :)
>
> We're guaranteed to learn something :)

Yep! I managed to get it working, but it is rather janky and
experimental. I don't think you should use this in your patch series
unless Miguel has commented on it.

Notable things in the diff below:
* the hostrustflags don't get the *provenance_casts lints (which is
  correct, I think, but probably not the way I did it with filter-out)
* the crates compiler_builtins, bindings, uapi, build_error, libmacros,
  ffi, etc do get them, but probably shouldn't?

---
Cheers,
Benno

diff --git a/Makefile b/Makefile
index 70bdbf2218fc..38a79337cd7b 100644
--- a/Makefile
+++ b/Makefile
@@ -473,6 +473,8 @@ export rust_common_flags := --edition=2021 \
 			    -Astable_features \
 			    -Dnon_ascii_idents \
 			    -Dunsafe_op_in_unsafe_fn \
+			    -Wfuzzy_provenance_casts \
+			    -Wlossy_provenance_casts \
 			    -Wmissing_docs \
 			    -Wrust_2018_idioms \
 			    -Wunreachable_pub \
@@ -493,7 +495,7 @@ KBUILD_HOSTCFLAGS   := $(KBUILD_USERHOSTCFLAGS) $(HOST_LFS_CFLAGS) \
 		       $(HOSTCFLAGS) -I $(srctree)/scripts/include
 KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS) \
 		       -I $(srctree)/scripts/include
-KBUILD_HOSTRUSTFLAGS := $(rust_common_flags) -O -Cstrip=debuginfo \
+KBUILD_HOSTRUSTFLAGS := $(filter-out -Wfuzzy_provenance_casts -Wlossy_provenance_casts,$(rust_common_flags)) -O -Cstrip=debuginfo \
 			-Zallow-features= $(HOSTRUSTFLAGS)
 KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
 KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
diff --git a/init/Kconfig b/init/Kconfig
index d0d021b3fa3b..82e28d6f7c3f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -132,6 +132,9 @@ config CC_HAS_COUNTED_BY
 config RUSTC_HAS_COERCE_POINTEE
 	def_bool RUSTC_VERSION >= 108400
 
+config RUSTC_HAS_STABLE_STRICT_PROVENANCE
+	def_bool RUSTC_VERSION >= 108400
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))
diff --git a/rust/Makefile b/rust/Makefile
index ea3849eb78f6..998b57c6e5f7 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -436,7 +436,8 @@ $(obj)/helpers/helpers.o: $(src)/helpers/helpers.c $(recordmcount_source) FORCE
 $(obj)/exports.o: private skip_gendwarfksyms = 1
 
 $(obj)/core.o: private skip_clippy = 1
-$(obj)/core.o: private skip_flags = -Wunreachable_pub
+$(obj)/core.o: private skip_flags = -Wunreachable_pub \
+    -Wfuzzy_provenance_casts -Wlossy_provenance_casts
 $(obj)/core.o: private rustc_objcopy = $(foreach sym,$(redirect-intrinsics),--redefine-sym $(sym)=__rust$(sym))
 $(obj)/core.o: private rustc_target_flags = $(core-cfgs)
 $(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs \
diff --git a/rust/bindings/lib.rs b/rust/bindings/lib.rs
index 014af0d1fc70..185bf29e44d9 100644
--- a/rust/bindings/lib.rs
+++ b/rust/bindings/lib.rs
@@ -9,6 +9,14 @@
 //! using this crate.
 
 #![no_std]
+#![cfg_attr(
+    not(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE),
+    feature(strict_provenance)
+)]
+#![cfg_attr(
+    CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE,
+    feature(strict_provenance_lints)
+)]
 // See <https://github.com/rust-lang/rust-bindgen/issues/1651>.
 #![cfg_attr(test, allow(deref_nullptr))]
 #![cfg_attr(test, allow(unaligned_references))]
diff --git a/rust/build_error.rs b/rust/build_error.rs
index fa24eeef9929..84e24598857f 100644
--- a/rust/build_error.rs
+++ b/rust/build_error.rs
@@ -18,6 +18,14 @@
 //! [const-context]: https://doc.rust-lang.org/reference/const_eval.html#const-context
 
 #![no_std]
+#![cfg_attr(
+    not(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE),
+    feature(strict_provenance)
+)]
+#![cfg_attr(
+    CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE,
+    feature(strict_provenance_lints)
+)]
 
 /// Panics if executed in [const context][const-context], or triggers a build error if not.
 ///
diff --git a/rust/compiler_builtins.rs b/rust/compiler_builtins.rs
index f14b8d7caf89..0dcb25a644f6 100644
--- a/rust/compiler_builtins.rs
+++ b/rust/compiler_builtins.rs
@@ -21,6 +21,14 @@
 
 #![allow(internal_features)]
 #![feature(compiler_builtins)]
+#![cfg_attr(
+    not(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE),
+    feature(strict_provenance)
+)]
+#![cfg_attr(
+    CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE,
+    feature(strict_provenance_lints)
+)]
 #![compiler_builtins]
 #![no_builtins]
 #![no_std]
diff --git a/rust/ffi.rs b/rust/ffi.rs
index 584f75b49862..28a5e9a09b70 100644
--- a/rust/ffi.rs
+++ b/rust/ffi.rs
@@ -8,6 +8,14 @@
 //! C ABI. The kernel does not use [`core::ffi`], so it can customise the mapping that deviates from
 //! the platform default.
 
+#![cfg_attr(
+    not(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE),
+    feature(strict_provenance)
+)]
+#![cfg_attr(
+    CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE,
+    feature(strict_provenance_lints)
+)]
 #![no_std]
 
 macro_rules! alias {
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 398242f92a96..6fd4fb2176aa 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -13,6 +13,14 @@
 
 #![no_std]
 #![feature(arbitrary_self_types)]
+#![cfg_attr(
+    not(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE),
+    feature(strict_provenance)
+)]
+#![cfg_attr(
+    CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE,
+    feature(strict_provenance_lints)
+)]
 #![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(derive_coerce_pointee))]
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(dispatch_from_dyn))]
diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
index 8c7b786377ee..91450de998d3 100644
--- a/rust/macros/lib.rs
+++ b/rust/macros/lib.rs
@@ -2,6 +2,15 @@
 
 //! Crate for all kernel procedural macros.
 
+#![cfg_attr(
+    not(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE),
+    feature(strict_provenance)
+)]
+#![cfg_attr(
+    CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE,
+    feature(strict_provenance_lints)
+)]
+
 // When fixdep scans this, it will find this string `CONFIG_RUSTC_VERSION_TEXT`
 // and thus add a dependency on `include/config/RUSTC_VERSION_TEXT`, which is
 // touched by Kconfig when the version string from the compiler changes.
diff --git a/rust/uapi/lib.rs b/rust/uapi/lib.rs
index 13495910271f..84ef3828e0d4 100644
--- a/rust/uapi/lib.rs
+++ b/rust/uapi/lib.rs
@@ -8,6 +8,14 @@
 //! userspace APIs.
 
 #![no_std]
+#![cfg_attr(
+    not(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE),
+    feature(strict_provenance)
+)]
+#![cfg_attr(
+    CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE,
+    feature(strict_provenance_lints)
+)]
 // See <https://github.com/rust-lang/rust-bindgen/issues/1651>.
 #![cfg_attr(test, allow(deref_nullptr))]
 #![cfg_attr(test, allow(unaligned_references))]
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 993708d11874..021ee36ae8f2 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -226,7 +226,10 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons
+# Lints were moved to `strict_provenance_lints` when `strict_provenance` was stabilized.
+#
+# See https://github.com/rust-lang/rust/commit/56ee492a6e7a917b2b3f888e33dd52a13d3ecb64.
+rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,$(if $(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE),strict_provenance_lints,strict_provenance)
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree