Re: [PATCH] rust: bitops: add helpers for _find_* functions

[Andreas Hindborg <a.hindborg@kernel.org>]

Alice Ryhl <aliceryhl@google.com> writes:

> On 32-bit ARM, _find_next_bit does not exist because ARM provides that
> function through a #define instead. This means that Rust, which calls
> the underscored version, fails to find it. This triggers errors:
>
> 	ld.lld: error: undefined symbol: _find_next_bit
> 	>>> referenced by bitmap.rs:459 (rust/kernel/bitmap.rs:459)
> 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_bitmap_borrow) in archive vmlinux.a
> 	>>> referenced by bitmap.rs:459 (rust/kernel/bitmap.rs:459)
> 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_bitmap_set_clear_find) in archive vmlinux.a
> 	>>> referenced by bitmap.rs:459 (rust/kernel/bitmap.rs:459)
> 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_bitmap_set_clear_find) in archive vmlinux.a
> 	>>> referenced 10 more times
>
> 	ld.lld: error: undefined symbol: _find_next_zero_bit
> 	>>> referenced by bitmap.rs:479 (rust/kernel/bitmap.rs:479)
> 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_bitmap_copy) in archive vmlinux.a
> 	>>> referenced by bitmap.rs:479 (rust/kernel/bitmap.rs:479)
> 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_bitmap_set_clear_find) in archive vmlinux.a
> 	>>> referenced by bitmap.rs:479 (rust/kernel/bitmap.rs:479)
> 	>>>               rust/kernel.o:(kernel::bitmap::tests::kunit_rust_wrapper_owned_bitmap_out_of_bounds) in archive vmlinux.a
> 	>>> referenced 6 more times
>
> To fix this, add Rust helpers in this particular case.
>
> Fixes: 6cf93a9ed39e ("rust: add bindings for bitops.h")
> Reported-by: Andreas Hindborg <a.hindborg@kernel.org>
> Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/x/topic/x/near/561677301
> Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/helpers/bitops.c | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
>
> diff --git a/rust/helpers/bitops.c b/rust/helpers/bitops.c
> index 5d0861d29d3f0d705a014ae4601685828405f33b..84061af591a261f7268ffe6535282bf3c7608e2d 100644
> --- a/rust/helpers/bitops.c
> +++ b/rust/helpers/bitops.c
> @@ -1,6 +1,7 @@
>  // SPDX-License-Identifier: GPL-2.0
>  
>  #include <linux/bitops.h>
> +#include <linux/find.h>
>  
>  void rust_helper___set_bit(unsigned long nr, unsigned long *addr)
>  {
> @@ -21,3 +22,34 @@ void rust_helper_clear_bit(unsigned long nr, volatile unsigned long *addr)
>  {
>  	clear_bit(nr, addr);
>  }
> +
> +/*
> + * Rust normally calls the single-underscore-prefixed version of these
> + * functions, which are not inlined. However, on some platforms, they do not
> + * exist. In those cases, provide a rust helper for the underscored version.
> + */
> +#ifdef find_next_zero_bit
> +__rust_helper unsigned long
> +rust_helper__find_next_zero_bit(const unsigned long *addr, unsigned long size,
> +				unsigned long offset)
> +{
> +	return find_next_zero_bit(addr, size, offset);
> +}
> +#endif /* find_next_zero_bit */
> +
> +#ifdef find_next_bit
> +__rust_helper unsigned long
> +rust_helper__find_next_bit(const unsigned long *addr, unsigned long size,
> +			   unsigned long offset)
> +{
> +	return find_next_bit(addr, size, offset);
> +}
> +#endif /* find_next_bit */
> +
> +#ifdef find_last_bit
> +__rust_helper unsigned long
> +rust_helper__find_last_bit(const unsigned long *addr, unsigned long size)
> +{
> +	return find_last_bit(addr, size);
> +}
> +#endif /* find_last_bit */
>
> ---
> base-commit: 54e3eae855629702c566bd2e130d9f40e7f35bde
> change-id: 20251203-bitops-find-helper-25ed1bbae700
>
> Best regards,
> -- 
> Alice Ryhl <aliceryhl@google.com>

I messed up when testing this patch. It actually does not solve the
issue.

It appears that bindgen will emit an extern "C" declaration for the
missing C functions, even though they are not in the symbol table. When
we add the rust helpers, the declarations emitted based on the
non-existent C functions will mask the rust helper functions.

We can circumvent this by using another name for our helper. While it
works, it is not fixing the root cause. I am not sure why bindgen emits
these functions even though they are not in the symbol table at the end.

At any rate, the following is a workaround:

diff --git a/rust/helpers/bitops.c b/rust/helpers/bitops.c
index 5d0861d29d3f0..4f15315bc47f2 100644
--- a/rust/helpers/bitops.c
+++ b/rust/helpers/bitops.c
@@ -21,3 +21,26 @@ void rust_helper_clear_bit(unsigned long nr, volatile unsigned long *addr)
 {
    clear_bit(nr, addr);
 }
+
+/*
+ * Rust normally calls the single-underscore-prefixed version of these
+ * functions, which are not inlined. However, on some platforms, they do not
+ * exist. In those cases, provide a rust helper for the underscored version.
+ */
+#ifdef find_next_zero_bit
+__rust_helper unsigned long
+rust_helper_r_find_next_zero_bit(const unsigned long *addr, unsigned long size,
+               unsigned long offset)
+{
+   return find_next_zero_bit(addr, size, offset);
+}
+#endif /* find_next_zero_bit */
+
+#ifdef find_next_bit
+__rust_helper unsigned long
+rust_helper_r_find_next_bit(const unsigned long *addr, unsigned long size,
+              unsigned long offset)
+{
+   return find_next_bit(addr, size, offset);
+}
+#endif /* find_next_bit */
diff --git a/rust/kernel/bitmap.rs b/rust/kernel/bitmap.rs
index aa8fc7bf06fc9..88af73edb0c21 100644
--- a/rust/kernel/bitmap.rs
+++ b/rust/kernel/bitmap.rs
@@ -456,7 +456,7 @@ pub fn next_bit(&self, start: usize) -> Option<usize> {
         );
         // SAFETY: `_find_next_bit` tolerates out-of-bounds arguments and returns a
         // value larger than or equal to `self.len()` in that case.
-        let index = unsafe { bindings::_find_next_bit(self.as_ptr(), self.len(), start) };
+        let index = unsafe { bindings::r_find_next_bit(self.as_ptr(), self.len(), start) };
         if index >= self.len() {
             None
         } else {
@@ -476,7 +476,7 @@ pub fn next_zero_bit(&self, start: usize) -> Option<usize> {
         );
         // SAFETY: `_find_next_zero_bit` tolerates out-of-bounds arguments and returns a
         // value larger than or equal to `self.len()` in that case.
-        let index = unsafe { bindings::_find_next_zero_bit(self.as_ptr(), self.len(), start) };
+        let index = unsafe { bindings::r_find_next_zero_bit(self.as_ptr(), self.len(), start) };
         if index >= self.len() {
             None
         } else {



Best regards,
Andreas Hindborg