make ARCH=arm64 rustdoc fails on a x86 host with SCS enabled

make ARCH=arm64 rustdoc fails on a x86 host with SCS enabled

[Konrad Dybcio]

Trying to build rustdoc as part of an arm64 kernel build on an x86
host results in a number of errors. Looks like we're ignoring the
-Zfixed-18 flag (arch/arm64/Makefile) at some point in the build
process, but I wasn't able to track it down it myself. Found on
next-20250722 (today).

Disabling SCS works around the issue, but that's less than ideal.

$ rustc --version
rustc 1.88.0 (6b00bc388 2025-06-23)

Error log below (CONFIG_SHADOW_CALL_STACK=y):

$ make ARCH=arm64 LLVM=1 -j24 rustdoc

[...]

error: mixing `-Zfixed-x18` will cause an ABI mismatch in crate `ffi`
 --> rust/ffi.rs:3:1
  |
3 | //! Foreign function interface (FFI) types.
  | ^
  |
  = help: the `-Zfixed-x18` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely
  = note: unset `-Zfixed-x18` in this crate is incompatible with `-Zfixed-x18=` in dependency `core`
  = help: set `-Zfixed-x18=` in this crate or unset `-Zfixed-x18` in `core`
  = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=fixed-x18` to silence this error

error: mixing `-Zfixed-x18` will cause an ABI mismatch in crate `ffi`
 --> rust/ffi.rs:3:1
  |
3 | //! Foreign function interface (FFI) types.
  | ^
  |
  = help: the `-Zfixed-x18` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely
  = note: unset `-Zfixed-x18` in this crate is incompatible with `-Zfixed-x18=` in dependency `compiler_builtins`
  = help: set `-Zfixed-x18=` in this crate or unset `-Zfixed-x18` in `compiler_builtins`
  = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=fixed-x18` to silence this error

error: aborting due to 2 previous errors
error: mixing `-Zfixed-x18` will cause an ABI mismatch in crate `compiler_builtins`
 --> rust/compiler_builtins.rs:3:1
  |
3 | //! Our own `compiler_builtins`.
  | ^

  |
  = help: the `-Zfixed-x18` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely
  = note: unset `-Zfixed-x18` in this crate is incompatible with `-Zfixed-x18=` in dependency `core`
  = help: set `-Zfixed-x18=` in this crate or unset `-Zfixed-x18` in `core`
  = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=fixed-x18` to silence this error

error: aborting due to 1 previous error

Konrad

Re: make ARCH=arm64 rustdoc fails on a x86 host with SCS enabled

[Miguel Ojeda]

On Tue, Jul 22, 2025 at 12:53 PM Konrad Dybcio
<konrad.dybcio@oss.qualcomm.com> wrote:
>
> Trying to build rustdoc as part of an arm64 kernel build on an x86
> host results in a number of errors. Looks like we're ignoring the
> -Zfixed-18 flag (arch/arm64/Makefile) at some point in the build
> process, but I wasn't able to track it down it myself. Found on
> next-20250722 (today).
>
> Disabling SCS works around the issue, but that's less than ideal.

Thanks, I can reproduce it -- I will send a patch.

Cheers,
Miguel

Re: make ARCH=arm64 rustdoc fails on a x86 host with SCS enabled

[Miguel Ojeda]

On Tue, Jul 22, 2025 at 2:40 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> Thanks, I can reproduce it -- I will send a patch.

It turns out it is a Rust compiler (`rustdoc`) bug -- filled:

    https://github.com/rust-lang/rust/issues/144521

I also sent a fix upstream, with a test similar to the kernel case:

    https://github.com/rust-lang/rust/pull/144523

And this is the patch to workaround it in the kernel for the time being:

    https://lore.kernel.org/rust-for-linux/20250727092317.2930617-1-ojeda@kernel.org/

Cheers,
Miguel

Re: make ARCH=arm64 rustdoc fails on a x86 host with SCS enabled

[Konrad Dybcio]

On 7/27/25 11:25 AM, Miguel Ojeda wrote:
> On Tue, Jul 22, 2025 at 2:40 PM Miguel Ojeda
> <miguel.ojeda.sandonis@gmail.com> wrote:
>>
>> Thanks, I can reproduce it -- I will send a patch.
> 
> It turns out it is a Rust compiler (`rustdoc`) bug -- filled:
> 
>     https://github.com/rust-lang/rust/issues/144521
> 
> I also sent a fix upstream, with a test similar to the kernel case:
> 
>     https://github.com/rust-lang/rust/pull/144523
> 
> And this is the patch to workaround it in the kernel for the time being:
> 
>     https://lore.kernel.org/rust-for-linux/20250727092317.2930617-1-ojeda@kernel.org/

Thanks for taking care of this!

Konrad