Re: [PATCH v2] rust: add flags for shadow call stack sanitizer

Miguel Ojeda
2024-03-05T12:14:32Z

On Tue, Mar 5, 2024 at 12:58 PM Alice Ryhl <aliceryhl@google.com> wrote:
>
> Add flags to support the shadow call stack sanitizer, both in the
> dynamic and non-dynamic modes.
>
> Right now, the compiler will emit the warning "unknown feature specified
> for `-Ctarget-feature`: `reserve-x18`". However, the compiler still
> passes it to the codegen backend, so the flag will work just fine. Once
> rustc starts recognizing the flag (or provides another way to enable the
> feature), it will stop emitting this warning. See [1] for the relevant
> issue.
>
> Currently, the compiler thinks that the aarch64-unknown-none target
> doesn't support -Zsanitizer=shadow-call-stack, so the build will fail if
> you enable shadow call stack in non-dynamic mode. However, I still think
> it is reasonable to add the flag now, as it will at least fail the build
> when using an invalid configuration, until the Rust compiler is fixed to
> list -Zsanitizer=shadow-call-stack as supported for the target. See [2]
> for the feature request to add this.
>
> I have tested this change with Rust Binder on an Android device using
> CONFIG_DYNAMIC_SCS. Without the -Ctarget-feature=+reserve-x18 flag, the
> phone crashes immediately on boot, and with the flag, the phone appears
> to work normally.
>
> This contains a TODO to add the -Zuse-sync-unwind=n flag. The flag
> defaults to n, so it isn't a problem today, but the flag is unstable, so
> the default could change in a future compiler release.
>
> Link: https://github.com/rust-lang/rust/issues/121970 [1]
> Link: https://github.com/rust-lang/rust/issues/121972 [2]
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> This patch raises the question of whether we should change the Rust
> aarch64 support to use a custom target.json specification. If we do
> that, then we can fix both the warning for dynamic SCS and the
> build-failure for non-dynamic SCS without waiting for a new version of
> rustc with the mentioned issues fixed.

If the arm64 maintainers are OK with the warning being triggered in that case:

Acked-by: Miguel Ojeda <ojeda@kernel.org>

Otherwise partially reverting to the `target.json` approach sounds good too.

I added the `-Zuse-sync-unwind=n` to the list at
https://github.com/Rust-for-Linux/linux/issues/2. Given the default is
what we want, I have put it in the "Good to have" section.

Cheers,
Miguel