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

[Will Deacon <will@kernel.org>]

On Tue, Mar 05, 2024 at 01:14:19PM +0100, Miguel Ojeda wrote:
> 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:

Sorry, I meant to reply on this at the time...

> 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.

I think we have time to do this properly, like we did for the clang
enablement a few years ago. In hindsight, avoiding hacks for the early
toolchains back then was a really good idea because it meant we could
rely on a solid baseline set of compiler features from the start.

So, please can we fix this in rustc and just have SCS dependent on that?

Cheers,

Will