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

[Alice Ryhl <aliceryhl@google.com>]

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.

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>
---
It's not 100% clear to me whether this patch is enough for full SCS
support in Rust. If there is some issue where this makes things compile
and work without actually applying SCS to the Rust code, please let me
know. Is there some way to verify that it is actually working?

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.
---
 Makefile            | 1 +
 arch/arm64/Makefile | 1 +
 2 files changed, 2 insertions(+)

diff --git a/Makefile b/Makefile
index 0e36eff14608..345066643a76 100644
--- a/Makefile
+++ b/Makefile
@@ -936,6 +936,7 @@ ifdef CONFIG_SHADOW_CALL_STACK
 ifndef CONFIG_DYNAMIC_SCS
 CC_FLAGS_SCS	:= -fsanitize=shadow-call-stack
 KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
+KBUILD_RUSTFLAGS += -Zsanitizer=shadow-call-stack
 endif
 export CC_FLAGS_SCS
 endif
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index a88cdf910687..df3f21370165 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -103,6 +103,7 @@ endif
 
 ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
 KBUILD_CFLAGS	+= -ffixed-x18
+KBUILD_RUSTFLAGS += -Ctarget-feature=+reserve-x18
 endif
 
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)

---
base-commit: 90d35da658da8cff0d4ecbb5113f5fac9d00eb72
change-id: 20240304-shadow-call-stack-9c197a4361d9

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>