Rollup merge of #128348 - dingxiangfei2009:allow-shadow-call-stack-sanitizer, r=tmandry

Unconditionally allow shadow call-stack sanitizer for AArch64

It is possible to do so whenever `-Z fixed-x18` is applied.

cc ``@Darksonn`` for context

The reasoning is that, as soon as reservation on `x18` is forced through the flag `fixed-x18`, on AArch64 the option to instrument with [Shadow Call Stack sanitizer](https://clang.llvm.org/docs/ShadowCallStack.html) is then applicable regardless of the target configuration.

At the every least, we would like to relax the restriction on specifically `aarch64-unknonw-none`. For this option, we can include a documentation change saying that users of compiled objects need to ensure that they are linked to runtime with Shadow Call Stack instrumentation support.

Related: #121972

3 files changed, 15 insertions, 1 deletions

diff --git a/src/doc/rustc/src/platform-support/android.md b/src/doc/rustc/src/platform-support/android.md
index 9ddf00e3a50..96499b0d801 100644
--- a/src/doc/rustc/src/platform-support/android.md
+++ b/src/doc/rustc/src/platform-support/android.md
@@ -61,3 +61,8 @@ Currently the `riscv64-linux-android` target requires the following architecture
 * `Zba` (address calculation instructions)
 * `Zbb` (base instructions)
 * `Zbs` (single-bit instructions)
+
+### aarch64-linux-android on Nightly compilers
+
+As soon as `-Zfixed-x18` compiler flag is supplied, the [`ShadowCallStack` sanitizer](https://releases.llvm.org/7.0.1/tools/clang/docs/ShadowCallStack.html)
+instrumentation is also made avaiable by supplying the second compiler flag `-Zsanitizer=shadow-call-stack`.
diff --git a/src/doc/unstable-book/src/compiler-flags/fixed-x18.md b/src/doc/unstable-book/src/compiler-flags/fixed-x18.md
index 8c8bff5fa29..b215bc84fbb 100644
--- a/src/doc/unstable-book/src/compiler-flags/fixed-x18.md
+++ b/src/doc/unstable-book/src/compiler-flags/fixed-x18.md
@@ -1,7 +1,7 @@
 # `fixed-x18`
 
 This option prevents the compiler from using the x18 register. It is only
-supported on aarch64.
+supported on `aarch64`.
 
 From the [ABI spec][arm-abi]:
 
@@ -23,6 +23,11 @@ Currently, the `-Zsanitizer=shadow-call-stack` flag is only supported on
 platforms that always treat x18 as a reserved register, and the `-Zfixed-x18`
 flag is not required to use the sanitizer on such platforms. However, the
 sanitizer may be supported on targets where this is not the case in the future.
+One way to do so now on Nightly compilers is to explicitly supply this `-Zfixed-x18`
+flag with `aarch64` targets, so that the sanitizer is available for instrumentation
+on targets like `aarch64-unknown-none`, for instance. However, discretion is still
+required to make sure that the runtime support is in place for this sanitizer
+to be effective.
 
 It is undefined behavior for `-Zsanitizer=shadow-call-stack` code to call into
 code where x18 is a temporary register. On the other hand, when you are *not*
diff --git a/src/doc/unstable-book/src/compiler-flags/sanitizer.md b/src/doc/unstable-book/src/compiler-flags/sanitizer.md
index 72b44e002b4..edc63a25ac1 100644
--- a/src/doc/unstable-book/src/compiler-flags/sanitizer.md
+++ b/src/doc/unstable-book/src/compiler-flags/sanitizer.md
@@ -787,6 +787,10 @@ A runtime must be provided by the application or operating system.
 
 See the [Clang ShadowCallStack documentation][clang-scs] for more details.
 
+* `aarch64-unknown-none`
+
+In addition to support from a runtime by the application or operating system, the `-Zfixed-x18` flag is also mandatory.
+
 # ThreadSanitizer
 
 ThreadSanitizer is a data race detection tool. It is supported on the following