riscv64imac: allow shadow call stack sanitizer

compiler/rustc_target/src/spec/targets/riscv64gc_unknown_none_elf.rs

@@ -28,7 +28,7 @@ pub fn target() -> Target {
             code_model: Some(CodeModel::Medium),
             emit_debug_gdb_scripts: false,
             eh_frame_header: false,
-            supported_sanitizers: SanitizerSet::KERNELADDRESS,
+            supported_sanitizers: SanitizerSet::KERNELADDRESS | SanitizerSet::SHADOWCALLSTACK,
             ..Default::default()
         },
     }

compiler/rustc_target/src/spec/targets/riscv64imac_unknown_none_elf.rs

@@ -27,7 +27,7 @@ pub fn target() -> Target {
             code_model: Some(CodeModel::Medium),
             emit_debug_gdb_scripts: false,
             eh_frame_header: false,
-            supported_sanitizers: SanitizerSet::KERNELADDRESS,
+            supported_sanitizers: SanitizerSet::KERNELADDRESS | SanitizerSet::SHADOWCALLSTACK,
             ..Default::default()
         },
     }

src/doc/unstable-book/src/compiler-flags/sanitizer.md

@@ -776,21 +776,42 @@ See the [Clang SafeStack documentation][clang-safestack] for more details.
 # ShadowCallStack
 
 ShadowCallStack provides backward edge control flow protection by storing a function's return address in a separately allocated 'shadow call stack' and loading the return address from that shadow call stack.
-
-ShadowCallStack requires a platform ABI which reserves `x18` as the instrumentation makes use of this register.
+To that end, implementation of this sanitizer requires reservation of one of the registers on the target platform.
+Software support from the operating system and runtime may be required depending on the target platform which is detailed in the remaining section.
+See the [Clang ShadowCallStack documentation][clang-scs] for more details.
 
 ShadowCallStack can be enabled with `-Zsanitizer=shadow-call-stack` option and is supported on the following targets:
 
-* `aarch64-linux-android`
+## AArch64 family
 
-A runtime must be provided by the application or operating system.
+ShadowCallStack requires on this platform ABI reservation of the register `x18` as the instrumentation makes use of this register.
+When `x18` is not reserved on the target AArch64 platform and is availabe as a scratch register, enabling ShadowCallStack leds to undefined behaviour.
+In other words, code that is calling into or called by functions instrumented with ShadowCallStack must reserve the `x18` register or preserve its value.
 
-See the [Clang ShadowCallStack documentation][clang-scs] for more details.
+### `aarch64-linux-android`
 
-* `aarch64-unknown-none`
+This target already reserves the `x18` register.
+A runtime must be provided by the application or operating system.
+If `bionic` is used on this target, the software support is provided.
+Otherwise, a runtime needs to prepare a memory region and points `x18` to the region which serves as the shadow call stack.
+
+### `aarch64-unknown-none`
 
 In addition to support from a runtime by the application or operating system, the `-Zfixed-x18` flag is also mandatory.
 
+## RISC-V 64 family
+
+ShadowCallStack uses either the `gp` register for software shadow stack, also known as `x3`, or the `ssp` register if [`Zicfiss`][riscv-zicfiss] extension is available.
+`gp`/`x3` is currently always reserved and available for ShadowCallStack instrumentation, and `ssp` in case of `Zicfiss` is only accessible through its dedicated shadow stack instructions.
+
+Support from the runtime and operating system is required when `gp`/`x3` is used for software shadow stack.
+A runtime must prepare a memory region and point `gp`/`x3` to the region before executing the code.
+
+The following targets support ShadowCallStack.
+
+* `riscv64imac-unknown-none-elf`
+* `riscv64gc-unknown-none-elf`
+
 # ThreadSanitizer
 
 ThreadSanitizer is a data race detection tool. It is supported on the following
@@ -912,3 +933,4 @@ Sanitizers produce symbolized stacktraces when llvm-symbolizer binary is in `PAT
 [clang-tsan]: https://clang.llvm.org/docs/ThreadSanitizer.html
 [linux-kasan]: https://www.kernel.org/doc/html/latest/dev-tools/kasan.html
 [llvm-memtag]: https://llvm.org/docs/MemTagSanitizer.html
+[riscv-zicfiss]: https://github.com/riscv/riscv-cfi/blob/3f8e450c481ac303bd5643444f7a89672f24476e/src/cfi_backward.adoc