Add support for control-flow protection

This change adds a flag for configuring control-flow protection in the
LLVM backend. In Clang, this flag is exposed as `-fcf-protection` with
options `none|branch|return|full`. This convention is followed for
`rustc`, though as a codegen option: `rustc -Z
cf-protection=<none|branch|return|full>`.

Co-authored-by: BlackHoleFox <[email protected]>

compiler/rustc_codegen_llvm/src/context.rs

@@ -21,7 +21,8 @@ use rustc_middle::ty::layout::{
 };
 use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
 use rustc_middle::{bug, span_bug};
-use rustc_session::config::{BranchProtection, CFGuard, CrateType, DebugInfo, PAuthKey, PacRet};
+use rustc_session::config::{BranchProtection, CFGuard, CFProtection};
+use rustc_session::config::{CrateType, DebugInfo, PAuthKey, PacRet};
 use rustc_session::Session;
 use rustc_span::source_map::Span;
 use rustc_span::symbol::Symbol;
@@ -287,6 +288,24 @@ pub unsafe fn create_module<'ll>(
         );
     }
 
+    // Pass on the control-flow protection flags to LLVM (equivalent to `-fcf-protection` in Clang).
+    if let CFProtection::Branch | CFProtection::Full = sess.opts.debugging_opts.cf_protection {
+        llvm::LLVMRustAddModuleFlag(
+            llmod,
+            llvm::LLVMModFlagBehavior::Override,
+            "cf-protection-branch\0".as_ptr().cast(),
+            1,
+        )
+    }
+    if let CFProtection::Return | CFProtection::Full = sess.opts.debugging_opts.cf_protection {
+        llvm::LLVMRustAddModuleFlag(
+            llmod,
+            llvm::LLVMModFlagBehavior::Override,
+            "cf-protection-return\0".as_ptr().cast(),
+            1,
+        )
+    }
+
     llmod
 }
 

compiler/rustc_session/src/config.rs

@@ -63,6 +63,22 @@ pub enum CFGuard {
     Checks,
 }
 
+/// The different settings that the `-Z cf-protection` flag can have.
+#[derive(Clone, Copy, PartialEq, Hash, Debug)]
+pub enum CFProtection {
+    /// Do not enable control-flow protection
+    None,
+
+    /// Emit control-flow protection for branches (enables indirect branch tracking).
+    Branch,
+
+    /// Emit control-flow protection for returns.
+    Return,
+
+    /// Emit control-flow protection for both branches and returns.
+    Full,
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Hash)]
 pub enum OptLevel {
     No,         // -O0
@@ -2630,11 +2646,11 @@ impl PpMode {
 /// we have an opt-in scheme here, so one is hopefully forced to think about
 /// how the hash should be calculated when adding a new command-line argument.
 crate mod dep_tracking {
-    use super::LdImpl;
     use super::{
-        BranchProtection, CFGuard, CrateType, DebugInfo, ErrorOutputType, InstrumentCoverage,
-        LinkerPluginLto, LocationDetail, LtoCli, OptLevel, OutputType, OutputTypes, Passes,
-        SourceFileHashAlgorithm, SwitchWithOptPath, SymbolManglingVersion, TrimmedDefPaths,
+        BranchProtection, CFGuard, CFProtection, CrateType, DebugInfo, ErrorOutputType,
+        InstrumentCoverage, LdImpl, LinkerPluginLto, LocationDetail, LtoCli, OptLevel, OutputType,
+        OutputTypes, Passes, SourceFileHashAlgorithm, SwitchWithOptPath, SymbolManglingVersion,
+        TrimmedDefPaths,
     };
     use crate::lint;
     use crate::options::WasiExecModel;
@@ -2715,6 +2731,7 @@ crate mod dep_tracking {
         NativeLibKind,
         SanitizerSet,
         CFGuard,
+        CFProtection,
         TargetTriple,
         Edition,
         LinkerPluginLto,

compiler/rustc_session/src/options.rs

@@ -380,6 +380,7 @@ mod desc {
     pub const parse_sanitizer_memory_track_origins: &str = "0, 1, or 2";
     pub const parse_cfguard: &str =
         "either a boolean (`yes`, `no`, `on`, `off`, etc), `checks`, or `nochecks`";
+    pub const parse_cfprotection: &str = "`none`|`no`|`n` (default), `branch`, `return`, or `full`|`yes`|`y` (equivalent to `branch` and `return`)";
     pub const parse_strip: &str = "either `none`, `debuginfo`, or `symbols`";
     pub const parse_linker_flavor: &str = ::rustc_target::spec::LinkerFlavor::one_of();
     pub const parse_optimization_fuel: &str = "crate=integer";
@@ -695,6 +696,25 @@ mod parse {
         true
     }
 
+    crate fn parse_cfprotection(slot: &mut CFProtection, v: Option<&str>) -> bool {
+        if v.is_some() {
+            let mut bool_arg = None;
+            if parse_opt_bool(&mut bool_arg, v) {
+                *slot = if bool_arg.unwrap() { CFProtection::Full } else { CFProtection::None };
+                return true;
+            }
+        }
+
+        *slot = match v {
+            None | Some("none") => CFProtection::None,
+            Some("branch") => CFProtection::Branch,
+            Some("return") => CFProtection::Return,
+            Some("full") => CFProtection::Full,
+            Some(_) => return false,
+        };
+        true
+    }
+
     crate fn parse_linker_flavor(slot: &mut Option<LinkerFlavor>, v: Option<&str>) -> bool {
         match v.and_then(LinkerFlavor::from_str) {
             Some(lf) => *slot = Some(lf),
@@ -1142,6 +1162,8 @@ options! {
         "select which borrowck is used (`mir` or `migrate`) (default: `migrate`)"),
     branch_protection: BranchProtection = (BranchProtection::default(), parse_branch_protection, [TRACKED],
         "set options for branch target identification and pointer authentication on AArch64"),
+    cf_protection: CFProtection = (CFProtection::None, parse_cfprotection, [TRACKED],
+        "instrument control-flow architecture protection"),
     cgu_partitioning_strategy: Option<String> = (None, parse_opt_string, [TRACKED],
         "the codegen unit partitioning strategy to use"),
     chalk: bool = (false, parse_bool, [TRACKED],

src/doc/unstable-book/src/compiler-flags/cf-protection.md

@@ -0,0 +1,40 @@
+# `cf-protection`
+
+This option enables control-flow enforcement technology (CET) on x86; a more detailed description of
+CET is available [here]. Similar to `clang`, this flag takes one of the following values:
+
+- `none` - Disable CET completely (this is the default).
+- `branch` - Enable indirect branch tracking (`IBT`).
+- `return` - Enable shadow stack (`SHSTK`).
+- `full` - Enable both `branch` and `return`.
+
+[here]: https://www.intel.com/content/www/us/en/develop/articles/technical-look-control-flow-enforcement-technology.html
+
+This flag only applies to the LLVM backend: it sets the `cf-protection-branch` and
+`cf-protection-return` flags on LLVM modules. Note, however, that all compiled modules linked
+together must have the flags set for the compiled output to be CET-enabled. Currently, Rust's
+standard library does not ship with CET enabled by default, so you may need to rebuild all standard
+modules with a `cargo` command like:
+
+```sh
+$ RUSTFLAGS="-Z cf-protection=full" RUSTC="rustc-custom" cargo +nightly build -Z build-std --target x86_64-unknown-linux-gnu
+```
+
+### Detection
+
+An ELF binary is CET-enabled if it has the `IBT` and `SHSTK` tags, e.g.:
+
+```sh
+$ readelf -a target/x86_64-unknown-linux-gnu/debug/example | grep feature:
+      Properties: x86 feature: IBT, SHSTK
+```
+
+### Troubleshooting
+
+To display modules that are not CET enabled, examine the linker errors available when `cet-report` is enabled:
+
+```sh
+$ RUSTC_LOG=rustc_codegen_ssa::back::link=info rustc-custom -v -Z cf-protection=full -C link-arg="-Wl,-z,cet-report=warning" -o example example.rs
+...
+/usr/bin/ld: /.../build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-d73f7266be14cb8b.rlib(std-d73f7266be14cb8b.std.f7443020-cgu.12.rcgu.o): warning: missing IBT and SHSTK properties
+```

src/test/codegen/cf-protection.rs

@@ -0,0 +1,38 @@
+// Test that the correct module flags are emitted with different control-flow protection flags.
+
+// revisions: undefined none branch return full
+// needs-llvm-components: x86
+// [undefined] compile-flags:
+// [none] compile-flags: -Z cf-protection=none
+// [branch] compile-flags: -Z cf-protection=branch
+// [return] compile-flags: -Z cf-protection=return
+// [full] compile-flags: -Z cf-protection=full
+// compile-flags: --target x86_64-unknown-linux-gnu
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang="sized"]
+trait Sized { }
+
+// A basic test function.
+pub fn test() {
+}
+
+// undefined-NOT: !"cf-protection-branch"
+// undefined-NOT: !"cf-protection-return"
+
+// none-NOT: !"cf-protection-branch"
+// none-NOT: !"cf-protection-return"
+
+// branch-NOT: !"cf-protection-return"
+// branch: !"cf-protection-branch", i32 1
+// branch-NOT: !"cf-protection-return"
+
+// return-NOT: !"cf-protection-branch"
+// return: !"cf-protection-return", i32 1
+// return-NOT: !"cf-protection-branch"
+
+// full: !"cf-protection-branch", i32 1
+// full: !"cf-protection-return", i32 1