-Zharden-sls flag (target modifier) added to enable mitigation agains…

…t straight line speculation (SLS)

compiler/rustc_session/src/config.rs

@@ -2944,11 +2944,11 @@ pub(crate) mod dep_tracking {
     use super::{
         AutoDiff, BranchProtection, CFGuard, CFProtection, CollapseMacroDebuginfo, CoverageOptions,
         CrateType, DebugInfo, DebugInfoCompression, ErrorOutputType, FmtDebug, FunctionReturn,
-        InliningThreshold, InstrumentCoverage, InstrumentXRay, LinkerPluginLto, LocationDetail,
-        LtoCli, MirStripDebugInfo, NextSolverConfig, OomStrategy, OptLevel, OutFileName,
-        OutputType, OutputTypes, PatchableFunctionEntry, Polonius, RemapPathScopeComponents,
-        ResolveDocLinks, SourceFileHashAlgorithm, SplitDwarfKind, SwitchWithOptPath,
-        SymbolManglingVersion, WasiExecModel,
+        HardenSls, InliningThreshold, InstrumentCoverage, InstrumentXRay, LinkerPluginLto,
+        LocationDetail, LtoCli, MirStripDebugInfo, NextSolverConfig, OomStrategy, OptLevel,
+        OutFileName, OutputType, OutputTypes, PatchableFunctionEntry, Polonius,
+        RemapPathScopeComponents, ResolveDocLinks, SourceFileHashAlgorithm, SplitDwarfKind,
+        SwitchWithOptPath, SymbolManglingVersion, WasiExecModel,
     };
     use crate::lint;
     use crate::utils::NativeLib;
@@ -3049,6 +3049,7 @@ pub(crate) mod dep_tracking {
         Polonius,
         InliningThreshold,
         FunctionReturn,
+        HardenSls,
         WasmCAbi,
         Align,
     );
@@ -3304,6 +3305,16 @@ pub enum FunctionReturn {
     ThunkExtern,
 }
 
+/// The different settings that the `-Zharden-sls` flag can have.
+#[derive(Clone, Copy, PartialEq, Hash, Debug, Default)]
+pub enum HardenSls {
+    #[default]
+    None,
+    All,
+    Return,
+    IndirectJmp,
+}
+
 /// Whether extra span comments are included when dumping MIR, via the `-Z mir-include-spans` flag.
 /// By default, only enabled in the NLL MIR dumps, and disabled in all other passes.
 #[derive(Clone, Copy, Default, PartialEq, Debug)]

compiler/rustc_session/src/options.rs

@@ -290,6 +290,7 @@ macro_rules! top_level_options {
             pub fn target_feature_flag_enabled(&self, flag: &str) -> bool {
                 match flag {
                     "x86-retpoline" => self.unstable_opts.x86_retpoline,
+                    "harden-sls" => !matches!(self.unstable_opts.harden_sls, HardenSls::None),
                     _ => false,
                 }
             }
@@ -307,6 +308,17 @@ macro_rules! top_level_options {
                         +retpoline-indirect-calls"
                     );
                 }
+                if let Some(features) = match unstable_opts.harden_sls {
+                    HardenSls::None => None,
+                    HardenSls::All => Some("+harden-sls-ijmp,+harden-sls-ret"),
+                    HardenSls::Return => Some("+harden-sls-ret"),
+                    HardenSls::IndirectJmp => Some("+harden-sls-ijmp"),
+                } {
+                    if !cg.target_feature.is_empty() {
+                        cg.target_feature.push(',');
+                    }
+                    cg.target_feature.push_str(features);
+                }
             }
         }
     );
@@ -812,6 +824,7 @@ mod desc {
         "either a boolean (`yes`, `no`, `on`, `off`, etc), or a non-negative number";
     pub(crate) const parse_llvm_module_flag: &str = "<key>:<type>:<value>:<behavior>. Type must currently be `u32`. Behavior should be one of (`error`, `warning`, `require`, `override`, `append`, `appendunique`, `max`, `min`)";
     pub(crate) const parse_function_return: &str = "`keep` or `thunk-extern`";
+    pub(crate) const parse_harden_sls: &str = "`none`, `all`, `return` or `indirect-jmp`";
     pub(crate) const parse_wasm_c_abi: &str = "`legacy` or `spec`";
     pub(crate) const parse_mir_include_spans: &str =
         "either a boolean (`yes`, `no`, `on`, `off`, etc), or `nll` (default: `nll`)";
@@ -1908,6 +1921,17 @@ pub mod parse {
         true
     }
 
+    pub(crate) fn parse_harden_sls(slot: &mut HardenSls, v: Option<&str>) -> bool {
+        match v {
+            Some("none") => *slot = HardenSls::None,
+            Some("all") => *slot = HardenSls::All,
+            Some("return") => *slot = HardenSls::Return,
+            Some("indirect-jmp") => *slot = HardenSls::IndirectJmp,
+            _ => return false,
+        }
+        true
+    }
+
     pub(crate) fn parse_wasm_c_abi(slot: &mut WasmCAbi, v: Option<&str>) -> bool {
         match v {
             Some("spec") => *slot = WasmCAbi::Spec,
@@ -2236,6 +2260,9 @@ options! {
     graphviz_font: String = ("Courier, monospace".to_string(), parse_string, [UNTRACKED],
         "use the given `fontname` in graphviz output; can be overridden by setting \
         environment variable `RUSTC_GRAPHVIZ_FONT` (default: `Courier, monospace`)"),
+    harden_sls: HardenSls = (HardenSls::None, parse_harden_sls, [TRACKED TARGET_MODIFIER],
+        "flag to mitigate against straight line speculation (SLS) [none|all|return|indirect-jmp] \
+        (default: none)"),
     has_thread_local: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "explicitly enable the `cfg(target_thread_local)` directive"),
     human_readable_cgu_names: bool = (false, parse_bool, [TRACKED],

compiler/rustc_target/src/target_features.rs

@@ -421,6 +421,22 @@ const X86_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("fma", Stable, &["avx"]),
     ("fxsr", Stable, &[]),
     ("gfni", Unstable(sym::avx512_target_feature), &["sse2"]),
+    (
+        "harden-sls-ijmp",
+        Stability::EnabledByTargetModifierFlag {
+            reason: "use `harden-sls` target modifier flag instead",
+            flag: "harden-sls",
+        },
+        &[],
+    ),
+    (
+        "harden-sls-ret",
+        Stability::EnabledByTargetModifierFlag {
+            reason: "use `harden-sls` target modifier flag instead",
+            flag: "harden-sls",
+        },
+        &[],
+    ),
     ("lahfsahf", Unstable(sym::lahfsahf_target_feature), &[]),
     ("lzcnt", Stable, &[]),
     ("movbe", Stable, &[]),

tests/codegen/harden-sls.rs

@@ -0,0 +1,33 @@
+// Test that the `harden-sls-ijmp`, `harden-sls-ret` target features is (not) emitted when
+// the `harden-sls=[none|all|return|indirect-jmp]` flag is (not) set.
+
+//@ revisions: none all return indirect_jmp
+//@ needs-llvm-components: x86
+//@ compile-flags: --target x86_64-unknown-linux-gnu
+//@ [none] compile-flags: -Zharden-sls=none
+//@ [all] compile-flags: -Zharden-sls=all
+//@ [return] compile-flags: -Zharden-sls=return
+//@ [indirect_jmp] compile-flags: -Zharden-sls=indirect-jmp
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+
+#[no_mangle]
+pub fn foo() {
+    // CHECK: @foo() unnamed_addr #0
+
+    // none-NOT: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+harden-sls-ijmp{{.*}} }
+    // none-NOT: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+harden-sls-ret{{.*}} }
+
+    // all: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+harden-sls-ijmp,+harden-sls-ret{{.*}} }
+
+    // return-NOT: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+harden-sls-ijmp{{.*}} }
+    // return: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+harden-sls-ret{{.*}} }
+
+    // indirect_jmp-NOT: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+harden-sls-ret{{.*}} }
+    // indirect_jmp: attributes #0 = { {{.*}}"target-features"="{{[^"]*}}+harden-sls-ijmp{{.*}} }
+}

tests/ui/target-feature/harden-sls-target-feature-flag.by_feature.stderr

@@ -0,0 +1,12 @@
+warning: target feature `harden-sls-ijmp` cannot be enabled with `-Ctarget-feature`: use `harden-sls` target modifier flag instead
+   |
+   = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
+
+warning: target feature `harden-sls-ret` cannot be enabled with `-Ctarget-feature`: use `harden-sls` target modifier flag instead
+   |
+   = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
+
+warning: 2 warnings emitted
+

tests/ui/target-feature/harden-sls-target-feature-flag.rs

@@ -0,0 +1,14 @@
+//@ revisions: by_flag by_feature
+//@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=lib
+//@ needs-llvm-components: x86
+//@ [by_flag]compile-flags: -Zharden-sls=all
+//@ [by_feature]compile-flags: -Ctarget-feature=+harden-sls-ijmp,+harden-sls-ret
+//@ [by_flag]build-pass
+// For now this is just a warning.
+//@ [by_feature]build-pass
+#![feature(no_core, lang_items)]
+#![no_std]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}