No branch protection metadata unless enabled

Even if we emit metadata disabling branch protection, this metadata may
conflict with other modules (e.g. during LTO) that have different branch
protection metadata set.

This is an unstable flag and feature, so ideally the flag not being
specified should act as if the feature wasn't implemented in the first
place.

Additionally this PR also ensures we emit an error if
`-Zbranch-protection` is set on targets other than the supported
aarch64. For now the error is being output from codegen, but ideally it
should be moved to earlier in the pipeline before stabilization.

compiler/rustc_codegen_llvm/src/context.rs

@@ -269,36 +269,36 @@ pub unsafe fn create_module<'ll>(
         }
     }
 
-    if sess.target.arch == "aarch64" {
-        let BranchProtection { bti, pac_ret: pac } = sess.opts.debugging_opts.branch_protection;
-
-        llvm::LLVMRustAddModuleFlag(
-            llmod,
-            llvm::LLVMModFlagBehavior::Error,
-            "branch-target-enforcement\0".as_ptr().cast(),
-            bti.into(),
-        );
-
-        llvm::LLVMRustAddModuleFlag(
-            llmod,
-            llvm::LLVMModFlagBehavior::Error,
-            "sign-return-address\0".as_ptr().cast(),
-            pac.is_some().into(),
-        );
-        let pac_opts = pac.unwrap_or(PacRet { leaf: false, key: PAuthKey::A });
-        llvm::LLVMRustAddModuleFlag(
-            llmod,
-            llvm::LLVMModFlagBehavior::Error,
-            "sign-return-address-all\0".as_ptr().cast(),
-            pac_opts.leaf.into(),
-        );
-        let is_bkey: bool = pac_opts.key != PAuthKey::A;
-        llvm::LLVMRustAddModuleFlag(
-            llmod,
-            llvm::LLVMModFlagBehavior::Error,
-            "sign-return-address-with-bkey\0".as_ptr().cast(),
-            is_bkey.into(),
-        );
+    if let Some(BranchProtection { bti, pac_ret }) = sess.opts.debugging_opts.branch_protection {
+        if sess.target.arch != "aarch64" {
+            sess.err("-Zbranch-protection is only supported on aarch64");
+        } else {
+            llvm::LLVMRustAddModuleFlag(
+                llmod,
+                llvm::LLVMModFlagBehavior::Error,
+                "branch-target-enforcement\0".as_ptr().cast(),
+                bti.into(),
+            );
+            llvm::LLVMRustAddModuleFlag(
+                llmod,
+                llvm::LLVMModFlagBehavior::Error,
+                "sign-return-address\0".as_ptr().cast(),
+                pac_ret.is_some().into(),
+            );
+            let pac_opts = pac_ret.unwrap_or(PacRet { leaf: false, key: PAuthKey::A });
+            llvm::LLVMRustAddModuleFlag(
+                llmod,
+                llvm::LLVMModFlagBehavior::Error,
+                "sign-return-address-all\0".as_ptr().cast(),
+                pac_opts.leaf.into(),
+            );
+            llvm::LLVMRustAddModuleFlag(
+                llmod,
+                llvm::LLVMModFlagBehavior::Error,
+                "sign-return-address-with-bkey\0".as_ptr().cast(),
+                u32::from(pac_opts.key == PAuthKey::B),
+            );
+        }
     }
 
     // Pass on the control-flow protection flags to LLVM (equivalent to `-fcf-protection` in Clang).

compiler/rustc_interface/src/tests.rs

@@ -721,7 +721,10 @@ fn test_debugging_options_tracking_hash() {
     tracked!(binary_dep_depinfo, true);
     tracked!(
         branch_protection,
-        BranchProtection { bti: true, pac_ret: Some(PacRet { leaf: true, key: PAuthKey::B }) }
+        Some(BranchProtection {
+            bti: true,
+            pac_ret: Some(PacRet { leaf: true, key: PAuthKey::B })
+        })
     );
     tracked!(chalk, true);
     tracked!(codegen_backend, Some("abc".to_string()));

compiler/rustc_session/src/lib.rs

@@ -2,6 +2,7 @@
 #![feature(derive_default_enum)]
 #![feature(min_specialization)]
 #![feature(once_cell)]
+#![feature(option_get_or_insert_default)]
 #![recursion_limit = "256"]
 #![cfg_attr(not(bootstrap), allow(rustc::potential_query_instability))]
 

compiler/rustc_session/src/options.rs

@@ -984,9 +984,10 @@ mod parse {
         true
     }
 
-    crate fn parse_branch_protection(slot: &mut BranchProtection, v: Option<&str>) -> bool {
+    crate fn parse_branch_protection(slot: &mut Option<BranchProtection>, v: Option<&str>) -> bool {
         match v {
             Some(s) => {
+                let slot = slot.get_or_insert_default();
                 for opt in s.split(',') {
                     match opt {
                         "bti" => slot.bti = true,
@@ -1161,7 +1162,7 @@ options! {
         (default: no)"),
     borrowck: String = ("migrate".to_string(), parse_string, [UNTRACKED],
         "select which borrowck is used (`mir` or `migrate`) (default: `migrate`)"),
-    branch_protection: BranchProtection = (BranchProtection::default(), parse_branch_protection, [TRACKED],
+    branch_protection: Option<BranchProtection> = (None, 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"),

src/test/codegen/branch-protection.rs

@@ -1,12 +1,12 @@
 // Test that the correct module flags are emitted with different branch protection flags.
 
-// revisions: bti pac-ret leaf b-key
+// revisions: BTI PACRET LEAF BKEY NONE
 // min-llvm-version: 12.0.0
 // needs-llvm-components: aarch64
-// [bti] compile-flags: -Z branch-protection=bti
-// [pac-ret] compile-flags: -Z branch-protection=pac-ret
-// [leaf] compile-flags: -Z branch-protection=pac-ret,leaf
-// [b-key] compile-flags: -Z branch-protection=pac-ret,b-key
+// [BTI] compile-flags: -Z branch-protection=bti
+// [PACRET] compile-flags: -Z branch-protection=pac-ret
+// [LEAF] compile-flags: -Z branch-protection=pac-ret,leaf
+// [BKEY] compile-flags: -Z branch-protection=pac-ret,b-key
 // compile-flags: --target aarch64-unknown-linux-gnu
 
 #![crate_type = "lib"]
@@ -20,22 +20,27 @@ trait Sized { }
 pub fn test() {
 }
 
-// bti: !"branch-target-enforcement", i32 1
-// bti: !"sign-return-address", i32 0
-// bti: !"sign-return-address-all", i32 0
-// bti: !"sign-return-address-with-bkey", i32 0
-
-// pac-ret: !"branch-target-enforcement", i32 0
-// pac-ret: !"sign-return-address", i32 1
-// pac-ret: !"sign-return-address-all", i32 0
-// pac-ret: !"sign-return-address-with-bkey", i32 0
-
-// leaf: !"branch-target-enforcement", i32 0
-// leaf: !"sign-return-address", i32 1
-// leaf: !"sign-return-address-all", i32 1
-// leaf: !"sign-return-address-with-bkey", i32 0
-
-// b-key: !"branch-target-enforcement", i32 0
-// b-key: !"sign-return-address", i32 1
-// b-key: !"sign-return-address-all", i32 0
-// b-key: !"sign-return-address-with-bkey", i32 1
+// BTI: !"branch-target-enforcement", i32 1
+// BTI: !"sign-return-address", i32 0
+// BTI: !"sign-return-address-all", i32 0
+// BTI: !"sign-return-address-with-bkey", i32 0
+
+// PACRET: !"branch-target-enforcement", i32 0
+// PACRET: !"sign-return-address", i32 1
+// PACRET: !"sign-return-address-all", i32 0
+// PACRET: !"sign-return-address-with-bkey", i32 0
+
+// LEAF: !"branch-target-enforcement", i32 0
+// LEAF: !"sign-return-address", i32 1
+// LEAF: !"sign-return-address-all", i32 1
+// LEAF: !"sign-return-address-with-bkey", i32 0
+
+// BKEY: !"branch-target-enforcement", i32 0
+// BKEY: !"sign-return-address", i32 1
+// BKEY: !"sign-return-address-all", i32 0
+// BKEY: !"sign-return-address-with-bkey", i32 1
+
+// NONE-NOT: branch-target-enforcement
+// NONE-NOT: sign-return-address
+// NONE-NOT: sign-return-address-all
+// NONE-NOT: sign-return-address-with-bkey

src/test/ui/invalid-compile-flags/branch-protection-missing-pac-ret.BADTARGET.stderr

@@ -0,0 +1,4 @@
+error: -Zbranch-protection is only supported on aarch64
+
+error: aborting due to previous error
+

src/test/ui/invalid-compile-flags/branch-protection-missing-pac-ret.rs

@@ -1 +1,14 @@
-// compile-flags: -Z branch-protection=leaf
+// revisions: BADFLAGS BADTARGET
+// [BADFLAGS] compile-flags: --target=aarch64-unknown-linux-gnu -Zbranch-protection=leaf
+// [BADFLAGS] check-fail
+// [BADFLAGS] needs-llvm-components: aarch64
+// [BADTARGET] compile-flags: --target=x86_64-unknown-linux-gnu -Zbranch-protection=bti
+// [BADTARGET] build-fail
+// [BADTARGET] needs-llvm-components: x86
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang="sized"]
+trait Sized { }