CFI: Support function pointers for trait methods

Adds support for both CFI and KCFI for attaching concrete and abstract
types to functions. KCFI does this through generation of `ReifyShim` on
any function pointer that could go in a vtable, and checking the
`ReifyReason` when emitting the instance. CFI does this by attaching
both the concrete and abstract type to every instance.

TypeID codegen tests are switched to be anchored on the left rather than
the right in order to allow emission of additional type attachments.

Fixes #115953

Author: maurer

compiler/rustc_middle/src/ty/instance.rs

@@ -502,6 +502,14 @@ impl<'tcx> Instance<'tcx> {
                     debug!(" => fn pointer created for virtual call");
                     resolved.def = InstanceDef::ReifyShim(def_id, reason);
                 }
+                // FIXME(maurer) only shim it if it is a vtable-safe function
+                _ if tcx.sess.is_sanitizer_kcfi_enabled()
+                    && tcx.associated_item(def_id).trait_item_def_id.is_some() =>
+                {
+                    // If this function could also go in a vtable, we need to `ReifyShim` it with
+                    // KCFI because it can only attach one type per function.
+                    resolved.def = InstanceDef::ReifyShim(resolved.def_id(), reason)
+                }
                 _ => {}
             }
 

compiler/rustc_symbol_mangling/src/typeid.rs

@@ -4,7 +4,7 @@
 /// For more information about LLVM CFI and cross-language LLVM CFI support for the Rust compiler,
 /// see design document in the tracking issue #89653.
 use bitflags::bitflags;
-use rustc_middle::ty::{Instance, Ty, TyCtxt};
+use rustc_middle::ty::{Instance, InstanceDef, ReifyReason, Ty, TyCtxt};
 use rustc_target::abi::call::FnAbi;
 use std::hash::Hasher;
 use twox_hash::XxHash64;
@@ -67,8 +67,13 @@ pub fn kcfi_typeid_for_fnabi<'tcx>(
 pub fn kcfi_typeid_for_instance<'tcx>(
     tcx: TyCtxt<'tcx>,
     instance: Instance<'tcx>,
-    options: TypeIdOptions,
+    mut options: TypeIdOptions,
 ) -> u32 {
+    // If we receive a `ReifyShim` intended to produce a function pointer, we need to remain
+    // concrete - abstraction is for vtables.
+    if matches!(instance.def, InstanceDef::ReifyShim(_, Some(ReifyReason::FnPtr))) {
+        options |= TypeIdOptions::NO_SELF_TYPE_ERASURE
+    }
     // A KCFI type metadata identifier is a 32-bit constant produced by taking the lower half of the
     // xxHash64 of the type metadata identifier. (See llvm/llvm-project@cff5bef.)
     let mut hash: XxHash64 = Default::default();

tests/ui/sanitizer/cfi-method-fn-ptr-cast.rs

@@ -1,11 +1,40 @@
 // Verifies that casting a method to a function pointer works.
-//
-// FIXME(#122848): Remove only-linux when fixed.
+
+//@ revisions: cfi kcfi
+// FIXME(#122848) Remove only-linux once OSX CFI binaries work
 //@ only-linux
-//@ needs-sanitizer-cfi
-//@ compile-flags: -Clto -Copt-level=0 -Cprefer-dynamic=off -Ctarget-feature=-crt-static -Zsanitizer=cfi
+//@ [cfi] needs-sanitizer-cfi
+//@ [kcfi] needs-sanitizer-kcfi
+//@ compile-flags: -C target-feature=-crt-static
+//@ [cfi] compile-flags: -C opt-level=0 -C codegen-units=1 -C lto
+//@ [cfi] compile-flags: -C prefer-dynamic=off
+//@ [cfi] compile-flags: -Z sanitizer=cfi
+//@ [kcfi] compile-flags: -Z sanitizer=kcfi
 //@ run-pass
 
+trait Foo {
+    fn foo(&self);
+    fn bar(&self);
+}
+
+struct S;
+
+impl Foo for S {
+    fn foo(&self) {}
+    #[track_caller]
+    fn bar(&self) {}
+}
+
+struct S2 {
+    f: fn(&S)
+}
+
+impl S2 {
+    fn foo(&self, s: &S) {
+        (self.f)(s)
+    }
+}
+
 trait Trait1 {
     fn foo(&self);
 }
@@ -20,4 +49,8 @@ fn main() {
     let type1 = Type1 {};
     let f = <Type1 as Trait1>::foo;
     f(&type1);
+    // Check again with different optimization barriers
+    S2 { f: <S as Foo>::foo }.foo(&S);
+    // Check mismatched #[track_caller]
+    S2 { f: <S as Foo>::bar }.foo(&S)
 }