Rollup merge of rust-lang#130824 - Darksonn:fix-function-return, r=we…

…sleywiser

Add missing module flags for `-Zfunction-return=thunk-extern`

This fixes a bug in the `-Zfunction-return=thunk-extern` flag. The flag needs to be passed onto LLVM to ensure that functions such as `asan.module_ctor` and `asan.module_dtor` that are created internally in LLVM have the mitigation applied to them.

This was originally discovered [in the Linux kernel](https://lore.kernel.org/all/CANiq72myZL4_poCMuNFevtpYYc0V0embjSuKb7y=C+m3vVA_8g@mail.gmail.com/).

Original flag PR: rust-lang#116892
PR for similar issue: rust-lang#129373
Tracking issue: rust-lang#116853

cc `@ojeda`
r? `@wesleywiser`

compiler/rustc_codegen_llvm/src/context.rs

@@ -19,7 +19,7 @@ use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
 use rustc_middle::{bug, span_bug};
 use rustc_session::Session;
 use rustc_session::config::{
-    BranchProtection, CFGuard, CFProtection, CrateType, DebugInfo, PAuthKey, PacRet,
+    BranchProtection, CFGuard, CFProtection, CrateType, DebugInfo, FunctionReturn, PAuthKey, PacRet,
 };
 use rustc_span::source_map::Spanned;
 use rustc_span::{DUMMY_SP, Span};
@@ -378,6 +378,18 @@ pub(crate) unsafe fn create_module<'ll>(
         }
     }
 
+    match sess.opts.unstable_opts.function_return {
+        FunctionReturn::Keep => {}
+        FunctionReturn::ThunkExtern => unsafe {
+            llvm::LLVMRustAddModuleFlagU32(
+                llmod,
+                llvm::LLVMModFlagBehavior::Override,
+                c"function_return_thunk_extern".as_ptr(),
+                1,
+            )
+        },
+    }
+
     match (sess.opts.unstable_opts.small_data_threshold, sess.target.small_data_threshold_support())
     {
         // Set up the small-data optimization limit for architectures that use

tests/codegen/function-return.rs

@@ -26,3 +26,9 @@ pub fn foo() {
     // keep-thunk-extern: attributes #0 = { {{.*}}fn_ret_thunk_extern{{.*}} }
     // thunk-extern-keep-NOT: fn_ret_thunk_extern
 }
+
+// unset-NOT: !{{[0-9]+}} = !{i32 4, !"function_return_thunk_extern", i32 1}
+// keep-NOT: !{{[0-9]+}} = !{i32 4, !"function_return_thunk_extern", i32 1}
+// thunk-extern: !{{[0-9]+}} = !{i32 4, !"function_return_thunk_extern", i32 1}
+// keep-thunk-extern: !{{[0-9]+}} = !{i32 4, !"function_return_thunk_extern", i32 1}
+// thunk-extern-keep-NOT: !{{[0-9]+}} = !{i32 4, !"function_return_thunk_extern", i32 1}