[clang] Allow the no_stack_protector attribute on local variables (PR #173311)
via cfe-commits
cfe-commits at lists.llvm.org
Mon Jan 12 16:53:06 PST 2026
https://github.com/cooperp updated https://github.com/llvm/llvm-project/pull/173311
>From 67b3337b4ffaa4da0866bef3b2ef6632ad21ac9f Mon Sep 17 00:00:00 2001
From: Peter Cooper <peter_cooper at apple.com>
Date: Sat, 8 Nov 2025 10:14:14 -0800
Subject: [PATCH] Allow the no_stack_protector attribute on local variables.
This attribute is currently only permitted on functions, but sometimes it can
be useful to opt out a specific local variable. That way the function can still
get a stack protector if other locals require it, but locals which are opted out
won't have the overhead of a stack protector if not needed.
---
clang/docs/ReleaseNotes.rst | 5 ++
clang/include/clang/Basic/Attr.td | 7 ++
clang/include/clang/Basic/AttrDocs.td | 11 +++
clang/lib/CodeGen/CGDecl.cpp | 8 +++
clang/test/CodeGen/stack-protector-vars.cpp | 68 +++++++++++++++++++
...a-attribute-supported-attributes-list.test | 1 +
6 files changed, 100 insertions(+)
create mode 100644 clang/test/CodeGen/stack-protector-vars.cpp
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8f7a0a2b304d0..41276403a38eb 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -387,6 +387,11 @@ Attribute Changes in Clang
multilibs for printf features. Requires cooperation with the libc
implementation.
+- Added new attribute ``stack_protector_ignore`` to opt specific local variables out of
+ the analysis which determines if a function should get a stack protector. A function
+ will still generate a stack protector if other local variables or command line flags
+ require it.
+
- On targets with Itanium C++ ABI, Clang now supports ``[[gnu:gcc_struct]]``
with the behavior similar to one existing in GCC. In particular, whenever
``-mms-bitfields`` command line option is provided (or if Microsoft-compatible
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index b017906a8d690..525a883f5987c 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -2731,6 +2731,13 @@ def NoStackProtector : InheritableAttr {
let SimpleHandler = 1;
}
+def StackProtectorIgnore : InheritableAttr {
+ let Spellings = [Clang<"stack_protector_ignore">];
+ let Subjects = SubjectList<[LocalVar]>;
+ let Documentation = [StackProtectorIgnoreDocs];
+ let SimpleHandler = 1;
+}
+
def StrictGuardStackCheck : InheritableAttr {
let Spellings = [Declspec<"strict_gs_check">];
let Subjects = SubjectList<[Function]>;
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index 812b48058d189..79b872c6bbe78 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -5736,6 +5736,17 @@ option.
}];
}
+def StackProtectorIgnoreDocs : Documentation {
+ let Category = DocCatFunction;
+ let Content = [{
+The ``stack_protector_ignore`` attribute skips analysis of the given local
+variable when determining if a function should use a stack protector.
+
+If ``-fstack-protector-all`` is specified then the given function will still
+use a stack protector, even if some/all variables have this attribute.
+ }];
+}
+
def StrictGuardStackCheckDocs : Documentation {
let Category = DocCatFunction;
let Content = [{
diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index 8b1cd83af2396..2d9278f599037 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -1635,6 +1635,14 @@ CodeGenFunction::EmitAutoVarAlloca(const VarDecl &D) {
assert(!emission.useLifetimeMarkers());
}
}
+
+ if (D.hasAttr<StackProtectorIgnoreAttr>()) {
+ if (auto *AI = dyn_cast<llvm::AllocaInst>(address.getBasePointer())) {
+ llvm::LLVMContext &Ctx = Builder.getContext();
+ auto *Operand = llvm::ConstantAsMetadata::get(Builder.getInt32(0));
+ AI->setMetadata("stack-protector", llvm::MDNode::get(Ctx, {Operand}));
+ }
+ }
} else {
EnsureInsertPoint();
diff --git a/clang/test/CodeGen/stack-protector-vars.cpp b/clang/test/CodeGen/stack-protector-vars.cpp
new file mode 100644
index 0000000000000..3baec1c24b2b6
--- /dev/null
+++ b/clang/test/CodeGen/stack-protector-vars.cpp
@@ -0,0 +1,68 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -emit-llvm -o - %s | FileCheck %s
+
+typedef __SIZE_TYPE__ size_t;
+
+int printf(const char * _Format, ...);
+char *strcpy(char *s1, const char *s2);
+
+struct S {
+ S();
+ int a[4];
+};
+
+// CHECK: define {{.*}} @_Z5test1PKc
+// CHECK: %a = alloca [1000 x i8], align {{.*}}, !stack-protector ![[A:.*]]
+void test1(const char *msg) {
+ __attribute__((stack_protector_ignore))
+ char a[1000];
+ strcpy(a, msg);
+ printf("%s\n", a);
+}
+
+// CHECK: define {{.*}} @_Z5test2
+// CHECK-NOT: %b = alloca [1000 x i8], align {{.*}}, !stack-protector
+void test2(const char *msg) {
+ char b[1000];
+ strcpy(b, msg);
+ printf("%s\n", b);
+}
+
+// CHECK: define {{.*}} @_Z5test3v
+// CHECK: %retval = alloca %struct.S, align {{.*}}, !stack-protector ![[A:.*]]
+S test3() {
+ __attribute__((stack_protector_ignore))
+ S s;
+ return s;
+}
+
+// CHECK: define {{.*}} @_Z5test4b
+// CHECK: %retval = alloca %struct.S, align {{.*}}, !stack-protector ![[A:.*]]
+// CHECK: call void @_ZN1SC1Ev
+S test4(bool b) {
+ __attribute__((stack_protector_ignore))
+ S s;
+ if ( b )
+ return s;
+ else
+ return s;
+}
+
+// CHECK: define {{.*}} @_Z5test5b
+// CHECK: %retval = alloca %struct.S, align {{.*}}
+// CHECK-NOT: stack-protector
+// CHECK: %s1 = alloca %struct.S, align {{.*}}, !stack-protector ![[A:.*]]
+// CHECK: %s2 = alloca %struct.S, align {{.*}}
+// CHECK-NOT: stack-protector
+// CHECK: call void @_ZN1SC1Ev
+// CHECK: call void @_ZN1SC1Ev
+S test5(bool b) {
+ __attribute__((stack_protector_ignore))
+ S s1;
+ S s2;
+ if ( b )
+ return s1;
+ else
+ return s2;
+}
+
+// CHECK: ![[A]] = !{i32 0}
diff --git a/clang/test/Misc/pragma-attribute-supported-attributes-list.test b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
index 3114d2fc9e693..d6d900f3caf84 100644
--- a/clang/test/Misc/pragma-attribute-supported-attributes-list.test
+++ b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
@@ -198,6 +198,7 @@
// CHECK-NEXT: Section (SubjectMatchRule_function, SubjectMatchRule_variable_is_global, SubjectMatchRule_objc_method, SubjectMatchRule_objc_property)
// CHECK-NEXT: SetTypestate (SubjectMatchRule_function_is_member)
// CHECK-NEXT: SpeculativeLoadHardening (SubjectMatchRule_function, SubjectMatchRule_objc_method)
+// CHECK-NEXT: StackProtectorIgnore (SubjectMatchRule_variable_is_local)
// CHECK-NEXT: StandaloneDebug (SubjectMatchRule_record)
// CHECK-NEXT: SwiftAsync (SubjectMatchRule_function, SubjectMatchRule_objc_method)
// CHECK-NEXT: SwiftAsyncContext (SubjectMatchRule_variable_is_parameter)
More information about the cfe-commits
mailing list