Allow the no_stack_protector attribute on local variables #173311
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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.
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@llvm/pr-subscribers-clang-codegen @llvm/pr-subscribers-clang Author: None (cooperp) ChangesThis 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. This is paired with an LLVM change which has already landed, which uses the new metadata to opt specific alloca instructions out of requiring a stack protector. Full diff: https://github.com/llvm/llvm-project/pull/173311.diff 5 Files Affected:
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index f85d2da21eab9..10cb3c64f54d2 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -2726,7 +2726,7 @@ def : MutualExclusions<[AlwaysInline, NotTailCalled]>;
def NoStackProtector : InheritableAttr {
let Spellings = [Clang<"no_stack_protector">, CXX11<"gnu", "no_stack_protector">,
C23<"gnu", "no_stack_protector">, Declspec<"safebuffers">];
- let Subjects = SubjectList<[Function]>;
+ let Subjects = SubjectList<[Function, LocalVar]>;
let Documentation = [NoStackProtectorDocs];
let SimpleHandler = 1;
}
diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index 8b1cd83af2396..ae8ca92d8860f 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -1604,6 +1604,14 @@ CodeGenFunction::EmitAutoVarAlloca(const VarDecl &D) {
allocaAlignment, D.getName(),
/*ArraySize=*/nullptr, &AllocaAddr);
+ if (D.hasAttr<NoStackProtectorAttr>()) {
+ 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}));
+ }
+ }
+
// Don't emit lifetime markers for MSVC catch parameters. The lifetime of
// the catch parameter starts in the catchpad instruction, and we can't
// insert code in those basic blocks.
diff --git a/clang/test/CodeGen/stack-protector-vars.c b/clang/test/CodeGen/stack-protector-vars.c
new file mode 100644
index 0000000000000..218cdc33b3010
--- /dev/null
+++ b/clang/test/CodeGen/stack-protector-vars.c
@@ -0,0 +1,25 @@
+// RUN: %clang_cc1 -emit-llvm -o - %s | FileCheck %s
+
+typedef __SIZE_TYPE__ size_t;
+
+int printf(const char * _Format, ...);
+char *strcpy(char *s1, const char *s2);
+
+// CHECK: define {{.*}}void @test1
+// CHECK: %a = alloca [1000 x i8], align 1, !stack-protector ![[A:.*]]
+void test1(const char *msg) {
+ __attribute__((no_stack_protector))
+ char a[1000];
+ strcpy(a, msg);
+ printf("%s\n", a);
+}
+
+// CHECK: define {{.*}}void @test2
+// CHECK-NOT: %b = alloca [1000 x i8], align 1, !stack-protector
+void test2(const char *msg) {
+ char b[1000];
+ strcpy(b, msg);
+ printf("%s\n", b);
+}
+
+// CHECK: ![[A]] = !{i32 0}
\ No newline at end of file
diff --git a/clang/test/Misc/pragma-attribute-supported-attributes-list.test b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
index 3114d2fc9e693..6420a298f4dc9 100644
--- a/clang/test/Misc/pragma-attribute-supported-attributes-list.test
+++ b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
@@ -138,7 +138,7 @@
// CHECK-NEXT: NoSanitizeThread (SubjectMatchRule_function)
// CHECK-NEXT: NoSpeculativeLoadHardening (SubjectMatchRule_function, SubjectMatchRule_objc_method)
// CHECK-NEXT: NoSplitStack (SubjectMatchRule_function)
-// CHECK-NEXT: NoStackProtector (SubjectMatchRule_function)
+// CHECK-NEXT: NoStackProtector (SubjectMatchRule_function, SubjectMatchRule_variable_is_local)
// CHECK-NEXT: NoThreadSafetyAnalysis (SubjectMatchRule_function)
// CHECK-NEXT: NoThrow (SubjectMatchRule_hasType_functionType)
// CHECK-NEXT: NoUwtable (SubjectMatchRule_hasType_functionType)
diff --git a/clang/test/Sema/no_stack_protector.c b/clang/test/Sema/no_stack_protector.c
index 1ecd46bc624ce..fab3df667883a 100644
--- a/clang/test/Sema/no_stack_protector.c
+++ b/clang/test/Sema/no_stack_protector.c
@@ -4,5 +4,10 @@
[[clang::no_stack_protector]] void test2(void) {}
void __attribute__((no_stack_protector)) foo(void) {}
-int __attribute__((no_stack_protector)) var; // expected-warning {{'no_stack_protector' attribute only applies to functions}}
+int __attribute__((no_stack_protector)) var; // expected-warning {{'no_stack_protector' attribute only applies to functions and local variables}}
void __attribute__((no_stack_protector(2))) bar(void) {} // expected-error {{'no_stack_protector' attribute takes no arguments}}
+
+void func()
+{
+ int __attribute__((no_stack_protector)) localvar;
+}
\ No newline at end of file
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🐧 Linux x64 Test Results
Failed Tests(click on a test name to see its output) ClangClang.CodeGen/stack-protector-vars.cIf these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the |
🪟 Windows x64 Test Results
Failed Tests(click on a test name to see its output) ClangClang.CodeGen/stack-protector-vars.cIf these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the |
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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.
This is paired with an LLVM change which has already landed, which uses the new metadata to opt specific alloca instructions out of requiring a stack protector.