[Clang] Implement function attribute no_stack_protector.

Summary:
This attribute tells clang to skip this function from stack protector
when -stack-protector option is passed.
GCC option for this is:
__attribute__((__optimize__("no-stack-protector"))) and the
equivalent clang syntax would be: __attribute__((no_stack_protector))

This is used in Linux kernel to selectively disable stack protector
in certain functions.

Reviewers: aaron.ballman, rsmith, rnk, probinson

Reviewed By: aaron.ballman

Subscribers: probinson, srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D46300

llvm-svn: 331925

clang/include/clang/Basic/Attr.td

@@ -1495,6 +1495,12 @@ def NotTailCalled : InheritableAttr {
   let Documentation = [NotTailCalledDocs];
 }
 
+def NoStackProtector : InheritableAttr {
+  let Spellings = [Clang<"no_stack_protector">];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [NoStackProtectorDocs];
+}
+
 def NoThrow : InheritableAttr {
   let Spellings = [GCC<"nothrow">, Declspec<"nothrow">];
   let Subjects = SubjectList<[Function]>;

clang/include/clang/Basic/AttrDocs.td

@@ -2740,6 +2740,28 @@ The syntax of the declare target directive is as follows:
   }];
 }
 
+def NoStackProtectorDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+Clang supports the ``__attribute__((no_stack_protector))`` attribute which disables
+the stack protector on the specified function. This attribute is useful for
+selectively disabling the stack protector on some functions when building with
+``-fstack-protector`` compiler option.
+
+For example, it disables the stack protector for the function ``foo`` but function
+``bar`` will still be built with the stack protector with the ``-fstack-protector``
+option.
+
+.. code-block:: c
+
+    int __attribute__((no_stack_protector))
+    foo (int x); // stack protection will be disabled for foo.
+
+    int bar(int y); // bar can be built with the stack protector.
+
+    }];
+}
+
 def NotTailCalledDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1142,12 +1142,14 @@ void CodeGenModule::SetLLVMFunctionAttributesForDefinition(const Decl *D,
   if (!hasUnwindExceptions(LangOpts))
     B.addAttribute(llvm::Attribute::NoUnwind);
 
+  if (!D || !D->hasAttr<NoStackProtectorAttr>()) {
     if (LangOpts.getStackProtector() == LangOptions::SSPOn)
       B.addAttribute(llvm::Attribute::StackProtect);
     else if (LangOpts.getStackProtector() == LangOptions::SSPStrong)
       B.addAttribute(llvm::Attribute::StackProtectStrong);
     else if (LangOpts.getStackProtector() == LangOptions::SSPReq)
       B.addAttribute(llvm::Attribute::StackProtectReq);
+  }
 
   if (!D) {
     // If we don't have a declaration to control inlining, the function isn't

clang/lib/Sema/SemaDeclAttr.cpp

@@ -6230,6 +6230,10 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case AttributeList::AT_NoInstrumentFunction: // Interacts with -pg.
     handleSimpleAttribute<NoInstrumentFunctionAttr>(S, D, AL);
     break;
+  case AttributeList::AT_NoStackProtector:
+    // Interacts with -fstack-protector options.
+    handleSimpleAttribute<NoStackProtectorAttr>(S, D, AL);
+    break;
   case AttributeList::AT_StdCall:
   case AttributeList::AT_CDecl:
   case AttributeList::AT_FastCall:

clang/test/CodeGen/stack-protector.c

@@ -22,6 +22,14 @@ void test1(const char *msg) {
   printf("%s\n", a);
 }
 
+// DEF: define {{.*}}void @test2(i8* %msg) #[[B:.*]] {
+__attribute__((no_stack_protector))
+void test2(const char *msg) {
+  char a[strlen(msg) + 1];
+  strcpy(a, msg);
+  printf("%s\n", a);
+}
+
 // NOSSP-NOT: attributes #[[A]] = {{.*}} ssp
 // SSP: attributes #[[A]] = {{.*}} ssp{{ }}
 // SSPSTRONG: attributes #[[A]] = {{.*}} sspstrong
@@ -33,3 +41,15 @@ void test1(const char *msg) {
 // SAFESTACK-SSP: attributes #[[A]] = {{.*}} safestack ssp{{ }}
 // SAFESTACK-SSPSTRONG: attributes #[[A]] = {{.*}} safestack sspstrong
 // SAFESTACK-SSPREQ: attributes #[[A]] = {{.*}} safestack sspreq
+
+// NOSSP-NOT: attributes #[[B]] = {{.*}} ssp
+// SSP-NOT: attributes #[[B]] = {{.*}} ssp{{ }}
+// SSPSTRONG-NOT: attributes #[[B]] = {{.*}} sspstrong
+// SSPREQ-NOT: attributes #[[B]] = {{.*}} sspreq
+
+// SAFESTACK-SSP: attributes #[[B]] = {{.*}} safestack
+// SAFESTACK-SSP-NOT: attributes #[[B]] = {{.*}} safestack ssp{{ }}
+// SAFESTACK-SSPSTRONG: attributes #[[B]] = {{.*}} safestack
+// SAFESTACK-SSPSTRONG-NOT: attributes #[[B]] = {{.*}} safestack sspstrong
+// SAFESTACK-SSPREQ: attributes #[[B]] = {{.*}} safestack
+// SAFESTACK-SSPREQ-NOT: attributes #[[B]] = {{.*}} safestack sspreq

clang/test/Sema/no_stack_protector.c

@@ -0,0 +1,5 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+void __attribute__((no_stack_protector)) foo() {}
+int __attribute__((no_stack_protector)) var; // expected-warning {{'no_stack_protector' attribute only applies to functions}}
+void  __attribute__((no_stack_protector(2))) bar() {} // expected-error {{'no_stack_protector' attribute takes no arguments}}