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When `-ftrivial-auto-var-init=` is enabled, allocas unconditionally
receive auto-initialization since [1].
In certain cases, it turns out, this is causing problems. For example,
when using alloca to add a random stack offset, as the Linux kernel does
on syscall entry [2]. In this case, none of the alloca'd stack memory is
ever used, and initializing it should be controllable; furthermore, it
is not always possible to safely call memset (see [2]).
Introduce `__builtin_alloca_uninitialized()`, which never performs
initialization when `-ftrivial-auto-var-init=` is enabled.
[1] https://reviews.llvm.org/D60548
[2] https://lkml.kernel.org/r/ybhtkujeejzcl...@elver.google.com
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D115440
Files:
clang/include/clang/Basic/Builtins.def
clang/lib/CodeGen/CGBuiltin.cpp
clang/lib/Sema/SemaChecking.cpp
clang/test/CodeGenCXX/trivial-auto-var-init.cpp
clang/test/Sema/warn-alloca.c
Index: clang/test/Sema/warn-alloca.c
===================================================================
--- clang/test/Sema/warn-alloca.c
+++ clang/test/Sema/warn-alloca.c
@@ -18,3 +18,10 @@
// expected-warning@-2 {{use of function '__builtin_alloca_with_align' is
discouraged; there is no way to check for failure but failure may still occur,
resulting in a possibly exploitable security vulnerability}}
#endif
}
+
+void test3(int a) {
+ __builtin_alloca_uninitialized(a);
+#ifndef SILENCE
+ // expected-warning@-2 {{use of function '__builtin_alloca_uninitialized' is
discouraged; there is no way to check for failure but failure may still occur,
resulting in a possibly exploitable security vulnerability}}
+#endif
+}
Index: clang/test/CodeGenCXX/trivial-auto-var-init.cpp
===================================================================
--- clang/test/CodeGenCXX/trivial-auto-var-init.cpp
+++ clang/test/CodeGenCXX/trivial-auto-var-init.cpp
@@ -201,6 +201,20 @@
used(ptr);
}
+// UNINIT-LABEL: test_alloca_uninitialized(
+// ZERO-LABEL: test_alloca_uninitialized(
+// ZERO: %[[SIZE:[a-z0-9]+]] = sext i32 %{{.*}} to i64
+// ZERO-NEXT: %[[ALLOCA:[a-z0-9]+]] = alloca i8, i64 %[[SIZE]], align
[[ALIGN:[0-9]+]]
+// ZERO-NOT: call void @llvm.memset
+// PATTERN-LABEL: test_alloca_uninitialized(
+// PATTERN: %[[SIZE:[a-z0-9]+]] = sext i32 %{{.*}} to i64
+// PATTERN-NEXT: %[[ALLOCA:[a-z0-9]+]] = alloca i8, i64 %[[SIZE]], align
[[ALIGN:[0-9]+]]
+// PATTERN-NOT: call void @llvm.memset
+void test_alloca_uninitialized(int size) {
+ void *ptr = __builtin_alloca_uninitialized(size);
+ used(ptr);
+}
+
// UNINIT-LABEL: test_struct_vla(
// ZERO-LABEL: test_struct_vla(
// ZERO: %[[SIZE:[0-9]+]] = mul nuw i64 %{{.*}}, 16
Index: clang/lib/Sema/SemaChecking.cpp
===================================================================
--- clang/lib/Sema/SemaChecking.cpp
+++ clang/lib/Sema/SemaChecking.cpp
@@ -1686,6 +1686,7 @@
if (SemaBuiltinAllocaWithAlign(TheCall))
return ExprError();
LLVM_FALLTHROUGH;
+ case Builtin::BI__builtin_alloca_uninitialized:
case Builtin::BI__builtin_alloca:
Diag(TheCall->getBeginLoc(), diag::warn_alloca)
<< TheCall->getDirectCallee();
Index: clang/lib/CodeGen/CGBuiltin.cpp
===================================================================
--- clang/lib/CodeGen/CGBuiltin.cpp
+++ clang/lib/CodeGen/CGBuiltin.cpp
@@ -3417,6 +3417,7 @@
case Builtin::BIalloca:
case Builtin::BI_alloca:
+ case Builtin::BI__builtin_alloca_uninitialized:
case Builtin::BI__builtin_alloca: {
Value *Size = EmitScalarExpr(E->getArg(0));
const TargetInfo &TI = getContext().getTargetInfo();
@@ -3427,7 +3428,8 @@
.getAsAlign();
AllocaInst *AI = Builder.CreateAlloca(Builder.getInt8Ty(), Size);
AI->setAlignment(SuitableAlignmentInBytes);
- initializeAlloca(*this, AI, Size, SuitableAlignmentInBytes);
+ if (BuiltinID != Builtin::BI__builtin_alloca_uninitialized)
+ initializeAlloca(*this, AI, Size, SuitableAlignmentInBytes);
return RValue::get(AI);
}
Index: clang/include/clang/Basic/Builtins.def
===================================================================
--- clang/include/clang/Basic/Builtins.def
+++ clang/include/clang/Basic/Builtins.def
@@ -640,6 +640,7 @@
BUILTIN(__builtin_shufflevector, "v." , "nct")
BUILTIN(__builtin_convertvector, "v." , "nct")
BUILTIN(__builtin_alloca, "v*z" , "Fn")
+BUILTIN(__builtin_alloca_uninitialized, "v*z", "Fn")
BUILTIN(__builtin_alloca_with_align, "v*zIz", "Fn")
BUILTIN(__builtin_call_with_static_chain, "v.", "nt")
Index: clang/test/Sema/warn-alloca.c
===================================================================
--- clang/test/Sema/warn-alloca.c
+++ clang/test/Sema/warn-alloca.c
@@ -18,3 +18,10 @@
// expected-warning@-2 {{use of function '__builtin_alloca_with_align' is discouraged; there is no way to check for failure but failure may still occur, resulting in a possibly exploitable security vulnerability}}
#endif
}
+
+void test3(int a) {
+ __builtin_alloca_uninitialized(a);
+#ifndef SILENCE
+ // expected-warning@-2 {{use of function '__builtin_alloca_uninitialized' is discouraged; there is no way to check for failure but failure may still occur, resulting in a possibly exploitable security vulnerability}}
+#endif
+}
Index: clang/test/CodeGenCXX/trivial-auto-var-init.cpp
===================================================================
--- clang/test/CodeGenCXX/trivial-auto-var-init.cpp
+++ clang/test/CodeGenCXX/trivial-auto-var-init.cpp
@@ -201,6 +201,20 @@
used(ptr);
}
+// UNINIT-LABEL: test_alloca_uninitialized(
+// ZERO-LABEL: test_alloca_uninitialized(
+// ZERO: %[[SIZE:[a-z0-9]+]] = sext i32 %{{.*}} to i64
+// ZERO-NEXT: %[[ALLOCA:[a-z0-9]+]] = alloca i8, i64 %[[SIZE]], align [[ALIGN:[0-9]+]]
+// ZERO-NOT: call void @llvm.memset
+// PATTERN-LABEL: test_alloca_uninitialized(
+// PATTERN: %[[SIZE:[a-z0-9]+]] = sext i32 %{{.*}} to i64
+// PATTERN-NEXT: %[[ALLOCA:[a-z0-9]+]] = alloca i8, i64 %[[SIZE]], align [[ALIGN:[0-9]+]]
+// PATTERN-NOT: call void @llvm.memset
+void test_alloca_uninitialized(int size) {
+ void *ptr = __builtin_alloca_uninitialized(size);
+ used(ptr);
+}
+
// UNINIT-LABEL: test_struct_vla(
// ZERO-LABEL: test_struct_vla(
// ZERO: %[[SIZE:[0-9]+]] = mul nuw i64 %{{.*}}, 16
Index: clang/lib/Sema/SemaChecking.cpp
===================================================================
--- clang/lib/Sema/SemaChecking.cpp
+++ clang/lib/Sema/SemaChecking.cpp
@@ -1686,6 +1686,7 @@
if (SemaBuiltinAllocaWithAlign(TheCall))
return ExprError();
LLVM_FALLTHROUGH;
+ case Builtin::BI__builtin_alloca_uninitialized:
case Builtin::BI__builtin_alloca:
Diag(TheCall->getBeginLoc(), diag::warn_alloca)
<< TheCall->getDirectCallee();
Index: clang/lib/CodeGen/CGBuiltin.cpp
===================================================================
--- clang/lib/CodeGen/CGBuiltin.cpp
+++ clang/lib/CodeGen/CGBuiltin.cpp
@@ -3417,6 +3417,7 @@
case Builtin::BIalloca:
case Builtin::BI_alloca:
+ case Builtin::BI__builtin_alloca_uninitialized:
case Builtin::BI__builtin_alloca: {
Value *Size = EmitScalarExpr(E->getArg(0));
const TargetInfo &TI = getContext().getTargetInfo();
@@ -3427,7 +3428,8 @@
.getAsAlign();
AllocaInst *AI = Builder.CreateAlloca(Builder.getInt8Ty(), Size);
AI->setAlignment(SuitableAlignmentInBytes);
- initializeAlloca(*this, AI, Size, SuitableAlignmentInBytes);
+ if (BuiltinID != Builtin::BI__builtin_alloca_uninitialized)
+ initializeAlloca(*this, AI, Size, SuitableAlignmentInBytes);
return RValue::get(AI);
}
Index: clang/include/clang/Basic/Builtins.def
===================================================================
--- clang/include/clang/Basic/Builtins.def
+++ clang/include/clang/Basic/Builtins.def
@@ -640,6 +640,7 @@
BUILTIN(__builtin_shufflevector, "v." , "nct")
BUILTIN(__builtin_convertvector, "v." , "nct")
BUILTIN(__builtin_alloca, "v*z" , "Fn")
+BUILTIN(__builtin_alloca_uninitialized, "v*z", "Fn")
BUILTIN(__builtin_alloca_with_align, "v*zIz", "Fn")
BUILTIN(__builtin_call_with_static_chain, "v.", "nt")
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