[clang] 732ad8e - [clang][auto-init] Provide __builtin_alloca*_uninitialized variants
Marco Elver via cfe-commits
cfe-commits at lists.llvm.org
Wed Jan 12 06:20:35 PST 2022
Author: Marco Elver
Date: 2022-01-12T15:13:10+01:00
New Revision: 732ad8ea62edc403727af57537b5d83dcfa937aa
URL: https://github.com/llvm/llvm-project/commit/732ad8ea62edc403727af57537b5d83dcfa937aa
DIFF: https://github.com/llvm/llvm-project/commit/732ad8ea62edc403727af57537b5d83dcfa937aa.diff
LOG: [clang][auto-init] Provide __builtin_alloca*_uninitialized variants
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()` (and
`__builtin_alloca_with_align_uninitialized`), which never performs
initialization when `-ftrivial-auto-var-init=` is enabled.
[1] https://reviews.llvm.org/D60548
[2] https://lkml.kernel.org/r/YbHTKUjEejZCLyhX@elver.google.com
Reviewed By: glider
Differential Revision: https://reviews.llvm.org/D115440
Added:
Modified:
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
Removed:
################################################################################
diff --git a/clang/include/clang/Basic/Builtins.def b/clang/include/clang/Basic/Builtins.def
index bfaa7e9f5a9fa..c7c47cf99abac 100644
--- a/clang/include/clang/Basic/Builtins.def
+++ b/clang/include/clang/Basic/Builtins.def
@@ -641,7 +641,9 @@ BUILTIN(__builtin_unreachable, "v", "nr")
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_alloca_with_align_uninitialized, "v*zIz", "Fn")
BUILTIN(__builtin_call_with_static_chain, "v.", "nt")
BUILTIN(__builtin_elementwise_abs, "v.", "nct")
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 7b5d202afbba2..e9dd41a7daa10 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -3427,6 +3427,7 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
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();
@@ -3437,10 +3438,12 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
.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);
}
+ case Builtin::BI__builtin_alloca_with_align_uninitialized:
case Builtin::BI__builtin_alloca_with_align: {
Value *Size = EmitScalarExpr(E->getArg(0));
Value *AlignmentInBitsValue = EmitScalarExpr(E->getArg(1));
@@ -3450,7 +3453,8 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
CGM.getContext().toCharUnitsFromBits(AlignmentInBits).getAsAlign();
AllocaInst *AI = Builder.CreateAlloca(Builder.getInt8Ty(), Size);
AI->setAlignment(AlignmentInBytes);
- initializeAlloca(*this, AI, Size, AlignmentInBytes);
+ if (BuiltinID != Builtin::BI__builtin_alloca_with_align_uninitialized)
+ initializeAlloca(*this, AI, Size, AlignmentInBytes);
return RValue::get(AI);
}
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 908cb78fbb0a7..d067ac31dc1e8 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -1750,10 +1750,12 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
return ExprError();
break;
case Builtin::BI__builtin_alloca_with_align:
+ case Builtin::BI__builtin_alloca_with_align_uninitialized:
if (SemaBuiltinAllocaWithAlign(TheCall))
return ExprError();
LLVM_FALLTHROUGH;
case Builtin::BI__builtin_alloca:
+ case Builtin::BI__builtin_alloca_uninitialized:
Diag(TheCall->getBeginLoc(), diag::warn_alloca)
<< TheCall->getDirectCallee();
break;
diff --git a/clang/test/CodeGenCXX/trivial-auto-var-init.cpp b/clang/test/CodeGenCXX/trivial-auto-var-init.cpp
index 513222cb3f1d1..58fba5d96b577 100644
--- a/clang/test/CodeGenCXX/trivial-auto-var-init.cpp
+++ b/clang/test/CodeGenCXX/trivial-auto-var-init.cpp
@@ -201,6 +201,34 @@ void test_alloca_with_align(int size) {
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_alloca_with_align_uninitialized(
+// ZERO-LABEL: test_alloca_with_align_uninitialized(
+// ZERO: %[[SIZE:[a-z0-9]+]] = sext i32 %{{.*}} to i64
+// ZERO-NEXT: %[[ALLOCA:[a-z0-9]+]] = alloca i8, i64 %[[SIZE]], align 128
+// ZERO-NOT: call void @llvm.memset
+// PATTERN-LABEL: test_alloca_with_align_uninitialized(
+// PATTERN: %[[SIZE:[a-z0-9]+]] = sext i32 %{{.*}} to i64
+// PATTERN-NEXT: %[[ALLOCA:[a-z0-9]+]] = alloca i8, i64 %[[SIZE]], align 128
+// PATTERN-NOT: call void @llvm.memset
+void test_alloca_with_align_uninitialized(int size) {
+ void *ptr = __builtin_alloca_with_align_uninitialized(size, 1024);
+ used(ptr);
+}
+
// UNINIT-LABEL: test_struct_vla(
// ZERO-LABEL: test_struct_vla(
// ZERO: %[[SIZE:[0-9]+]] = mul nuw i64 %{{.*}}, 16
diff --git a/clang/test/Sema/warn-alloca.c b/clang/test/Sema/warn-alloca.c
index 0990e11ef2fa2..07fe0f920304d 100644
--- a/clang/test/Sema/warn-alloca.c
+++ b/clang/test/Sema/warn-alloca.c
@@ -18,3 +18,17 @@ void test2(int a) {
// expected-warning at -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 at -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
+}
+
+void test4(int a) {
+ __builtin_alloca_with_align_uninitialized(a, 32);
+#ifndef SILENCE
+ // expected-warning at -2 {{use of function '__builtin_alloca_with_align_uninitialized' is discouraged; there is no way to check for failure but failure may still occur, resulting in a possibly exploitable security vulnerability}}
+#endif
+}
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