https://github.com/amane-ame updated 
https://github.com/llvm/llvm-project/pull/119563

From 659eda3ec76b63418f8b621b004728d9d7bf26ad Mon Sep 17 00:00:00 2001
From: amane-ame <i...@amane-a.me>
Date: Wed, 11 Dec 2024 22:17:51 +0800
Subject: [PATCH 1/4] [clang] Fix crashes when passing VLA to va_arg

---
 clang/lib/CodeGen/CGExprAgg.cpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
index 2ad6587089f101..a4111cb65c8b1c 100644
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -2201,6 +2201,8 @@ void CodeGenFunction::EmitAggregateCopy(LValue Dest, 
LValue Src, QualType Ty,
     // But note that getTypeInfo returns 0 for a VLA.
     if (auto *VAT = dyn_cast_or_null<VariableArrayType>(
             getContext().getAsArrayType(Ty))) {
+      assert(Ty->isVariableArrayType());
+      EmitVariablyModifiedType(Ty);
       QualType BaseEltTy;
       SizeVal = emitArrayLength(VAT, BaseEltTy, DestPtr);
       TypeInfo = getContext().getTypeInfoInChars(BaseEltTy);

From 5937db790ff0a59ea5bf18cb008d38a4524dc7dc Mon Sep 17 00:00:00 2001
From: amane-ame <i...@amane-a.me>
Date: Thu, 12 Dec 2024 13:50:13 +0800
Subject: [PATCH 2/4] [clang] Add a testcase for passing VLA to va_arg

---
 clang/test/CodeGen/varargs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/clang/test/CodeGen/varargs.c b/clang/test/CodeGen/varargs.c
index 625399b87f7ad7..b7b1b52156be37 100644
--- a/clang/test/CodeGen/varargs.c
+++ b/clang/test/CodeGen/varargs.c
@@ -20,4 +20,7 @@ void vla(int n, ...)
   __builtin_va_list ap;
   void *p;
   p = __builtin_va_arg(ap, typeof (int (*)[++n])); // CHECK: add nsw i32 
{{.*}}, 1
+  // Don't crash on some undefined behaviors.
+  p = __builtin_va_arg(ap, typeof (int [++n]));
+  p = __builtin_va_arg(ap, typeof (int [n][n]));
 }

From df9f8f61ee21b81c9cfd300d113afea9298b8067 Mon Sep 17 00:00:00 2001
From: amane-ame <i...@amane-a.me>
Date: Thu, 12 Dec 2024 13:52:59 +0800
Subject: [PATCH 3/4] [clang] Move the parsing of VLA in va_arg to EmitVAArg

---
 clang/lib/CodeGen/CGCall.cpp    | 2 ++
 clang/lib/CodeGen/CGExprAgg.cpp | 2 --
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index 3cefc9da66ddb8..4e2812c62f4357 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -6121,6 +6121,8 @@ RValue CodeGenFunction::EmitVAArg(VAArgExpr *VE, Address 
&VAListAddr,
   VAListAddr = VE->isMicrosoftABI() ? EmitMSVAListRef(VE->getSubExpr())
                                     : EmitVAListRef(VE->getSubExpr());
   QualType Ty = VE->getType();
+  if (Ty->isVariableArrayType())
+    EmitVariablyModifiedType(Ty);
   if (VE->isMicrosoftABI())
     return CGM.getABIInfo().EmitMSVAArg(*this, VAListAddr, Ty, Slot);
   return CGM.getABIInfo().EmitVAArg(*this, VAListAddr, Ty, Slot);
diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
index a4111cb65c8b1c..2ad6587089f101 100644
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -2201,8 +2201,6 @@ void CodeGenFunction::EmitAggregateCopy(LValue Dest, 
LValue Src, QualType Ty,
     // But note that getTypeInfo returns 0 for a VLA.
     if (auto *VAT = dyn_cast_or_null<VariableArrayType>(
             getContext().getAsArrayType(Ty))) {
-      assert(Ty->isVariableArrayType());
-      EmitVariablyModifiedType(Ty);
       QualType BaseEltTy;
       SizeVal = emitArrayLength(VAT, BaseEltTy, DestPtr);
       TypeInfo = getContext().getTypeInfoInChars(BaseEltTy);

From b38c1d1ee20d3308a4120c3b95a167a936314a6b Mon Sep 17 00:00:00 2001
From: amane-ame <i...@amane-a.me>
Date: Thu, 12 Dec 2024 15:43:35 +0800
Subject: [PATCH 4/4] [clang] Emit an undefined-behavior warning for passing
 VLA to va_arg

---
 clang/include/clang/Basic/DiagnosticSemaKinds.td | 4 ++++
 clang/lib/Sema/SemaExpr.cpp                      | 7 +++++++
 clang/test/CodeGen/varargs.c                     | 6 +++---
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td 
b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 0a245e2077f68f..3a352f23faa353 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -10497,6 +10497,10 @@ def 
warn_second_parameter_to_va_arg_ownership_qualified : Warning<
 def warn_second_parameter_to_va_arg_never_compatible : Warning<
   "second argument to 'va_arg' is of promotable type %0; this va_arg has "
   "undefined behavior because arguments will be promoted to %1">, 
InGroup<Varargs>;
+def warn_second_parameter_to_va_arg_vla : Warning<
+  "second argument to 'va_arg' is of variable length array type %0; "
+  "this va_arg has undefined behavior because arguments will never "
+  "be compatible with variable length array type">, InGroup<Varargs>;
 
 def warn_return_missing_expr : Warning<
   "non-void %select{function|method}1 %0 should return a value">, DefaultError,
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 1524622408a18a..ad84f38b75cfcf 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -16538,6 +16538,13 @@ ExprResult Sema::BuildVAArgExpr(SourceLocation 
BuiltinLoc,
         << TInfo->getTypeLoc().getSourceRange();
     }
 
+    if (TInfo->getType()->isVariableArrayType()) {
+      DiagRuntimeBehavior(TInfo->getTypeLoc().getBeginLoc(), E,
+                  PDiag(diag::warn_second_parameter_to_va_arg_vla)
+                          << TInfo->getType()
+                          << TInfo->getTypeLoc().getSourceRange());
+    }
+
     // Check for va_arg where arguments of the given type will be promoted
     // (i.e. this va_arg is guaranteed to have undefined behavior).
     QualType PromoteType;
diff --git a/clang/test/CodeGen/varargs.c b/clang/test/CodeGen/varargs.c
index b7b1b52156be37..8e474ef35b12f3 100644
--- a/clang/test/CodeGen/varargs.c
+++ b/clang/test/CodeGen/varargs.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple i386-unknown-unknown -emit-llvm -o - %s | FileCheck 
%s
+// RUN: %clang_cc1 -triple i386-unknown-unknown -emit-llvm -verify -o - %s | 
FileCheck %s
 
 // PR6433 - Don't crash on va_arg(typedef).
 typedef double gdouble;
@@ -21,6 +21,6 @@ void vla(int n, ...)
   void *p;
   p = __builtin_va_arg(ap, typeof (int (*)[++n])); // CHECK: add nsw i32 
{{.*}}, 1
   // Don't crash on some undefined behaviors.
-  p = __builtin_va_arg(ap, typeof (int [++n]));
-  p = __builtin_va_arg(ap, typeof (int [n][n]));
+  p = __builtin_va_arg(ap, typeof (int [++n])); // expected-warning{{second 
argument to 'va_arg' is of variable length array type 'typeof(int[++n])'}}
+  p = __builtin_va_arg(ap, typeof (int [n][n])); // expected-warning{{second 
argument to 'va_arg' is of variable length array type 'typeof(int[n][n])'}}
 }

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