diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 20b499462ae94..33f28f251f5d0 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -8114,6 +8114,12 @@ def ext_gnu_ptr_func_arith : Extension<
   "arithmetic on%select{ a|}0 pointer%select{|s}0 to%select{ the|}2 function "
   "type%select{|s}2 %1%select{| and %3}2 is a GNU extension">,
   InGroup<GNUPointerArith>;
+def ext_gnu_counted_by_void_ptr
+    : Extension<
+          "'%select{counted_by|sized_by|counted_by_or_null|sized_by_or_null}0' "
+          "on a pointer to void is a GNU extension, treated as "
+          "'%select{sized_by|sized_by|sized_by_or_null|sized_by_or_null}0'">,
+      InGroup<GNUPointerArith>;
 def err_readonly_message_assignment : Error<
   "assigning to 'readonly' return result of an Objective-C message not allowed">;
 def ext_c2y_increment_complex : Extension<
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index fd14cd6926fe2..aaf8d59414498 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -1211,11 +1211,13 @@ llvm::Value *CodeGenFunction::emitCountedByPointerSize(
         getContext().getTypeSizeInChars(ElementTy->getPointeeType());
 
     if (ElementSize.isZero()) {
-      // This might be a __sized_by on a 'void *', which counts bytes, not
-      // elements.
+      // This might be a __sized_by (or __counted_by in GNU mode) on a
+      // 'void *', which counts bytes, not elements.
       auto *CAT = ElementTy->getAs<CountAttributedType>();
       if (!CAT || (CAT->getKind() != CountAttributedType::SizedBy &&
-                   CAT->getKind() != CountAttributedType::SizedByOrNull))
+                   CAT->getKind() != CountAttributedType::SizedByOrNull &&
+                   CAT->getKind() != CountAttributedType::CountedBy &&
+                   CAT->getKind() != CountAttributedType::CountedByOrNull))
         // Okay, not sure what it is now.
         // FIXME: Should this be an assert?
         return std::optional<CharUnits>();
diff --git a/clang/lib/Sema/SemaBoundsSafety.cpp b/clang/lib/Sema/SemaBoundsSafety.cpp
index 39ab13653f5fe..abc7397aaa888 100644
--- a/clang/lib/Sema/SemaBoundsSafety.cpp
+++ b/clang/lib/Sema/SemaBoundsSafety.cpp
@@ -132,9 +132,20 @@ bool Sema::CheckCountedByAttrOnField(FieldDecl *FD, Expr *E, bool CountInBytes,
     // `BoundsSafetyCheckUseOfCountAttrPtr`
     //
     // * When the pointee type is always an incomplete type (e.g.
-    // `void`) the attribute is disallowed by this method because we know the
-    // type can never be completed so there's no reason to allow it.
-    InvalidTypeKind = CountedByInvalidPointeeTypeKind::INCOMPLETE;
+    // `void` in strict C mode) the attribute is disallowed by this method
+    // because we know the type can never be completed so there's no reason
+    // to allow it.
+    //
+    // Exception: In GNU mode, void has an implicit size of 1 byte for pointer
+    // arithmetic. Therefore, counted_by on void* is allowed as a GNU extension
+    // and behaves equivalently to sized_by (treating the count as bytes).
+    bool IsVoidPtrInGNUMode = PointeeTy->isVoidType() && getLangOpts().GNUMode;
+    if (IsVoidPtrInGNUMode) {
+      // Emit a warning that this is a GNU extension
+      Diag(FD->getBeginLoc(), diag::ext_gnu_counted_by_void_ptr) << Kind;
+    } else {
+      InvalidTypeKind = CountedByInvalidPointeeTypeKind::INCOMPLETE;
+    }
   } else if (PointeeTy->isSizelessType()) {
     InvalidTypeKind = CountedByInvalidPointeeTypeKind::SIZELESS;
   } else if (PointeeTy->isFunctionType()) {
@@ -272,6 +283,11 @@ GetCountedByAttrOnIncompletePointee(QualType Ty, NamedDecl **ND) {
   if (!PointeeTy->isIncompleteType(ND))
     return {};
 
+  // If counted_by is on void*, it was already validated at declaration time
+  // as a GNU extension. No need to re-check GNU mode here.
+  if (PointeeTy->isVoidType())
+    return {};
+
   return {CATy, PointeeTy};
 }
 
diff --git a/clang/test/CodeGen/attr-counted-by-void-ptr-gnu.c b/clang/test/CodeGen/attr-counted-by-void-ptr-gnu.c
new file mode 100644
index 0000000000000..e22aad306f60c
--- /dev/null
+++ b/clang/test/CodeGen/attr-counted-by-void-ptr-gnu.c
@@ -0,0 +1,65 @@
+// RUN: %clang_cc1 -std=gnu11 -triple x86_64-unknown-linux-gnu -O2 -emit-llvm -o - %s | FileCheck %s
+
+// Test that counted_by on void* in GNU mode treats void as having size 1 (byte count)
+
+#define __counted_by(f)  __attribute__((counted_by(f)))
+#define __sized_by(f)  __attribute__((sized_by(f)))
+
+struct with_counted_by_void {
+  int count;
+  void* buf __counted_by(count);
+};
+
+struct with_sized_by_void {
+  int size;
+  void* buf __sized_by(size);
+};
+
+struct with_counted_by_int {
+  int count;
+  int* buf __counted_by(count);
+};
+
+// CHECK-LABEL: define dso_local {{.*}}@test_counted_by_void(
+// CHECK:         %[[COUNT:.*]] = load i32, ptr %s
+// CHECK:         %[[NARROW:.*]] = tail call i32 @llvm.smax.i32(i32 %[[COUNT]], i32 0)
+// CHECK:         %[[ZEXT:.*]] = zext nneg i32 %[[NARROW]] to i64
+// CHECK:         ret i64 %[[ZEXT]]
+//
+// Verify: counted_by on void* returns the count directly (count * 1 byte)
+long long test_counted_by_void(struct with_counted_by_void *s) {
+  return __builtin_dynamic_object_size(s->buf, 0);
+}
+
+// CHECK-LABEL: define dso_local {{.*}}@test_sized_by_void(
+// CHECK:         %[[SIZE:.*]] = load i32, ptr %s
+// CHECK:         %[[NARROW:.*]] = tail call i32 @llvm.smax.i32(i32 %[[SIZE]], i32 0)
+// CHECK:         %[[ZEXT:.*]] = zext nneg i32 %[[NARROW]] to i64
+// CHECK:         ret i64 %[[ZEXT]]
+//
+// Verify: sized_by on void* returns the size directly
+long long test_sized_by_void(struct with_sized_by_void *s) {
+  return __builtin_dynamic_object_size(s->buf, 0);
+}
+
+// CHECK-LABEL: define dso_local {{.*}}@test_counted_by_int(
+// CHECK:         %[[COUNT:.*]] = load i32, ptr %s
+// CHECK:         %[[SEXT:.*]] = sext i32 %[[COUNT]] to i64
+// CHECK:         %[[SIZE:.*]] = shl nsw i64 %[[SEXT]], 2
+// CHECK:         ret i64
+//
+// Verify: counted_by on int* returns count * sizeof(int) = count * 4
+long long test_counted_by_int(struct with_counted_by_int *s) {
+  return __builtin_dynamic_object_size(s->buf, 0);
+}
+
+// CHECK-LABEL: define dso_local ptr @test_void_ptr_arithmetic(
+// CHECK:         %[[BUF:.*]] = load ptr, ptr
+// CHECK:         %[[EXT:.*]] = sext i32 %offset to i64
+// CHECK:         %[[PTR:.*]] = getelementptr inbounds i8, ptr %[[BUF]], i64 %[[EXT]]
+// CHECK:         ret ptr %[[PTR]]
+//
+// Verify: pointer arithmetic on void* uses i8 (byte offsets), not i32 or other sizes
+void* test_void_ptr_arithmetic(struct with_counted_by_void *s, int offset) {
+  return s->buf + offset;  // GNU extension: void* arithmetic
+}
diff --git a/clang/test/Sema/attr-counted-by-late-parsed-struct-ptrs.c b/clang/test/Sema/attr-counted-by-late-parsed-struct-ptrs.c
index 8d4e0c510603a..b6c2ba68732ee 100644
--- a/clang/test/Sema/attr-counted-by-late-parsed-struct-ptrs.c
+++ b/clang/test/Sema/attr-counted-by-late-parsed-struct-ptrs.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fexperimental-late-parse-attributes -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c11 -fexperimental-late-parse-attributes -fsyntax-only -verify %s
 
 #define __counted_by(f)  __attribute__((counted_by(f)))
 
diff --git a/clang/test/Sema/attr-counted-by-or-null-last-field.c b/clang/test/Sema/attr-counted-by-or-null-last-field.c
index 60a1f571b19e9..4cf8cb64557d2 100644
--- a/clang/test/Sema/attr-counted-by-or-null-last-field.c
+++ b/clang/test/Sema/attr-counted-by-or-null-last-field.c
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify=expected,immediate %s
-// RUN: %clang_cc1 -fsyntax-only -fexperimental-late-parse-attributes -verify=expected,late %s
+// RUN: %clang_cc1 -std=c11 -fsyntax-only -verify=expected,immediate %s
+// RUN: %clang_cc1 -std=c11 -fsyntax-only -fexperimental-late-parse-attributes -verify=expected,late %s
 
 #define __counted_by_or_null(f)  __attribute__((counted_by_or_null(f)))
 
diff --git a/clang/test/Sema/attr-counted-by-or-null-late-parsed-struct-ptrs.c b/clang/test/Sema/attr-counted-by-or-null-late-parsed-struct-ptrs.c
index 2150c81f9e9be..29e235463f822 100644
--- a/clang/test/Sema/attr-counted-by-or-null-late-parsed-struct-ptrs.c
+++ b/clang/test/Sema/attr-counted-by-or-null-late-parsed-struct-ptrs.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fexperimental-late-parse-attributes -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c11 -fexperimental-late-parse-attributes -fsyntax-only -verify %s
 
 #define __counted_by_or_null(f)  __attribute__((counted_by_or_null(f)))
 #define __counted_by(f)  __attribute__((counted_by(f)))
diff --git a/clang/test/Sema/attr-counted-by-or-null-struct-ptrs.c b/clang/test/Sema/attr-counted-by-or-null-struct-ptrs.c
index 0bb09059c97f9..7fe43a20c0c6e 100644
--- a/clang/test/Sema/attr-counted-by-or-null-struct-ptrs.c
+++ b/clang/test/Sema/attr-counted-by-or-null-struct-ptrs.c
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
-// RUN: %clang_cc1 -fexperimental-late-parse-attributes -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c11 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c11 -fexperimental-late-parse-attributes -fsyntax-only -verify %s
 
 #define __counted_by_or_null(f)  __attribute__((counted_by_or_null(f)))
 #define __counted_by(f)  __attribute__((counted_by(f)))
diff --git a/clang/test/Sema/attr-counted-by-struct-ptrs.c b/clang/test/Sema/attr-counted-by-struct-ptrs.c
index c05d18262e2b7..d75533d7d135b 100644
--- a/clang/test/Sema/attr-counted-by-struct-ptrs.c
+++ b/clang/test/Sema/attr-counted-by-struct-ptrs.c
@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
-// RUN: %clang_cc1 -fsyntax-only -fexperimental-late-parse-attributes %s -verify
+// RUN: %clang_cc1 -std=c11 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c11 -fsyntax-only -fexperimental-late-parse-attributes %s -verify
 
 #define __counted_by(f)  __attribute__((counted_by(f)))
 
diff --git a/clang/test/Sema/attr-counted-by-void-ptr-gnu.c b/clang/test/Sema/attr-counted-by-void-ptr-gnu.c
new file mode 100644
index 0000000000000..c6aa371434bc4
--- /dev/null
+++ b/clang/test/Sema/attr-counted-by-void-ptr-gnu.c
@@ -0,0 +1,65 @@
+// RUN: %clang_cc1 -std=gnu11 -fsyntax-only -verify=expected-nowarn %s
+// RUN: %clang_cc1 -std=gnu11 -Wpointer-arith -fsyntax-only -verify=expected-warn %s
+// RUN: %clang_cc1 -std=c11 -fsyntax-only -verify=expected-strict %s
+
+// expected-nowarn-no-diagnostics
+
+#define __counted_by(f)  __attribute__((counted_by(f)))
+#define __counted_by_or_null(f)  __attribute__((counted_by_or_null(f)))
+#define __sized_by(f)  __attribute__((sized_by(f)))
+
+//==============================================================================
+// Test: counted_by on void* is allowed in GNU mode, rejected in strict mode
+//==============================================================================
+
+struct test_void_ptr_gnu {
+  int count;
+  // expected-warn-warning@+2{{'counted_by' on a pointer to void is a GNU extension, treated as 'sized_by'}}
+  // expected-strict-error@+1{{'counted_by' cannot be applied to a pointer with pointee of unknown size because 'void' is an incomplete type}}
+  void* buf __counted_by(count);
+};
+
+struct test_const_void_ptr_gnu {
+  int count;
+  // expected-warn-warning@+2{{'counted_by' on a pointer to void is a GNU extension, treated as 'sized_by'}}
+  // expected-strict-error@+1{{'counted_by' cannot be applied to a pointer with pointee of unknown size because 'const void' is an incomplete type}}
+  const void* buf __counted_by(count);
+};
+
+struct test_volatile_void_ptr_gnu {
+  int count;
+  // expected-warn-warning@+2{{'counted_by' on a pointer to void is a GNU extension, treated as 'sized_by'}}
+  // expected-strict-error@+1{{'counted_by' cannot be applied to a pointer with pointee of unknown size because 'volatile void' is an incomplete type}}
+  volatile void* buf __counted_by(count);
+};
+
+struct test_const_volatile_void_ptr_gnu {
+  int count;
+  // expected-warn-warning@+2{{'counted_by' on a pointer to void is a GNU extension, treated as 'sized_by'}}
+  // expected-strict-error@+1{{'counted_by' cannot be applied to a pointer with pointee of unknown size because 'const volatile void' is an incomplete type}}
+  const volatile void* buf __counted_by(count);
+};
+
+// Verify sized_by still works the same way (always allowed, no warning)
+struct test_sized_by_void_ptr {
+  int size;
+  void* buf __sized_by(size);  // OK in both modes, no warning
+};
+
+//==============================================================================
+// Test: counted_by_or_null on void* behaves the same
+//==============================================================================
+
+struct test_void_ptr_or_null_gnu {
+  int count;
+  // expected-warn-warning@+2{{'counted_by_or_null' on a pointer to void is a GNU extension, treated as 'sized_by_or_null'}}
+  // expected-strict-error@+1{{'counted_by_or_null' cannot be applied to a pointer with pointee of unknown size because 'void' is an incomplete type}}
+  void* buf __counted_by_or_null(count);
+};
+
+struct test_const_void_ptr_or_null_gnu {
+  int count;
+  // expected-warn-warning@+2{{'counted_by_or_null' on a pointer to void is a GNU extension, treated as 'sized_by_or_null'}}
+  // expected-strict-error@+1{{'counted_by_or_null' cannot be applied to a pointer with pointee of unknown size because 'const void' is an incomplete type}}
+  const void* buf __counted_by_or_null(count);
+};