[clang] [Clang][Sema]: Allow flexible arrays in unions and alone in structs (PR #84428)
Kees Cook via cfe-commits
cfe-commits at lists.llvm.org
Fri Mar 8 14:17:06 PST 2024
https://github.com/kees updated https://github.com/llvm/llvm-project/pull/84428
>From eb5138b45fa450737600050ad8dabdcb27513d42 Mon Sep 17 00:00:00 2001
From: Kees Cook <keescook at chromium.org>
Date: Thu, 7 Mar 2024 17:03:09 -0800
Subject: [PATCH] [Clang][Sema]: Allow flexible arrays in unions and alone in
structs
GNU and MSVC have extensions where flexible array members (or their
equivalent) can be in unions or alone in structs. This is already fully
supported in Clang through the 0-sized array ("fake flexible array")
extension or when C99 flexible array members have been syntactically
obfuscated.
Clang needs to explicitly allow these extensions directly for C99
flexible arrays, since they are common code patterns in active use by the
Linux kernel (and other projects). Such projects have been using either
0-sized arrays (which is considered deprecated in favor of C99 flexible
array members) or via obfuscated syntax, both of which complicate their
code bases.
For example, these do not error by default:
union one {
int a;
int b[0];
};
union two {
int a;
struct {
struct { } __empty;
int b[];
};
};
But this does:
union three {
int a;
int b[];
};
Remove the default error diagnostics for this but continue to provide
warnings under Microsoft or GNU extensions checks. This will allow for
a seamless transition for code bases away from 0-sized arrays without
losing existing code patterns. Add explicit checking for the warnings
under various constructions.
Fixes #84565
---
clang/docs/ReleaseNotes.rst | 3 ++
.../clang/Basic/DiagnosticSemaKinds.td | 5 --
clang/lib/Sema/SemaDecl.cpp | 8 +--
clang/test/C/drs/dr5xx.c | 2 +-
clang/test/Sema/flexible-array-in-union.c | 53 +++++++++++++++++--
clang/test/Sema/transparent-union.c | 4 +-
6 files changed, 57 insertions(+), 18 deletions(-)
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 1b901a27fd19d1..960ab7e021cf2f 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -214,6 +214,9 @@ Improvements to Clang's diagnostics
- Clang now diagnoses lambda function expressions being implicitly cast to boolean values, under ``-Wpointer-bool-conversion``.
Fixes #GH82512.
+- ``-Wmicrosoft`` or ``-Wgnu`` is now required to diagnose C99 flexible
+ array members in a union or alone in a struct.
+
Improvements to Clang's time-trace
----------------------------------
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index c8dfdc08f5ea07..f09121b8c7ec8f 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6447,9 +6447,6 @@ def ext_c99_flexible_array_member : Extension<
def err_flexible_array_virtual_base : Error<
"flexible array member %0 not allowed in "
"%select{struct|interface|union|class|enum}1 which has a virtual base class">;
-def err_flexible_array_empty_aggregate : Error<
- "flexible array member %0 not allowed in otherwise empty "
- "%select{struct|interface|union|class|enum}1">;
def err_flexible_array_has_nontrivial_dtor : Error<
"flexible array member %0 of type %1 with non-trivial destruction">;
def ext_flexible_array_in_struct : Extension<
@@ -6464,8 +6461,6 @@ def ext_flexible_array_empty_aggregate_ms : Extension<
"flexible array member %0 in otherwise empty "
"%select{struct|interface|union|class|enum}1 is a Microsoft extension">,
InGroup<MicrosoftFlexibleArray>;
-def err_flexible_array_union : Error<
- "flexible array member %0 in a union is not allowed">;
def ext_flexible_array_union_ms : Extension<
"flexible array member %0 in a union is a Microsoft extension">,
InGroup<MicrosoftFlexibleArray>;
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 67e56a917a51de..053122b588246b 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -19357,15 +19357,11 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl,
} else if (Record->isUnion())
DiagID = getLangOpts().MicrosoftExt
? diag::ext_flexible_array_union_ms
- : getLangOpts().CPlusPlus
- ? diag::ext_flexible_array_union_gnu
- : diag::err_flexible_array_union;
+ : diag::ext_flexible_array_union_gnu;
else if (NumNamedMembers < 1)
DiagID = getLangOpts().MicrosoftExt
? diag::ext_flexible_array_empty_aggregate_ms
- : getLangOpts().CPlusPlus
- ? diag::ext_flexible_array_empty_aggregate_gnu
- : diag::err_flexible_array_empty_aggregate;
+ : diag::ext_flexible_array_empty_aggregate_gnu;
if (DiagID)
Diag(FD->getLocation(), DiagID)
diff --git a/clang/test/C/drs/dr5xx.c b/clang/test/C/drs/dr5xx.c
index 68bcef78baccd7..13464f78b6a654 100644
--- a/clang/test/C/drs/dr5xx.c
+++ b/clang/test/C/drs/dr5xx.c
@@ -29,7 +29,7 @@ void dr502(void) {
*/
struct t {
int i;
- struct { int a[]; }; /* expected-error {{flexible array member 'a' not allowed in otherwise empty struct}}
+ struct { int a[]; }; /* expected-warning {{flexible array member 'a' in otherwise empty struct is a GNU extension}}
c89only-warning {{flexible array members are a C99 feature}}
expected-warning {{'' may not be nested in a struct due to flexible array member}}
*/
diff --git a/clang/test/Sema/flexible-array-in-union.c b/clang/test/Sema/flexible-array-in-union.c
index 5fabfbe0b1eaab..a4b2c5ff27184d 100644
--- a/clang/test/Sema/flexible-array-in-union.c
+++ b/clang/test/Sema/flexible-array-in-union.c
@@ -1,13 +1,58 @@
-// RUN: %clang_cc1 %s -verify=c -fsyntax-only
+// RUN: %clang_cc1 %s -verify -fsyntax-only
// RUN: %clang_cc1 %s -verify -fsyntax-only -x c++
-// RUN: %clang_cc1 %s -verify -fsyntax-only -fms-compatibility
// RUN: %clang_cc1 %s -verify -fsyntax-only -fms-compatibility -x c++
+// RUN: %clang_cc1 %s -verify=gnu -fsyntax-only -Wgnu-flexible-array-union-member -Wgnu-empty-struct
+// RUN: %clang_cc1 %s -verify=microsoft -fsyntax-only -fms-compatibility -Wmicrosoft
// The test checks that an attempt to initialize union with flexible array
// member with an initializer list doesn't crash clang.
-union { char x[]; } r = {0}; // c-error {{flexible array member 'x' in a union is not allowed}}
+union { char x[]; } r = {0}; /* gnu-warning {{flexible array member 'x' in a union is a GNU extension}}
+ microsoft-warning {{flexible array member 'x' in a union is a Microsoft extension}}
+ */
-// expected-no-diagnostics
+struct already_hidden {
+ int a;
+ union {
+ int b;
+ struct {
+ struct { } __empty; // gnu-warning {{empty struct is a GNU extension}}
+ char x[];
+ };
+ };
+};
+
+struct still_zero_sized {
+ struct { } __unused; // gnu-warning {{empty struct is a GNU extension}}
+ int x[];
+};
+
+struct warn1 {
+ int a;
+ union {
+ int b;
+ char x[]; /* gnu-warning {{flexible array member 'x' in a union is a GNU extension}}
+ microsoft-warning {{flexible array member 'x' in a union is a Microsoft extension}}
+ */
+ };
+};
+
+struct warn2 {
+ int x[]; /* gnu-warning {{flexible array member 'x' in otherwise empty struct is a GNU extension}}
+ microsoft-warning {{flexible array member 'x' in otherwise empty struct is a Microsoft extension}}
+ */
+};
+union warn3 {
+ short x[]; /* gnu-warning {{flexible array member 'x' in a union is a GNU extension}}
+ microsoft-warning {{flexible array member 'x' in a union is a Microsoft extension}}
+ */
+};
+
+struct quiet1 {
+ int a;
+ short x[];
+};
+
+// expected-no-diagnostics
diff --git a/clang/test/Sema/transparent-union.c b/clang/test/Sema/transparent-union.c
index c134a7a9b1c4d0..f02c2298b51ce1 100644
--- a/clang/test/Sema/transparent-union.c
+++ b/clang/test/Sema/transparent-union.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wgnu-flexible-array-union-member %s
typedef union {
int *ip;
float *fp;
@@ -131,7 +131,7 @@ union pr15134v2 {
union pr30520v { void b; } __attribute__((transparent_union)); // expected-error {{field has incomplete type 'void'}}
-union pr30520a { int b[]; } __attribute__((transparent_union)); // expected-error {{flexible array member 'b' in a union is not allowed}}
+union pr30520a { int b[]; } __attribute__((transparent_union)); // expected-warning {{flexible array member 'b' in a union is a GNU extension}}
// expected-note at +1 2 {{forward declaration of 'struct stb'}}
union pr30520s { struct stb b; } __attribute__((transparent_union)); // expected-error {{field has incomplete type 'struct stb'}}
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