From b875343873a584965daf507d73ff1fe71eab1953 Mon Sep 17 00:00:00 2001
From: Yonghong Song <yhs@fb.com>
Date: Mon, 20 Sep 2021 16:23:46 -0700
Subject: [PATCH] [Clang] Ignore BTFTag attr if used as a type attribute

Currently, linux kernel has a __user attribute ([1]) defined as
   __attribute__((noderef, address_space(__user)))
which is used by sparse tool ([2]) to do some
type checking of pointers to user space memory.
During normal compilation, __user will be defined
to nothing so it won't have an impact on compilation.

The btf_tag attribute, which is motivated by
carrying linux kernel annotations into dwarf/BTF,
is introduced in [3]. We intended to define __user as
   __attribute__((btf_tag("user")))
so such information will be encoded in dwarf/BTF
and can be used later by bpf verification or other
tracing tools.

But linux kernel __user attribute is also used during
type conversion which btf_tag doesn't support ([4]) since
such type conversion is only used for compiler analysis
and not encoded in dwarf/btf. Theoretically, it is
possible for clang to understand these tags and
do a sparse-like type checking work. But I would like
to leave that to future work and for now suggest simply
ignore these btf_tag attributes if they are used
as type attributes.

  [1] https://github.com/torvalds/linux/blob/master/include/linux/compiler_types.h#L10
  [2] https://sparse.docs.kernel.org/en/latest/
  [3] https://reviews.llvm.org/D106614
  [4] https://github.com/torvalds/linux/blob/master/fs/binfmt_flat.c#L135

Differential Revision: https://reviews.llvm.org/D110116
---
 clang/include/clang/Basic/Attr.td     | 2 +-
 clang/include/clang/Basic/AttrDocs.td | 4 ++++
 clang/lib/AST/TypePrinter.cpp         | 1 +
 clang/lib/Sema/SemaType.cpp           | 6 ++++++
 clang/test/Sema/attr-btf_tag.c        | 8 ++++++++
 5 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index a540395..00e847b 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -1835,7 +1835,7 @@ def BPFPreserveAccessIndex : InheritableAttr,
   let LangOpts = [COnly];
 }
 
-def BTFTag : InheritableAttr {
+def BTFTag : DeclOrTypeAttr {
   let Spellings = [Clang<"btf_tag">];
   let Args = [StringArgument<"BTFTag">];
   let Subjects = SubjectList<[Var, Function, Record, Field], ErrorDiag>;
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index b2b41c3..d3b0de0 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -2019,6 +2019,10 @@ targets. This attribute may be attached to a struct/union, struct/union field,
 function, function parameter or variable declaration. If -g is specified,
 the ``ARGUMENT`` info will be preserved in IR and be emitted to dwarf.
 For BPF targets, the ``ARGUMENT`` info will be emitted to .BTF ELF section too.
+
+The attribute can also be used as a type qualifier. Right now it is accepted
+and silently ignored in order to permit the linux kernel to build with the
+attribute.
   }];
 }
 
diff --git a/clang/lib/AST/TypePrinter.cpp b/clang/lib/AST/TypePrinter.cpp
index 3c7a6b8..2805916 100644
--- a/clang/lib/AST/TypePrinter.cpp
+++ b/clang/lib/AST/TypePrinter.cpp
@@ -1704,6 +1704,7 @@ void TypePrinter::printAttributedAfter(const AttributedType *T,
   case attr::UPtr:
   case attr::AddressSpace:
   case attr::CmseNSCall:
+  case attr::BTFTag:
     llvm_unreachable("This attribute should have been handled already");
 
   case attr::NSReturnsRetained:
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index d2d5428..11a6d3f 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -8127,6 +8127,12 @@ static void processTypeAttrs(TypeProcessingState &state, QualType &type,
     case ParsedAttr::IgnoredAttribute:
       break;
 
+    case ParsedAttr::AT_BTFTag:
+      // FIXME: Linux kernel may also use this attribute for type casting check,
+      // which clang doesn's support for now. Let us ignore them so linux kernel
+      // build won't break.
+      attr.setUsedAsTypeAttr();
+      break;
     case ParsedAttr::AT_MayAlias:
       // FIXME: This attribute needs to actually be handled, but if we ignore
       // it it breaks large amounts of Linux software.
diff --git a/clang/test/Sema/attr-btf_tag.c b/clang/test/Sema/attr-btf_tag.c
index 88452fa..2ba2515 100644
--- a/clang/test/Sema/attr-btf_tag.c
+++ b/clang/test/Sema/attr-btf_tag.c
@@ -40,3 +40,11 @@ int __tag2 __tag3 foo(struct t1 *arg, struct t2 *arg2);
 int __tag1 foo(struct t1 *arg __tag1, struct t2 *arg2) {
   return arg->a + arg2->a;
 }
+
+void * convert(long arg) {
+  /* FIXME: the attribute __tag1 is accepted but didn't really do type conversion
+   * or enforce type checking. This is to permit linux kernel build with btf_tag
+   * attribute.
+   */
+  return (void __tag1 *)arg;
+}
-- 
2.7.4