[clang] b875343 - [Clang] Ignore BTFTag attr if used as a type attribute

[Yonghong Song via cfe-commits]

Author: Yonghong Song
Date: 2021-09-22T13:48:29-07:00
New Revision: b875343873a584965daf507d73ff1fe71eab1953

URL: https://github.com/llvm/llvm-project/commit/b875343873a584965daf507d73ff1fe71eab1953
DIFF: https://github.com/llvm/llvm-project/commit/b875343873a584965daf507d73ff1fe71eab1953.diff

LOG: [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

Added: 
    

Modified: 
    clang/include/clang/Basic/Attr.td
    clang/include/clang/Basic/AttrDocs.td
    clang/lib/AST/TypePrinter.cpp
    clang/lib/Sema/SemaType.cpp
    clang/test/Sema/attr-btf_tag.c

Removed: 
    


################################################################################
diff  --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index a54039501a7f7..00e847bc5d2fb 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 b2b41c3bb4197..d3b0de06e7172 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 3c7a6b8b9e953..2805916b1f7bb 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 d2d54281c63e0..11a6d3faf2dd6 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 88452fa875b1a..2ba2515868344 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;
+}