[PATCH] D110116: [Clang] Ignore BTFTag attr if used as a type attribute

[Yonghong Song via Phabricator via cfe-commits]

yonghong-song created this revision.
yonghong-song added a reviewer: aaron.ballman.
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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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D110116

Files:
  clang/lib/Sema/SemaType.cpp
  clang/test/Sema/attr-btf_tag.c


Index: clang/test/Sema/attr-btf_tag.c
===================================================================
--- clang/test/Sema/attr-btf_tag.c
+++ clang/test/Sema/attr-btf_tag.c
@@ -40,3 +40,7 @@
 int __tag1 foo(struct t1 *arg __tag1, struct t2 *arg2) {
   return arg->a + arg2->a;
 }
+
+void __tag1 * convert(long arg) {
+  return (void __tag1 *)arg;
+}
Index: clang/lib/Sema/SemaType.cpp
===================================================================
--- clang/lib/Sema/SemaType.cpp
+++ clang/lib/Sema/SemaType.cpp
@@ -8129,6 +8129,12 @@
     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.


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