[RFC PATCH bpf-next 1/3] compiler_types: define __user as __attribute__((btf_type_tag("user")))

[Yonghong Song <yhs@fb.com>]

If pahole and compiler supports btf_type_tag attributes,
during kernel build, we can define __user as
__attribute__((btf_type_tag("user"))). This will encode __user
information in BTF. Such information, encoded in BTF
as BTF_KIND_TYPE_TAG, can help bpf verifier to
ensure proper memory dereference mechanism depending
on user memory or kernel memory.

The encoded __user info is also useful for other tracing
facility where instead of to require user to specify
kernel/user address type, the kernel can detect it
by itself with btf.

The following is an example with latest upstream clang
(clang14, [1]) and latest pahole:
  [$ ~] cat test.c
  #define __tag1 __attribute__((btf_type_tag("tag1")))
  int foo(int __tag1 *arg) {
          return *arg;
  }
  [$ ~] clang -O2 -g -c test.c
  [$ ~] pahole -JV test.o
  ...
  [1] INT int size=4 nr_bits=32 encoding=SIGNED
  [2] TYPE_TAG tag1 type_id=1
  [3] PTR (anon) type_id=2
  [4] FUNC_PROTO (anon) return=1 args=(3 arg)
  [5] FUNC foo type_id=4
  [$ ~]

You can see for the function argument "int __tag1 *arg",
its type is described as
  PTR -> TYPE_TAG(tag1) -> INT

The kernel can take advantage of this information
to bpf verification or other use cases.

Current btf_type_tag is only supported in clang (>= clang14).
gcc support is also proposed and under development ([2]).

  [1] https://reviews.llvm.org/D111199
  [2] https://www.spinics.net/lists/bpf/msg45773.html

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 include/linux/compiler_types.h | 2 ++
 lib/Kconfig.debug              | 5 +++++
 2 files changed, 7 insertions(+)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 1d32f4c03c9e..65725e183a38 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -31,6 +31,8 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 # define __kernel
 # ifdef STRUCTLEAK_PLUGIN
 #  define __user	__attribute__((user))
+# elif defined(CONFIG_PAHOLE_HAS_BTF_TAG) && __has_attribute(btf_type_tag)
+#  define __user	__attribute__((btf_type_tag("user")))
 # else
 #  define __user
 # endif
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 9ef7ce18b4f5..bf21b501c66d 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -324,6 +324,11 @@ config DEBUG_INFO_BTF
 config PAHOLE_HAS_SPLIT_BTF
 	def_bool $(success, test `$(PAHOLE) --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/'` -ge "119")
 
+config PAHOLE_HAS_BTF_TAG
+	depends on DEBUG_INFO_BTF
+	depends on CC_IS_CLANG
+	def_bool $(success, test `$(PAHOLE) --version | sed -E 's/v([0-9]+)\.([0-9]+)/\1\2/'` -ge "122")
+
 config DEBUG_INFO_BTF_MODULES
 	def_bool y
 	depends on DEBUG_INFO_BTF && MODULES && PAHOLE_HAS_SPLIT_BTF
-- 
2.30.2