[PATCH v4] compiler_types: Allow opting out of __counted_by and __counted_by_ptr

[Bill Wendling <morbo@google.com>]

Code that runs outside the kernel proper, such as the EFI stub, gets
nothing out of the counted_by annotations: the bounds checks they feed
(FORTIFY_SOURCE, UBSAN_BOUNDS) are already disabled there.

The annotations can also break the build. A __counted_by_ptr() that
names a member declared after the pointer needs Clang's
'-fexperimental-late-parse-attributes', which the top-level Makefile
adds to 'KBUILD_CFLAGS'. The x86 EFI stub builds its own cflags and does
not get that flag, so it fails as soon as such a struct is pulled in
through a common header.

Overriding the macros from a Makefile doesn't work:
'compiler_types.h' is pulled in with '-include', which is processed
after all -D/-U options, so it re-establishes the definitions. Follow
the '__NO_FORTIFY' precedent instead: let a build define
'__NO_COUNTED_BY' to turn the corresponding annotation into a no-op.

Assisted-by: LLM
Signed-off-by: Bill Wendling <morbo@google.com>
---
v2: Only specify __NO_COUNTED_BY_PTR. Leave __NO_COUNTED_BY until we need it.
v3: Add the version to the Subject line.
v4: Use __NO_COUNTED_BY to reduce churn later on.
---
 include/linux/compiler_types.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index c5921f139007..05e2b637f1cd 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -371,8 +371,13 @@ struct ftrace_likely_data {
  *
  * __bdos on clang < 19.1.3 can be off by 4:
  * https://github.com/llvm/llvm-project/pull/112636
+ *
+ * Code that runs outside the kernel proper (e.g. the EFI stub) can define
+ * __NO_COUNTED_BY to drop the annotation, since it gains nothing from the
+ * bounds checks. __NO_COUNTED_BY should be used sparingly, because there are
+ * better options for opting out of bounds checking.
  */
-#ifdef CONFIG_CC_HAS_COUNTED_BY
+#if defined(CONFIG_CC_HAS_COUNTED_BY) && !defined(__NO_COUNTED_BY)
 # define __counted_by(member)		__attribute__((__counted_by__(member)))
 #else
 # define __counted_by(member)
-- 
2.56.0.rc1.315.gc6ed9934b7-goog