[PATCH v3 14/14] compiler_attributes.h: add note about __section

[Nick Desaulniers]

GCC unescapes escaped string section names while Clang does not. Because
__section uses the `#` stringification operator for the section name, it
doesn't need to be escaped.

Instead, we should:
1. Prefer __section(.section_name_no_quotes).
2. Only use __attribute__((__section__(".section"))) when creating the
section name via C preprocessor (see the definition of __define_initcall
in arch/um/include/shared/init.h).

This antipattern was found with:
$ grep -e __section\(\" -e __section__\(\" -r

See the discussions in:
Link: https://bugs.llvm.org/show_bug.cgi?id=42950
Link: https://marc.info/?l=linux-netdev&m=156412960619946&w=2
Link: https://github.com/ClangBuiltLinux/linux/issues/619
Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
 include/linux/compiler_attributes.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 6b318efd8a74..f8c008d7f616 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -225,6 +225,16 @@
 #define __pure                          __attribute__((__pure__))
 
 /*
+ *  Note: Since this macro makes use of the "stringification operator" `#`, a
+ *        quoted string literal should not be passed to it. eg.
+ *        prefer:
+ *        __section(.foo)
+ *        to:
+ *        __section(".foo")
+ *        unless the section name is dynamically built up, in which case the
+ *        verbose __attribute__((__section__(".foo" x))) should be preferred.
+ *        See also: https://bugs.llvm.org/show_bug.cgi?id=42950
+ *
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-section-function-attribute
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-section-variable-attribute
  * clang: https://clang.llvm.org/docs/AttributeReference.html#section-declspec-allocate
-- 
2.23.0.187.g17f5b7556c-goog