linux-kernel - [PATCH v3 02/14] include/linux/compiler.h: prefer __section from compiler

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Message-Id: <20190828225535.49592-3-ndesaulniers@google.com>
Date:   Wed, 28 Aug 2019 15:55:23 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     miguel.ojeda.sandonis@...il.com
Cc:     sedat.dilek@...il.com, will@...nel.org, jpoimboe@...hat.com,
        naveen.n.rao@...ux.vnet.ibm.com, davem@...emloft.net,
        paul.burton@...s.com, clang-built-linux@...glegroups.com,
        linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: [PATCH v3 02/14] include/linux/compiler.h: prefer __section from compiler_attributes.h

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.

This fixes an Oops observed in distro's that use systemd and not
net.core.bpf_jit_enable=1, when their kernels are compiled with Clang.

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
Acked-by: Will Deacon <will@...nel.org>
Reported-by: Sedat Dilek <sedat.dilek@...il.com>
Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
Tested-by: Sedat Dilek <sedat.dilek@...il.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
 include/linux/compiler.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index f0fd5636fddb..5e88e7e33abe 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -24,7 +24,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 			long ______r;					\
 			static struct ftrace_likely_data		\
 				__aligned(4)				\
-				__section("_ftrace_annotated_branch")	\
+				__section(_ftrace_annotated_branch)	\
 				______f = {				\
 				.data.func = __func__,			\
 				.data.file = __FILE__,			\
@@ -60,7 +60,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 #define __trace_if_value(cond) ({			\
 	static struct ftrace_branch_data		\
 		__aligned(4)				\
-		__section("_ftrace_branch")		\
+		__section(_ftrace_branch)		\
 		__if_trace = {				\
 			.func = __func__,		\
 			.file = __FILE__,		\
@@ -118,7 +118,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	".popsection\n\t"
 
 /* Annotate a C jump table to allow objtool to follow the code flow */
-#define __annotate_jump_table __section(".rodata..c_jump_table")
+#define __annotate_jump_table __section(.rodata..c_jump_table)
 
 #else
 #define annotate_reachable()
@@ -298,7 +298,7 @@ unsigned long read_word_at_a_time(const void *addr)
  * visible to the compiler.
  */
 #define __ADDRESSABLE(sym) \
-	static void * __section(".discard.addressable") __used \
+	static void * __section(.discard.addressable) __used \
 		__PASTE(__addressable_##sym, __LINE__) = (void *)&sym;
 
 /**
-- 
2.23.0.187.g17f5b7556c-goog