linux-kernel - [PATCH v2] objtool, module: discard objtool annotation sections for modules

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Message-ID: <20170301180444.lhd53c5tibc4ns77@treble>
Date:   Wed, 1 Mar 2017 12:04:44 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jessica Yu <jeyu@...hat.com>
Subject: [PATCH v2] objtool, module: discard objtool annotation sections for
 modules


The '__unreachable' and '__func_stack_frame_non_standard' sections are
only used at compile time.  They're discarded for vmlinux but they
should also be discarded for modules.

Since this is a recurring pattern, prefix the section names with
".discard.".  It's a nice convention and vmlinux.lds.h already discards
such sections.

Also remove the 'a' (allocatable) flag from the __unreachable section
since it doesn't make sense for a discarded section.

Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Fixes: d1091c7fa3d5 ("objtool: Improve detection of BUG() and other dead ends")
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 arch/x86/kernel/vmlinux.lds.S | 2 --
 include/linux/compiler-gcc.h  | 2 +-
 include/linux/frame.h         | 2 +-
 scripts/mod/modpost.c         | 1 +
 scripts/module-common.lds     | 5 ++++-
 tools/objtool/builtin-check.c | 7 ++++---
 6 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index ad0118f..c74ae9c 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -345,8 +345,6 @@ SECTIONS
 	DISCARDS
 	/DISCARD/ : {
 		*(.eh_frame)
-		*(__func_stack_frame_non_standard)
-		*(__unreachable)
 	}
 }
 
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index b6bb901..0efef9c 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -200,7 +200,7 @@
 #ifdef CONFIG_STACK_VALIDATION
 #define annotate_unreachable() ({					\
 	asm("%c0:\t\n"							\
-	    ".pushsection __unreachable, \"a\"\t\n"			\
+	    ".pushsection .discard.unreachable\t\n"			\
 	    ".long %c0b - .\t\n"					\
 	    ".popsection\t\n" : : "i" (__LINE__));			\
 })
diff --git a/include/linux/frame.h b/include/linux/frame.h
index e6baaba..d772c61 100644
--- a/include/linux/frame.h
+++ b/include/linux/frame.h
@@ -11,7 +11,7 @@
  * For more information, see tools/objtool/Documentation/stack-validation.txt.
  */
 #define STACK_FRAME_NON_STANDARD(func) \
-	static void __used __section(__func_stack_frame_non_standard) \
+	static void __used __section(.discard.func_stack_frame_non_standard) \
 		*__func_stack_frame_non_standard_##func = func
 
 #else /* !CONFIG_STACK_VALIDATION */
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 4dedd0d..30d752a 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -854,6 +854,7 @@ static const char *const section_white_list[] =
 	".cmem*",			/* EZchip */
 	".fmt_slot*",			/* EZchip */
 	".gnu.lto*",
+	".discard.*",
 	NULL
 };
 
diff --git a/scripts/module-common.lds b/scripts/module-common.lds
index 73a2c7d..cf7e52e 100644
--- a/scripts/module-common.lds
+++ b/scripts/module-common.lds
@@ -4,7 +4,10 @@
  * combine them automatically.
  */
 SECTIONS {
-	/DISCARD/ : { *(.discard) }
+	/DISCARD/ : {
+		*(.discard)
+		*(.discard.*)
+	}
 
 	__ksymtab		0 : { *(SORT(___ksymtab+*)) }
 	__ksymtab_gpl		0 : { *(SORT(___ksymtab_gpl+*)) }
diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
index 5fc52ee..bd12eb1 100644
--- a/tools/objtool/builtin-check.c
+++ b/tools/objtool/builtin-check.c
@@ -339,13 +339,14 @@ static int add_dead_ends(struct objtool_file *file)
 	struct instruction *insn;
 	bool found;
 
-	sec = find_section_by_name(file->elf, ".rela__unreachable");
+	sec = find_section_by_name(file->elf, ".rela.discard.unreachable");
 	if (!sec)
 		return 0;
 
 	list_for_each_entry(rela, &sec->rela_list, list) {
 		if (rela->sym->type != STT_SECTION) {
-			WARN("unexpected relocation symbol type in .rela__unreachable");
+			WARN("unexpected relocation symbol type in %s",
+			     sec->name);
 			return -1;
 		}
 		insn = find_insn(file, rela->sym->sec, rela->addend);
@@ -1272,7 +1273,7 @@ int cmd_check(int argc, const char **argv)
 
 	INIT_LIST_HEAD(&file.insn_list);
 	hash_init(file.insn_hash);
-	file.whitelist = find_section_by_name(file.elf, "__func_stack_frame_non_standard");
+	file.whitelist = find_section_by_name(file.elf, ".discard.func_stack_frame_non_standard");
 	file.rodata = find_section_by_name(file.elf, ".rodata");
 	file.ignore_unreachables = false;
 	file.c_file = find_section_by_name(file.elf, ".comment");
-- 
2.7.4