debug: Fix WARN_ON_ONCE() for modules

Mike Galbraith reported a situation where a WARN_ON_ONCE() call in DRM
code turned into an oops.  As it turns out, WARN_ON_ONCE() seems to be
completely broken when called from a module.

The bug was introduced with the following commit:

  19d436268d ("debug: Add _ONCE() logic to report_bug()")

That commit changed WARN_ON_ONCE() to move its 'once' logic into the bug
trap handler.  It requires a writable bug table so that the BUGFLAG_DONE
bit can be written to the flags to indicate the first warning has
occurred.

The bug table was made writable for vmlinux, which relies on
vmlinux.lds.S and vmlinux.lds.h for laying out the sections.  However,
it wasn't made writable for modules, which rely on the ELF section
header flags.

Reported-by: Mike Galbraith <efault@gmx.de>
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 19d436268d ("debug: Add _ONCE() logic to report_bug()")
Link: http://lkml.kernel.org/r/a53b04235a65478dd9afc51f5b329fdc65c84364.1500095401.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/arm/include/asm/bug.h

@@ -37,7 +37,7 @@ do {								\
 		".pushsection .rodata.str, \"aMS\", %progbits, 1\n" \
 		"2:\t.asciz " #__file "\n" 			\
 		".popsection\n" 				\
-		".pushsection __bug_table,\"a\"\n"		\
+		".pushsection __bug_table,\"aw\"\n"		\
 		".align 2\n"					\
 		"3:\t.word 1b, 2b\n"				\
 		"\t.hword " #__line ", 0\n"			\

arch/arm64/include/asm/bug.h

@@ -36,7 +36,7 @@
 #ifdef CONFIG_GENERIC_BUG
 
 #define __BUG_ENTRY(flags) 				\
-		".pushsection __bug_table,\"a\"\n\t"	\
+		".pushsection __bug_table,\"aw\"\n\t"	\
 		".align 2\n\t"				\
 	"0:	.long 1f - 0b\n\t"			\
 _BUGVERBOSE_LOCATION(__FILE__, __LINE__)		\

arch/blackfin/include/asm/bug.h

@@ -21,7 +21,7 @@
 #define _BUG_OR_WARN(flags)						\
 	asm volatile(							\
 		"1:	.hword	%0\n"					\
-		"	.section __bug_table,\"a\",@progbits\n"		\
+		"	.section __bug_table,\"aw\",@progbits\n"	\
 		"2:	.long	1b\n"					\
 		"	.long	%1\n"					\
 		"	.short	%2\n"					\
@@ -38,7 +38,7 @@
 #define _BUG_OR_WARN(flags)						\
 	asm volatile(							\
 		"1:	.hword	%0\n"					\
-		"	.section __bug_table,\"a\",@progbits\n"		\
+		"	.section __bug_table,\"aw\",@progbits\n"	\
 		"2:	.long	1b\n"					\
 		"	.short	%1\n"					\
 		"	.org	2b + %2\n"				\

arch/mn10300/include/asm/bug.h

@@ -21,7 +21,7 @@ do {								\
 	asm volatile(						\
 		"	syscall 15			\n"	\
 		"0:					\n"	\
-		"	.section __bug_table,\"a\"	\n"	\
+		"	.section __bug_table,\"aw\"	\n"	\
 		"	.long 0b,%0,%1			\n"	\
 		"	.previous			\n"	\
 		:						\

arch/parisc/include/asm/bug.h

@@ -27,7 +27,7 @@
 	do {								\
 		asm volatile("\n"					\
 			     "1:\t" PARISC_BUG_BREAK_ASM "\n"		\
-			     "\t.pushsection __bug_table,\"a\"\n"	\
+			     "\t.pushsection __bug_table,\"aw\"\n"	\
 			     "2:\t" ASM_WORD_INSN "1b, %c0\n"		\
 			     "\t.short %c1, %c2\n"			\
 			     "\t.org 2b+%c3\n"				\
@@ -50,7 +50,7 @@
 	do {								\
 		asm volatile("\n"					\
 			     "1:\t" PARISC_BUG_BREAK_ASM "\n"		\
-			     "\t.pushsection __bug_table,\"a\"\n"	\
+			     "\t.pushsection __bug_table,\"aw\"\n"	\
 			     "2:\t" ASM_WORD_INSN "1b, %c0\n"		\
 			     "\t.short %c1, %c2\n"			\
 			     "\t.org 2b+%c3\n"				\
@@ -64,7 +64,7 @@
 	do {								\
 		asm volatile("\n"					\
 			     "1:\t" PARISC_BUG_BREAK_ASM "\n"		\
-			     "\t.pushsection __bug_table,\"a\"\n"	\
+			     "\t.pushsection __bug_table,\"aw\"\n"	\
 			     "2:\t" ASM_WORD_INSN "1b\n"		\
 			     "\t.short %c0\n"				\
 			     "\t.org 2b+%c1\n"				\

arch/powerpc/include/asm/bug.h

@@ -18,7 +18,7 @@
 #include <asm/asm-offsets.h>
 #ifdef CONFIG_DEBUG_BUGVERBOSE
 .macro EMIT_BUG_ENTRY addr,file,line,flags
-	 .section __bug_table,"a"
+	 .section __bug_table,"aw"
 5001:	 PPC_LONG \addr, 5002f
 	 .short \line, \flags
 	 .org 5001b+BUG_ENTRY_SIZE
@@ -29,7 +29,7 @@
 .endm
 #else
 .macro EMIT_BUG_ENTRY addr,file,line,flags
-	 .section __bug_table,"a"
+	 .section __bug_table,"aw"
 5001:	 PPC_LONG \addr
 	 .short \flags
 	 .org 5001b+BUG_ENTRY_SIZE
@@ -42,14 +42,14 @@
    sizeof(struct bug_entry), respectively */
 #ifdef CONFIG_DEBUG_BUGVERBOSE
 #define _EMIT_BUG_ENTRY				\
-	".section __bug_table,\"a\"\n"		\
+	".section __bug_table,\"aw\"\n"		\
 	"2:\t" PPC_LONG "1b, %0\n"		\
 	"\t.short %1, %2\n"			\
 	".org 2b+%3\n"				\
 	".previous\n"
 #else
 #define _EMIT_BUG_ENTRY				\
-	".section __bug_table,\"a\"\n"		\
+	".section __bug_table,\"aw\"\n"		\
 	"2:\t" PPC_LONG "1b\n"			\
 	"\t.short %2\n"				\
 	".org 2b+%3\n"				\

arch/s390/include/asm/bug.h

@@ -14,7 +14,7 @@
 		".section .rodata.str,\"aMS\",@progbits,1\n"	\
 		"2:	.asciz	\""__FILE__"\"\n"		\
 		".previous\n"					\
-		".section __bug_table,\"a\"\n"			\
+		".section __bug_table,\"aw\"\n"			\
 		"3:	.long	1b-3b,2b-3b\n"			\
 		"	.short	%0,%1\n"			\
 		"	.org	3b+%2\n"			\
@@ -30,7 +30,7 @@
 	asm volatile(					\
 		"0:	j	0b+2\n"			\
 		"1:\n"					\
-		".section __bug_table,\"a\"\n"		\
+		".section __bug_table,\"aw\"\n"		\
 		"2:	.long	1b-2b\n"		\
 		"	.short	%0\n"			\
 		"	.org	2b+%1\n"		\

arch/sh/include/asm/bug.h

@@ -24,14 +24,14 @@
  */
 #ifdef CONFIG_DEBUG_BUGVERBOSE
 #define _EMIT_BUG_ENTRY				\
-	"\t.pushsection __bug_table,\"a\"\n"	\
+	"\t.pushsection __bug_table,\"aw\"\n"	\
 	"2:\t.long 1b, %O1\n"			\
 	"\t.short %O2, %O3\n"			\
 	"\t.org 2b+%O4\n"			\
 	"\t.popsection\n"
 #else
 #define _EMIT_BUG_ENTRY				\
-	"\t.pushsection __bug_table,\"a\"\n"	\
+	"\t.pushsection __bug_table,\"aw\"\n"	\
 	"2:\t.long 1b\n"			\
 	"\t.short %O3\n"			\
 	"\t.org 2b+%O4\n"			\

arch/x86/include/asm/bug.h

@@ -35,7 +35,7 @@
 #define _BUG_FLAGS(ins, flags)						\
 do {									\
 	asm volatile("1:\t" ins "\n"					\
-		     ".pushsection __bug_table,\"a\"\n"			\
+		     ".pushsection __bug_table,\"aw\"\n"		\
 		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
 		     "\t"  __BUG_REL(%c0) "\t# bug_entry::file\n"	\
 		     "\t.word %c1"        "\t# bug_entry::line\n"	\
@@ -52,7 +52,7 @@ do {									\
 #define _BUG_FLAGS(ins, flags)						\
 do {									\
 	asm volatile("1:\t" ins "\n"					\
-		     ".pushsection __bug_table,\"a\"\n"			\
+		     ".pushsection __bug_table,\"aw\"\n"		\
 		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
 		     "\t.word %c0"        "\t# bug_entry::flags\n"	\
 		     "\t.org 2b+%c1\n"					\