Revert "powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with asm goto"

This partly reverts commit 1e688dd.

That commit aimed at optimising the code around generation of
WARN_ON/BUG_ON but this leads to a lot of dead code erroneously
generated by GCC.

That dead code becomes a problem when we start using objtool validation
because objtool will abort validation with a warning as soon as it
detects unreachable code. This is because unreachable code might
be the indication that objtool doesn't properly decode object text.

     text	   data	    bss	    dec	    hex	filename
  9551585	3627834	 224376	13403795	 cc8693	vmlinux.before
  9535281	3628358	 224376	13388015	 cc48ef	vmlinux.after

Once this change is reverted, in a standard configuration (pmac32 +
function tracer) the text is reduced by 16k which is around 1.7%

We already had problem with it when starting to use objtool on powerpc
as a replacement for recordmcount, see commit 93e3f45 ("powerpc:
Fix __WARN_FLAGS() for use with Objtool")

There is also a problem with at least GCC 12, on ppc64_defconfig +
CONFIG_CC_OPTIMIZE_FOR_SIZE=y + CONFIG_DEBUG_SECTION_MISMATCH=y :

    LD      .tmp_vmlinux.kallsyms1
  powerpc64-linux-ld: net/ipv4/tcp_input.o:(__ex_table+0xc4): undefined reference to `.L2136'
  make[2]: *** [scripts/Makefile.vmlinux:36: vmlinux] Error 1
  make[1]: *** [/home/chleroy/linux-powerpc/Makefile:1238: vmlinux] Error 2

Taking into account that other problems are encountered with that
'asm goto' in WARN_ON(), including build failures, keeping that
change is not worth it allthough it is primarily a compiler bug.

Revert it for now.

mpe: Retain EMIT_WARN_ENTRY as a synonym for EMIT_BUG_ENTRY to reduce
churn, as there are now nearly as many uses of EMIT_WARN_ENTRY as
EMIT_BUG_ENTRY.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Naveen N Rao <naveen@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230712134552.534955-1-mpe@ellerman.id.au

Author: chleroy

arch/powerpc/include/asm/bug.h

@@ -4,14 +4,13 @@
 #ifdef __KERNEL__
 
 #include <asm/asm-compat.h>
-#include <asm/extable.h>
 
 #ifdef CONFIG_BUG
 
 #ifdef __ASSEMBLY__
 #include <asm/asm-offsets.h>
 #ifdef CONFIG_DEBUG_BUGVERBOSE
-.macro __EMIT_BUG_ENTRY addr,file,line,flags
+.macro EMIT_BUG_ENTRY addr,file,line,flags
 	 .section __bug_table,"aw"
 5001:	 .4byte \addr - .
 	 .4byte 5002f - .
@@ -23,7 +22,7 @@
 	 .previous
 .endm
 #else
-.macro __EMIT_BUG_ENTRY addr,file,line,flags
+.macro EMIT_BUG_ENTRY addr,file,line,flags
 	 .section __bug_table,"aw"
 5001:	 .4byte \addr - .
 	 .short \flags
@@ -32,18 +31,6 @@
 .endm
 #endif /* verbose */
 
-.macro EMIT_WARN_ENTRY addr,file,line,flags
-	EX_TABLE(\addr,\addr+4)
-	__EMIT_BUG_ENTRY \addr,\file,\line,\flags
-.endm
-
-.macro EMIT_BUG_ENTRY addr,file,line,flags
-	.if \flags & 1 /* BUGFLAG_WARNING */
-	.err /* Use EMIT_WARN_ENTRY for warnings */
-	.endif
-	__EMIT_BUG_ENTRY \addr,\file,\line,\flags
-.endm
-
 #else /* !__ASSEMBLY__ */
 /* _EMIT_BUG_ENTRY expects args %0,%1,%2,%3 to be FILE, LINE, flags and
    sizeof(struct bug_entry), respectively */
@@ -73,16 +60,6 @@
 		  "i" (sizeof(struct bug_entry)),	\
 		  ##__VA_ARGS__)
 
-#define WARN_ENTRY(insn, flags, label, ...)		\
-	asm_volatile_goto(				\
-		"1:	" insn "\n"			\
-		EX_TABLE(1b, %l[label])			\
-		_EMIT_BUG_ENTRY				\
-		: : "i" (__FILE__), "i" (__LINE__),	\
-		  "i" (flags),				\
-		  "i" (sizeof(struct bug_entry)),	\
-		  ##__VA_ARGS__ : : label)
-
 /*
  * BUG_ON() and WARN_ON() do their best to cooperate with compile-time
  * optimisations. However depending on the complexity of the condition
@@ -95,16 +72,7 @@
 } while (0)
 #define HAVE_ARCH_BUG
 
-#define __WARN_FLAGS(flags) do {				\
-	__label__ __label_warn_on;				\
-								\
-	WARN_ENTRY("twi 31, 0, 0", BUGFLAG_WARNING | (flags), __label_warn_on); \
-	barrier_before_unreachable();				\
-	__builtin_unreachable();				\
-								\
-__label_warn_on:						\
-	break;							\
-} while (0)
+#define __WARN_FLAGS(flags) BUG_ENTRY("twi 31, 0, 0", BUGFLAG_WARNING | (flags))
 
 #ifdef CONFIG_PPC64
 #define BUG_ON(x) do {						\
@@ -117,25 +85,15 @@ __label_warn_on:						\
 } while (0)
 
 #define WARN_ON(x) ({						\
-	bool __ret_warn_on = false;				\
-	do {							\
-		if (__builtin_constant_p((x))) {		\
-			if (!(x)) 				\
-				break; 				\
+	int __ret_warn_on = !!(x);				\
+	if (__builtin_constant_p(__ret_warn_on)) {		\
+		if (__ret_warn_on)				\
 			__WARN();				\
-			__ret_warn_on = true;			\
-		} else {					\
-			__label__ __label_warn_on;		\
-								\
-			WARN_ENTRY(PPC_TLNEI " %4, 0",		\
-				   BUGFLAG_WARNING | BUGFLAG_TAINT(TAINT_WARN),	\
-				   __label_warn_on,		\
-				   "r" ((__force long)(x)));	\
-			break;					\
-__label_warn_on:						\
-			__ret_warn_on = true;			\
-		}						\
-	} while (0);						\
+	} else {						\
+		BUG_ENTRY(PPC_TLNEI " %4, 0",			\
+			  BUGFLAG_WARNING | BUGFLAG_TAINT(TAINT_WARN),	\
+			  "r" (__ret_warn_on));	\
+	}							\
 	unlikely(__ret_warn_on);				\
 })
 
@@ -148,14 +106,13 @@ __label_warn_on:						\
 #ifdef __ASSEMBLY__
 .macro EMIT_BUG_ENTRY addr,file,line,flags
 .endm
-.macro EMIT_WARN_ENTRY addr,file,line,flags
-.endm
 #else /* !__ASSEMBLY__ */
 #define _EMIT_BUG_ENTRY
-#define _EMIT_WARN_ENTRY
 #endif
 #endif /* CONFIG_BUG */
 
+#define EMIT_WARN_ENTRY EMIT_BUG_ENTRY
+
 #include <asm-generic/bug.h>
 
 #ifndef __ASSEMBLY__

arch/powerpc/kernel/traps.c

@@ -1508,13 +1508,8 @@ static void do_program_check(struct pt_regs *regs)
 
 		if (!(regs->msr & MSR_PR) &&  /* not user-mode */
 		    report_bug(bugaddr, regs) == BUG_TRAP_TYPE_WARN) {
-			const struct exception_table_entry *entry;
-
-			entry = search_exception_tables(bugaddr);
-			if (entry) {
-				regs_set_return_ip(regs, extable_fixup(entry) + regs->nip - bugaddr);
-				return;
-			}
+			regs_add_return_ip(regs, 4);
+			return;
 		}
 
 		if (cpu_has_feature(CPU_FTR_DEXCR_NPHIE) && user_mode(regs)) {