bug: refactor away warn_slowpath_fmt_taint()

Patch series "Clean up WARN() "cut here" handling", v2.

Christophe Leroy noticed that the fix for missing "cut here" in the WARN()
case was adding explicit printk() calls instead of teaching the exception
handler to add it.  This refactors the bug/warn infrastructure to pass
this information as a new BUGFLAG.

Longer details repeated from the last patch in the series:

bug: move WARN_ON() "cut here" into exception handler

The original cleanup of "cut here" missed the WARN_ON() case (that does
not have a printk message), which was fixed recently by adding an explicit
printk of "cut here".  This had the downside of adding a printk() to every
WARN_ON() caller, which reduces the utility of using an instruction
exception to streamline the resulting code.  By making this a new BUGFLAG,
all of these can be removed and "cut here" can be handled by the exception
handler.

This was very pronounced on PowerPC, but the effect can be seen on x86 as
well.  The resulting text size of a defconfig build shows some small
savings from this patch:

   text    data     bss     dec     hex filename
19691167        5134320 1646664 26472151        193eed7 vmlinux.before
19676362        5134260 1663048 26473670        193f4c6 vmlinux.after

This change also opens the door for creating something like BUG_MSG(),
where a custom printk() before issuing BUG(), without confusing the "cut
here" line.

This patch (of 7):

There's no reason to have specialized helpers for passing the warn taint
down to __warn().  Consolidate and refactor helper macros, removing
__WARN_printf() and warn_slowpath_fmt_taint().

Link: http://lkml.kernel.org/r/20190819234111.9019-2-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Drew Davenport <ddavenport@chromium.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/bug.h

@@ -90,24 +90,19 @@ struct bug_entry {
  * Use the versions with printk format strings to provide better diagnostics.
  */
 #ifndef __WARN_TAINT
-extern __printf(3, 4)
-void warn_slowpath_fmt(const char *file, const int line,
-		       const char *fmt, ...);
 extern __printf(4, 5)
-void warn_slowpath_fmt_taint(const char *file, const int line, unsigned taint,
-			     const char *fmt, ...);
+void warn_slowpath_fmt(const char *file, const int line, unsigned taint,
+		       const char *fmt, ...);
 extern void warn_slowpath_null(const char *file, const int line);
 #define WANT_WARN_ON_SLOWPATH
 #define __WARN()		warn_slowpath_null(__FILE__, __LINE__)
-#define __WARN_printf(arg...)	warn_slowpath_fmt(__FILE__, __LINE__, arg)
 #define __WARN_printf_taint(taint, arg...)				\
-	warn_slowpath_fmt_taint(__FILE__, __LINE__, taint, arg)
+	warn_slowpath_fmt(__FILE__, __LINE__, taint, arg)
 #else
 extern __printf(1, 2) void __warn_printk(const char *fmt, ...);
 #define __WARN() do { \
 	printk(KERN_WARNING CUT_HERE); __WARN_TAINT(TAINT_WARN); \
 } while (0)
-#define __WARN_printf(arg...)	__WARN_printf_taint(TAINT_WARN, arg)
 #define __WARN_printf_taint(taint, arg...)				\
 	do { __warn_printk(arg); __WARN_TAINT(taint); } while (0)
 #endif
@@ -132,7 +127,7 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
 #define WARN(condition, format...) ({					\
 	int __ret_warn_on = !!(condition);				\
 	if (unlikely(__ret_warn_on))					\
-		__WARN_printf(format);					\
+		__WARN_printf_taint(TAINT_WARN, format);		\
 	unlikely(__ret_warn_on);					\
 })
 #endif

kernel/panic.c

@@ -600,20 +600,8 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
 }
 
 #ifdef WANT_WARN_ON_SLOWPATH
-void warn_slowpath_fmt(const char *file, int line, const char *fmt, ...)
-{
-	struct warn_args args;
-
-	args.fmt = fmt;
-	va_start(args.args, fmt);
-	__warn(file, line, __builtin_return_address(0), TAINT_WARN, NULL,
-	       &args);
-	va_end(args.args);
-}
-EXPORT_SYMBOL(warn_slowpath_fmt);
-
-void warn_slowpath_fmt_taint(const char *file, int line,
-			     unsigned taint, const char *fmt, ...)
+void warn_slowpath_fmt(const char *file, int line, unsigned taint,
+		       const char *fmt, ...)
 {
 	struct warn_args args;
 
@@ -622,7 +610,7 @@ void warn_slowpath_fmt_taint(const char *file, int line,
 	__warn(file, line, __builtin_return_address(0), taint, NULL, &args);
 	va_end(args.args);
 }
-EXPORT_SYMBOL(warn_slowpath_fmt_taint);
+EXPORT_SYMBOL(warn_slowpath_fmt);
 
 void warn_slowpath_null(const char *file, int line)
 {