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Subject: Re: [PATCH v2 7/7] bug: Move WARN_ON() "cut here" into exception
 handler
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Drew Davenport <ddavenport@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Steven Rostedt (VMware)" <rostedt@goodmis.org>,
        Feng Tang <feng.tang@intel.com>,
        Petr Mladek <pmladek@suse.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Borislav Petkov <bp@suse.de>,
        YueHaibing <yuehaibing@huawei.com>, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <201908200943.601DD59DCE@keescook>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <ef7097d4-d924-4053-fd50-77128f198ae7@c-s.fr>
Date:   Fri, 23 Aug 2019 16:56:31 +0200
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In-Reply-To: 20190819234111.9019-8-keescook@chromium.org

Le 20/08/2019 à 18:47, Kees Cook a écrit :
> The original clean up 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.
> 
> Reported-by: Christophe Leroy <christophe.leroy@c-s.fr>
> Fixes: Fixes: 6b15f678fb7d ("include/asm-generic/bug.h: fix "cut here" for WARN_ON for __WARN_TAINT architectures")
> Signed-off-by: Kees Cook <keescook@chromium.org>

Tested-by: Christophe Leroy <christophe.leroy@c-s.fr>

> ---
> v2:
>   - rename BUGFLAG_PRINTK to BUGFLAG_NO_CUT_HERE (peterz, christophe)
> ---
>   include/asm-generic/bug.h |  8 +++-----
>   lib/bug.c                 | 11 +++++++++--
>   2 files changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
> index 588dd59a5b72..a21e83f8a274 100644
> --- a/include/asm-generic/bug.h
> +++ b/include/asm-generic/bug.h
> @@ -10,6 +10,7 @@
>   #define BUGFLAG_WARNING		(1 << 0)
>   #define BUGFLAG_ONCE		(1 << 1)
>   #define BUGFLAG_DONE		(1 << 2)
> +#define BUGFLAG_NO_CUT_HERE	(1 << 3)	/* CUT_HERE already sent */
>   #define BUGFLAG_TAINT(taint)	((taint) << 8)
>   #define BUG_GET_TAINT(bug)	((bug)->flags >> 8)
>   #endif
> @@ -86,13 +87,10 @@ void warn_slowpath_fmt(const char *file, const int line, unsigned taint,
>   	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_FLAGS(BUGFLAG_TAINT(TAINT_WARN));		\
> -	} while (0)
> +#define __WARN()		__WARN_FLAGS(BUGFLAG_TAINT(TAINT_WARN))
>   #define __WARN_printf(taint, arg...) do {				\
>   		__warn_printk(arg);					\
> -		__WARN_FLAGS(BUGFLAG_TAINT(taint));			\
> +		__WARN_FLAGS(BUGFLAG_NO_CUT_HERE | BUGFLAG_TAINT(taint));\
>   	} while (0)
>   #define WARN_ON_ONCE(condition) ({				\
>   	int __ret_warn_on = !!(condition);			\
> diff --git a/lib/bug.c b/lib/bug.c
> index 1077366f496b..8c98af0bf585 100644
> --- a/lib/bug.c
> +++ b/lib/bug.c
> @@ -181,6 +181,15 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
>   		}
>   	}
>   
> +	/*
> +	 * BUG() and WARN_ON() families don't print a custom debug message
> +	 * before triggering the exception handler, so we must add the
> +	 * "cut here" line now. WARN() issues its own "cut here" before the
> +	 * extra debugging message it writes before triggering the handler.
> +	 */
> +	if ((bug->flags & BUGFLAG_NO_CUT_HERE) == 0)
> +		printk(KERN_DEFAULT CUT_HERE);
> +
>   	if (warning) {
>   		/* this is a WARN_ON rather than BUG/BUG_ON */
>   		__warn(file, line, (void *)bugaddr, BUG_GET_TAINT(bug), regs,
> @@ -188,8 +197,6 @@ enum bug_trap_type report_bug(unsigned long bugaddr, struct pt_regs *regs)
>   		return BUG_TRAP_TYPE_WARN;
>   	}
>   
> -	printk(KERN_DEFAULT CUT_HERE);
> -
>   	if (file)
>   		pr_crit("kernel BUG at %s:%u!\n", file, line);
>   	else
>