linux-kernel - Re: [PATCH 2/3] vsprintf: print <no-symbol> if symbol not found

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Date:   Sun, 17 Dec 2017 16:04:14 -0800
From:   Joe Perches <joe@...ches.com>
To:     "Tobin C. Harding" <me@...in.cc>,
        kernel-hardening@...ts.openwall.com
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Alexei Starovoitov <ast@...nel.org>,
        linux-kernel@...r.kernel.org,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/3] vsprintf: print <no-symbol> if symbol not found

On Mon, 2017-12-18 at 10:53 +1100, Tobin C. Harding wrote:
> Depends on: commit bd6b239cdbb2 ("kallsyms: don't leak address when
> symbol not found")
> 
> Currently vsprintf for specifiers %p[SsB] relies on the behaviour of
> kallsyms (sprint_symbol()) and prints the actual address if a symbol is
> not found. Previous patch changes this behaviour so tha sprint_symbol()

tha->that

> returns an error if symbol not found. With this patch in place we can
> print a sanitized message '<no-symbol>' instead of leaking the address.
> 
> Print '<no-symbol>' for printk specifier %s[sSB] if no symbol is found.

%s->%ps

> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
[]
> @@ -460,6 +460,8 @@ char *kvasprintf(gfp_t gfp, const char *fmt, va_list args);
>  extern __printf(2, 0)
>  const char *kvasprintf_const(gfp_t gfp, const char *fmt, va_list args);
>  
> +extern int string_is_no_symbol(const char *s);
> +
[]
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> 
> +#define PRINTK_NO_SYMBOL_STR "<no-symbol>"

"<symbol unavailable>" ?  "not found"?

[]

> +int string_is_no_symbol(const char *s)
> +{
> +	return !!strstr(s, PRINTK_NO_SYMBOL_STR);
> +}
> +EXPORT_SYMBOL(string_is_no_symbol);

Why should string_is_no_symbol be exported?