linux-kernel - Re: [lkp-robot] [include/linux/string.h] 6974f0c455: kernel_BUG_at

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Date:   Thu, 20 Jul 2017 00:41:36 -0400
From:   Daniel Micay <danielmicay@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        kernel test robot <xiaolong.ye@...el.com>,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...db.de>,
        Mark Rutland <mark.rutland@....com>,
        Daniel Axtens <dja@...ens.net>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>
Subject: Re: [lkp-robot] [include/linux/string.h] 6974f0c455:
 kernel_BUG_at_lib/string.c

> So the fortify_string code has decided that only a single-byte (or
> empty) memcpy is ok.
> 
> And that, in turn, seems to be because we're copying from
> optprobe_template_entry, which is declared as
> 
>     extern __visible kprobe_opcode_t optprobe_template_entry;
> 
> so the fortify code decides it's a single character.
> 
> Does just changing all those things to be declared as arrays fix
> things?

Yeah, that fixes it because GCC will consider the size of 'char foo[]'
unknown (i.e. (size_t)-1 from __builtin_object_size).

GCC doesn't know this essentially constant value at compile-time so it
wasn't a compile-time error:

#define TMPL_END_IDX \
	((long)&optprobe_template_end - (long)&optprobe_template_entry)

-fsanitize=object-size works the same way for pointer dereferences so
replacing might fix some issues for CONFIG_UBSAN_SANITIZE_ALL. I guess
that's way too noisy at the moment thus the !COMPILE_TEST.