linux-kernel - Re: [PATCH] copy_{to,from}_user(): fix compile-time sanity checks with gcc -Og

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Date:   Sat, 3 Nov 2018 00:03:58 +0000
From:   Changbin Du <changbin.du@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Changbin Du <changbin.du@...il.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Alex Bennée <alex.bennee@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] copy_{to,from}_user(): fix compile-time sanity checks
 with gcc -Og

On Fri, Nov 02, 2018 at 04:56:58PM +0100, Arnd Bergmann wrote:
> When CONFIG_CC_OPTIMIZE_FOR_DEBUGGING is set, we get countless warnings
> like
> 
> In function 'check_copy_size',
>     inlined from 'copy_from_user' at include/linux/uaccess.h:146:6,
>     inlined from '__copy_siginfo_from_user' at kernel/signal.c:3032:6:
> include/linux/thread_info.h:147:4: error: call to '__bad_copy_to' declared with attribute error: copy destination size is too small
> 
> It seems that constant propagation doesn't work well enough to make
> this code reliable, so turn it off for that configuration.
> 
This is caused by __compiletime_warning() and fixed by below change
already. Could you try the latest kbuild tree?

--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -86,8 +86,10 @@
 #define __compiletime_object_size(obj) __builtin_object_size(obj, 0)

 #ifndef __CHECKER__
+#ifndef CONFIG_CC_OPTIMIZE_FOR_DEBUGGING
 #define __compiletime_warning(message) __attribute__((warning(message)))
 #define __compiletime_error(message) __attribute__((error(message)))
+#endif

> Cc: Changbin Du<changbin.du@...il.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  include/linux/thread_info.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
> index 8d8821b3689a..762f379bdf5d 100644
> --- a/include/linux/thread_info.h
> +++ b/include/linux/thread_info.h
> @@ -138,6 +138,11 @@ static __always_inline bool
>  check_copy_size(const void *addr, size_t bytes, bool is_source)
>  {
>  	int sz = __compiletime_object_size(addr);
> +
> +	/* constant propagation doesn't work well with -Og */
> +	if (IS_ENABLED(CONFIG_CC_OPTIMIZE_FOR_DEBUGGING))
> +		return true;
> +
>  	if (unlikely(sz >= 0 && sz < bytes)) {
>  		if (!__builtin_constant_p(bytes))
>  			copy_overflow(sz, bytes);
> -- 
> 2.18.0
> 

-- 
Thanks,
Changbin Du