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Date: Tue, 28 Mar 2017 16:29:42 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Peter Zijlstra <peterz@infradead.org>, kasan-dev@googlegroups.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kasan: avoid -Wmaybe-uninitialized warning
Message-Id: <20170328162942.eb08b50af725428a4be25f2b@linux-foundation.org>
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On Thu, 23 Mar 2017 16:04:09 +0100 Arnd Bergmann <arnd@arndb.de> wrote:

> gcc-7 produces this warning:
> 
> mm/kasan/report.c: In function 'kasan_report':
> mm/kasan/report.c:351:3: error: 'info.first_bad_addr' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>    print_shadow_for_address(info->first_bad_addr);
>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> mm/kasan/report.c:360:27: note: 'info.first_bad_addr' was declared here
> 
> The code seems fine as we only print info.first_bad_addr when there is a shadow,
> and we always initialize it in that case, but this is relatively hard
> for gcc to figure out after the latest rework. Adding an intialization
> in the other code path gets rid of the warning.
> 
> ...
>
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -109,6 +109,8 @@ const char *get_wild_bug_type(struct kasan_access_info *info)
>  {
>  	const char *bug_type = "unknown-crash";
>  
> +	info->first_bad_addr = (void *)(-1ul);
> +
>  	if ((unsigned long)info->access_addr < PAGE_SIZE)
>  		bug_type = "null-ptr-deref";
>  	else if ((unsigned long)info->access_addr < TASK_SIZE)

A weird, ugly and seemingly-unneeded statement should have a comment
explaining its existence, no?

Fortunately it is no longer needed.  We now have:

static void print_error_description(struct kasan_access_info *info)
{
	const char *bug_type = "unknown-crash";
	u8 *shadow_addr;

	info->first_bad_addr = find_first_bad_addr(info->access_addr,
						info->access_size);

	shadow_addr = (u8 *)kasan_mem_to_shadow(info->first_bad_addr);

	...