linux-kernel - Re: [BUG REPORT] use of unreachable() masks uninitialized variables warnings

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Date:	Thu, 11 Feb 2016 11:12:12 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	"Theodore Ts'o" <tytso@....edu>, Jeff Merkey <linux.mdb@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG REPORT] use of unreachable() masks uninitialized variables warnings

On 2/11/16, Theodore Ts'o <tytso@....edu> wrote:
> On Wed, Feb 10, 2016 at 08:13:09PM -0700, Jeff Merkey wrote:
>> Here are the sources of several bugs I have seen recently in ext4 I am
>> pretty sure with a null bh.  One good check is to set the BUG() macro
>> NOT TO call unreachable() as a build test since the compiler will
>> ignore uninitialized variables in a function if someone calls BUG()
>> even conditionally, and never report them during build.
>>
>> The following are from v4.4.1 with a BUG() macro with the call to
>> unreachable() removed:
>
> I checked all of the fs/ext4 warnings you listed and they are all
> false positives.
>
>> In file included from fs/ext4/file.c:30:0:
>> fs/ext4/ext4_jbd2.h: In function ‘ext4_inode_journal_mode’:
>> fs/ext4/ext4_jbd2.h:409:1: warning: control reaches end of non-void
>> function [-Wreturn-type]
>>  }
>
> This is from a:
>
> 	if (foo) {
> 		...
> 		return foobie;
> 	} else if (bar) {
> 		...
> 		return barbie;
> 	} else {
> 		BUG();
> 	}
>
> construct.
>
>> fs/ext4/inode.c: In function ‘ext4_map_blocks’:
>> fs/ext4/inode.c:548:5: warning: ‘retval’ may be used uninitialized in
>> this function [-Wmaybe-uninitialized]
>>   if (retval > 0 && map->m_flags & EXT4_MAP_MAPPED) {
>>      ^
>> fs/ext4/extents.c:2305:14: warning: ‘len’ may be used uninitialized in
>> this function [-Wmaybe-uninitialized]
>>   ext4_lblk_t len;
>>               ^
>
> All of the may be used uninitialized warnings are from a:
>
> 	if (foo) {
> 		...
> 		retval = xxx;
> 	} else if (bar) {
> 		...
> 		retval = yyy;
> 	} else {
> 		BUG();
> 	}
>
> construct.
>
> It may be that there are some false warnings, but they certainly weren't
> from warnings you've listed from ext4.
>
> Cheers,
>
> 						- Ted
>

Hi Ted,

That's good to know, they could be false positives, but it was kind of
wierd behavior caused by that macro.

Jeff