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

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Date:	Thu, 11 Feb 2016 09:06:37 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Jeff Merkey <linux.mdb@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG REPORT] use of unreachable() masks uninitialized variables
 warnings

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