linux-kernel - Re: [PATCH] ext4: use BUG() instead of BUG

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Message-ID: <20190325181422.GH1173@magnolia>
Date:   Mon, 25 Mar 2019 11:14:22 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "Theodore Ts'o" <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Eric Whitney <enwlinux@...il.com>,
        Sean Fu <fxinrong@...il.com>, Jan Kara <jack@...e.cz>,
        Eric Biggers <ebiggers@...gle.com>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: use BUG() instead of BUG_ON(1)

On Mon, Mar 25, 2019 at 02:00:25PM +0100, Arnd Bergmann wrote:
> BUG_ON(1) leads to bogus warnings from clang when
> CONFIG_PROFILE_ANNOTATED_BRANCHES is set:
> 
>  fs/ext4/inode.c:544:4: error: variable 'retval' is used uninitialized whenever 'if' condition is false
>       [-Werror,-Wsometimes-uninitialized]
>                         BUG_ON(1);
>                         ^~~~~~~~~
>  include/asm-generic/bug.h:61:36: note: expanded from macro 'BUG_ON'
>                                    ^~~~~~~~~~~~~~~~~~~
>  include/linux/compiler.h:48:23: note: expanded from macro 'unlikely'
>                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>  fs/ext4/inode.c:591:6: note: uninitialized use occurs here
>         if (retval > 0 && map->m_flags & EXT4_MAP_MAPPED) {
>             ^~~~~~
>  fs/ext4/inode.c:544:4: note: remove the 'if' if its condition is always true
>                         BUG_ON(1);
>                         ^
>  include/asm-generic/bug.h:61:32: note: expanded from macro 'BUG_ON'
>                                ^
>  fs/ext4/inode.c:502:12: note: initialize the variable 'retval' to silence this warning
> 
> Change it to BUG() so clang can see that this code path can never
> continue.

I grok that most of these look like "should never get here" assertions,
but shouldn't we be converting these BUG*() calls to "shut down fs,
bounce error back to userspace" instead of killing the whole kernel?

(He says knowing that ripping all of those out is its own project... :P)

--D

> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  fs/ext4/extents_status.c | 4 ++--
>  fs/ext4/inode.c          | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
> index 2b439afafe13..023a3eb3afa3 100644
> --- a/fs/ext4/extents_status.c
> +++ b/fs/ext4/extents_status.c
> @@ -711,7 +711,7 @@ static void ext4_es_insert_extent_ind_check(struct inode *inode,
>  			 * We don't need to check unwritten extent because
>  			 * indirect-based file doesn't have it.
>  			 */
> -			BUG_ON(1);
> +			BUG();
>  		}
>  	} else if (retval == 0) {
>  		if (ext4_es_is_written(es)) {
> @@ -780,7 +780,7 @@ static int __es_insert_extent(struct inode *inode, struct extent_status *newes)
>  			}
>  			p = &(*p)->rb_right;
>  		} else {
> -			BUG_ON(1);
> +			BUG();
>  			return -EINVAL;
>  		}
>  	}
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index b32a57bc5d5d..190f0478582a 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -541,7 +541,7 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
>  			map->m_len = retval;
>  			retval = 0;
>  		} else {
> -			BUG_ON(1);
> +			BUG();
>  		}
>  #ifdef ES_AGGRESSIVE_TEST
>  		ext4_map_blocks_es_recheck(handle, inode, map,
> @@ -1876,7 +1876,7 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
>  		else if (ext4_es_is_unwritten(&es))
>  			map->m_flags |= EXT4_MAP_UNWRITTEN;
>  		else
> -			BUG_ON(1);
> +			BUG();
>  
>  #ifdef ES_AGGRESSIVE_TEST
>  		ext4_map_blocks_es_recheck(NULL, inode, map, &orig_map, 0);
> -- 
> 2.20.0
>