linux-kernel - Re: [PATCH linux] ext4: Delete useless ret assignment

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Date:   Thu, 6 Jan 2022 11:58:43 +0100
From:   Lukas Czerner <lczerner@...hat.com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     cgel.zte@...il.com, Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        luo penghao <luo.penghao@....com.cn>,
        Zeal Robot <zealci@....com.cn>,
        Harshad Shirwadkar <harshadshirwadkar@...il.com>
Subject: Re: [PATCH linux] ext4: Delete useless ret assignment

On Wed, Jan 05, 2022 at 11:44:39PM -0500, Theodore Ts'o wrote:
> On Thu, Dec 30, 2021 at 06:29:05AM +0000, cgel.zte@...il.com wrote:
> > From: luo penghao <luo.penghao@....com.cn>
> > 
> > The assignments in these two places will be overwritten by new
> > assignments later, so they should be deleted.
> > 
> > The clang_analyzer complains as follows:
> > 
> > fs/ext4/fast_commit.c
> > 
> > Value stored to 'ret' is never read
> 
> I suspect the right answer here is that we *should* be checking the
> return value, and reflecting the error up to caller, if appropriate.
> 
> Harshad, what do you think?

Indeed we absolutely *must* be checking the return value and bail out
otherwise we risk overwriting kernel memory among other possible
problems.

See ext4_fc_record_modified_inode() where we increment
fc_modified_inodes_size before the actual reallocation which in case of
allocation failure will leave us with elevated fc_modified_inodes_size
and the next call to ext4_fc_record_modified_inode() can modify
fc_modified_inodes[] out of bounds.

In addition to checking the return value we should probably also move
incrementing the fc_modified_inodes_size until after the successful
reallocation in order to avoid such pitfalls.

Thanks!
-Lukas

> 
> 					- Ted
> 
> > 
> > Reported-by: Zeal Robot <zealci@....com.cn>
> > Signed-off-by: luo penghao <luo.penghao@....com.cn>
> > ---
> >  fs/ext4/fast_commit.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
> > index 8ea5a81..8d5d044 100644
> > --- a/fs/ext4/fast_commit.c
> > +++ b/fs/ext4/fast_commit.c
> > @@ -1660,7 +1660,7 @@ static int ext4_fc_replay_add_range(struct super_block *sb,
> >  		return 0;
> >  	}
> >  
> > -	ret = ext4_fc_record_modified_inode(sb, inode->i_ino);
> > +	ext4_fc_record_modified_inode(sb, inode->i_ino);
> >  
> >  	start = le32_to_cpu(ex->ee_block);
> >  	start_pblk = ext4_ext_pblock(ex);
> > @@ -1785,7 +1785,7 @@ ext4_fc_replay_del_range(struct super_block *sb, struct ext4_fc_tl *tl,
> >  		return 0;
> >  	}
> >  
> > -	ret = ext4_fc_record_modified_inode(sb, inode->i_ino);
> > +	ext4_fc_record_modified_inode(sb, inode->i_ino);
> >  
> >  	jbd_debug(1, "DEL_RANGE, inode %ld, lblk %d, len %d\n",
> >  			inode->i_ino, le32_to_cpu(lrange.fc_lblk),
> > -- 
> > 2.15.2
> > 
> > 
>