linux-kernel - Re: [PATCH 1/2] f2fs: fix to avoid potential negative .f

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Message-ID: <20190430025406.GA17299@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Mon, 29 Apr 2019 19:54:06 -0700
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <chao@...nel.org>
Cc:     Chao Yu <yuchao0@...wei.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, drosen@...gle.com
Subject: Re: [PATCH 1/2] f2fs: fix to avoid potential negative .f_bfree

On 04/29, Chao Yu wrote:
> On 2019-4-28 21:47, Jaegeuk Kim wrote:
> > On 04/26, Chao Yu wrote:
> >> When calculating .f_bfree value in f2fs_statfs(), sbi->unusable_block_count
> >> can be increased after the judgment condition, result in overflow of
> >> .f_bfree in later calculation. This patch fixes to use a temporary signed
> >> variable to save the calculation result of .f_bfree.
> >>
> >> 	if (unlikely(buf->f_bfree <= sbi->unusable_block_count))
> >>  		buf->f_bfree = 0;
> >>  	else
> >> 		buf->f_bfree -= sbi->unusable_block_count;
> > 
> > Do we just need stat_lock for this?
> 
> Like we access other stat value in statfs(), we just need the instantaneous
> value of .unusable_block_count, so we don't need additional stat_lock, right?

What I've concerend is whether or not this fixes all the inconsistent values.
The original intention was providing stats in best effort, so we wouldn't use
any lock.

> 
> Thanks,
> 
> > 
> >>
> >> Signed-off-by: Chao Yu <yuchao0@...wei.com>
> >> ---
> >>  fs/f2fs/super.c | 7 +++++--
> >>  1 file changed, 5 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> >> index 2376bb01b5c4..fcc9793dbc2c 100644
> >> --- a/fs/f2fs/super.c
> >> +++ b/fs/f2fs/super.c
> >> @@ -1216,6 +1216,7 @@ static int f2fs_statfs(struct dentry *dentry, struct kstatfs *buf)
> >>  	u64 id = huge_encode_dev(sb->s_bdev->bd_dev);
> >>  	block_t total_count, user_block_count, start_count;
> >>  	u64 avail_node_count;
> >> +	long long bfree;
> >>  
> >>  	total_count = le64_to_cpu(sbi->raw_super->block_count);
> >>  	user_block_count = sbi->user_block_count;
> >> @@ -1226,10 +1227,12 @@ static int f2fs_statfs(struct dentry *dentry, struct kstatfs *buf)
> >>  	buf->f_blocks = total_count - start_count;
> >>  	buf->f_bfree = user_block_count - valid_user_blocks(sbi) -
> >>  						sbi->current_reserved_blocks;
> >> -	if (unlikely(buf->f_bfree <= sbi->unusable_block_count))
> >> +
> >> +	bfree = buf->f_bfree - sbi->unusable_block_count;
> >> +	if (unlikely(bfree < 0))
> >>  		buf->f_bfree = 0;
> >>  	else
> >> -		buf->f_bfree -= sbi->unusable_block_count;
> >> +		buf->f_bfree = bfree;
> >>  
> >>  	if (buf->f_bfree > F2FS_OPTION(sbi).root_reserved_blocks)
> >>  		buf->f_bavail = buf->f_bfree -
> >> -- 
> >> 2.18.0.rc1