Re: [PATCH] xfs: ensure f_bfree returned by statfs() is non-negative

[Brian Foster <bfoster@redhat.com>]

On Mon, May 11, 2020 at 10:45:24AM +0800, Zheng Bin wrote:
> Construct an img like this:
> 
> dd if=/dev/zero of=xfs.img bs=1M count=20
> mkfs.xfs -d agcount=1 xfs.img
> xfs_db -x xfs.img
> sb 0
> write fdblocks 0
> agf 0
> write freeblks 0
> write longest 0
> quit
> 
> mount it, df -h /mnt(xfs mount point), will show this:
> Filesystem      Size  Used Avail Use% Mounted on
> /dev/loop0       17M  -64Z  -32K 100% /mnt
> 
> Reported-by: Hulk Robot <hulkci@huawei.com>
> Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
> ---

Seems reasonable to me:

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  fs/xfs/xfs_super.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index e80bd2c4c279..aae469f73efe 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -807,7 +807,8 @@ xfs_fs_statfs(
>  	statp->f_blocks = sbp->sb_dblocks - lsize;
>  	spin_unlock(&mp->m_sb_lock);
> 
> -	statp->f_bfree = fdblocks - mp->m_alloc_set_aside;
> +	/* make sure statp->f_bfree does not underflow */
> +	statp->f_bfree = max_t(int64_t, fdblocks - mp->m_alloc_set_aside, 0);
>  	statp->f_bavail = statp->f_bfree;
> 
>  	fakeinos = XFS_FSB_TO_INO(mp, statp->f_bfree);
> --
> 2.26.0.106.g9fadedd
>