linux-kernel - Re: [RFC PATCH] fs: introduce check for drop/inc

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Message-ID: <20231017005747.GB11424@frogsfrogsfrogs>
Date:   Mon, 16 Oct 2023 17:57:47 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     cheng.lin130@....com.cn
Cc:     brauner@...nel.org, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        david@...morbit.com, hch@...radead.org, jiang.yong5@....com.cn,
        wang.liang82@....com.cn, liu.dong3@....com.cn
Subject: Re: [RFC PATCH] fs: introduce check for drop/inc_nlink

On Fri, Oct 13, 2023 at 05:40:57PM +0800, cheng.lin130@....com.cn wrote:
> > On Fri, Oct 13, 2023 at 03:27:30PM +0800, cheng.lin130@....com.cn wrote:
> > > From: Cheng Lin <cheng.lin130@....com.cn>
> > >
> > > Avoid inode nlink overflow or underflow.
> > >
> > > Signed-off-by: Cheng Lin <cheng.lin130@....com.cn>
> > > ---
> > I'm very confused. There's no explanation why that's needed. As it
> > stands it's not possible to provide a useful review.
> > I'm not saying it's wrong. I just don't understand why and even if this
> > should please show up in the commit message.
> In an xfs issue, there was an nlink underflow of a directory inode. There
> is a key information in the kernel messages, that is the WARN_ON from
> drop_nlink(). However, VFS did not prevent the underflow. I'm not sure
> if this behavior is inadvertent or specifically designed. As an abnormal
> situation, perhaps prohibiting nlink overflow or underflow is a better way
> to handle it.
> Request for your comment.

I was trying to steer you towards modifying vfs_rmdir and vfs_unlink to
check that i_nlink of the files involved aren't somehow already zero
and returning -EFSCORRUPTED if they are.  Not messing around with
drop_nlink.

--D