linux-kernel - Re: [PATCH v2] 9p: retrieve fid from file when file instance exist.

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Message-ID: <20200701105927.GA20762@nautica>
Date:   Wed, 1 Jul 2020 12:59:27 +0200
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Jianyong Wu <jianyong.wu@....com>
Cc:     ericvh@...il.com, lucho@...kov.net,
        v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        Steve.Capper@....com, Kaly.Xin@....com, justin.he@....com,
        wei.chen@....com
Subject: Re: [PATCH v2] 9p: retrieve fid from file when file instance exist.

Jianyong Wu wrote on Wed, Jul 01, 2020:
> In the current setattr implementation in 9p, fid is always retrieved
> from dentry no matter file instance exists or not. There may be
> some info related to opened file instance dropped. so it's better
> to retrieve fid from file instance if file instance is passed to setattr.
> 
> for example:
> fd=open("tmp", O_RDWR);
> ftruncate(fd, 10);
> 
> The file context related with fd will be lost as fid is always
> retrieved from dentry, then the backend can't get the info of
> file context. It is against the original intention of user and
> may lead to bug.

Thanks for the commit message - still feels a bit odd but at least
correct enough for me :)

> Signed-off-by: Jianyong Wu <jianyong.wu@....com>
> ---
>  fs/9p/vfs_inode.c      | 6 +++++-
>  fs/9p/vfs_inode_dotl.c | 6 +++++-
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
> index c9255d399917..b33574d347fa 100644
> --- a/fs/9p/vfs_inode.c
> +++ b/fs/9p/vfs_inode.c
> @@ -1100,7 +1100,11 @@ static int v9fs_vfs_setattr(struct dentry *dentry, struct iattr *iattr)
>  
>  	retval = -EPERM;
>  	v9ses = v9fs_dentry2v9ses(dentry);
> -	fid = v9fs_fid_lookup(dentry);
> +	if (iattr->ia_valid & ATTR_FILE) {
> +		fid = iattr->ia_file->private_data;
> +		WARN_ON(!fid);

That would crash in 9p_client_wstat a few lines below with the current
else ; so I'm not sure WARN_ON is appropriate with this code.

the snippet I had suggested had v9fs_fid_lookup in a different if, not
as a else statement to avoid this crash (and then warning is OK)

> +	} else
> +		fid = v9fs_fid_lookup(dentry);

-- 
Dominique