Re: [PATCH v2] ubifs: make ubifs_[get|set]xattr atomic

[Richard Weinberger <richard@nod.at>]

Am 31.07.2015 um 03:12 schrieb Dongsheng Yang:
> This commit make the ubifs_[get|set]xattr protected by ui_mutex.
> 
> Originally, there is a possibility that ubifs_getxattr to get
> a wrong value.
> 
>     P1				    P2
> ----------			----------
> ubifs_getxattr			ubifs_setxattr
> 				   - kfree()
>    - memcpy()
> 				   - kmemdup()
> 
> Then ubifs_getxattr() would get a non-sense data. To solve this
> problem, this commit make the xattr of ubifs_inode updated in
> atomic.

so, ui->data needs protection?
The comment in fs/ubifs/ubifs.h does not mention ->data.
I'm asking because I want to make sure that ui_mutex is the correct lock to take.

> Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
> ---
> 	-v2:
> 		Add more description in commit message
>  fs/ubifs/xattr.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/ubifs/xattr.c b/fs/ubifs/xattr.c
> index 96f3448..dec1afd 100644
> --- a/fs/ubifs/xattr.c
> +++ b/fs/ubifs/xattr.c
> @@ -208,6 +208,7 @@ static int change_xattr(struct ubifs_info *c, struct inode *host,
>  	if (err)
>  		return err;
>  
> +	mutex_lock(&ui->ui_mutex);
>  	kfree(ui->data);
>  	ui->data = kmemdup(value, size, GFP_NOFS);
>  	if (!ui->data) {
> @@ -216,6 +217,7 @@ static int change_xattr(struct ubifs_info *c, struct inode *host,
>  	}
>  	inode->i_size = ui->ui_size = size;
>  	ui->data_len = size;
> +	mutex_unlock(&ui->ui_mutex);

You also need a mutex_unlock(&ui->ui_mutex) under out_free, otherwise
the if (!ui->data) { branch will trigger a deadlock.

Thanks,
//richard