linux-kernel - Re: [PATCH] fs/9p/xattr.c: catch the error of p9_client

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Date:   Wed, 25 Jul 2018 05:32:00 +0200
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     piaojun <piaojun@...wei.com>
Cc:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        Eric Van Hensbergen <ericvh@...il.com>,
        Ron Minnich <rminnich@...dia.gov>,
        Latchesar Ionkov <lucho@...kov.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        v9fs-developer@...ts.sourceforge.net, groug@...d.org
Subject: Re: [PATCH] fs/9p/xattr.c: catch the error of p9_client_clunk when
 setting xattr failed

piaojun wrote on Wed, Jul 25, 2018:
> In my testing, v9fs_fid_xattr_set will return successfully even if the
> backend ext4 filesystem has no space to store xattr key-value. That will
> cause inconsistent behavior between front end and back end. The reason is
> that lsetxattr will be triggered by p9_client_clunk, and unfortunately we
> did not catch the error. This patch will catch the error to notify upper
> caller.
> 
> p9_client_clunk (in 9p)
>   p9_client_rpc(clnt, P9_TCLUNK, "d", fid->fid);
>     v9fs_clunk (in qemu)
>       put_fid
>         free_fid
>           v9fs_xattr_fid_clunk
>             v9fs_co_lsetxattr
>               s->ops->lsetxattr
>                 ext4_xattr_user_set (in host ext4 filesystem)
> 
> Signed-off-by: Jun Piao <piaojun@...wei.com>

Good catch!

LGTM on its own but see comment below for further error checking.
Please let me know if you want to do this in a v2 or submit a separate
patch altogether, I can pick this one up as it is.

> ---
>  fs/9p/xattr.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/9p/xattr.c b/fs/9p/xattr.c
> index f329eee..352abc3 100644
> --- a/fs/9p/xattr.c
> +++ b/fs/9p/xattr.c
> @@ -105,7 +105,7 @@ int v9fs_fid_xattr_set(struct p9_fid *fid, const char *name,
>  {
>  	struct kvec kvec = {.iov_base = (void *)value, .iov_len = value_len};
>  	struct iov_iter from;
> -	int retval;
> +	int retval, err;
> 
>  	iov_iter_kvec(&from, WRITE | ITER_KVEC, &kvec, 1, value_len);
> 
> @@ -126,7 +126,9 @@ int v9fs_fid_xattr_set(struct p9_fid *fid, const char *name,
>  			 retval);
>  	else
>  		p9_client_write(fid, 0, &from, &retval);

I'm not sure what to do about this but it's also possible for
p9_client_write to not write the full length without setting and error.

We should probably compare the return value of p9_client_write and
value_len to detect short writes and return an error in this case.

> -	p9_client_clunk(fid);
> +	err = p9_client_clunk(fid);
> +	if (!retval && err)
> +		retval = err;
>  	return retval;
>  }
> 
> --