linux-kernel - Re: [PATCH 2/6] nfsd: swap fs root in NFSd kthreads

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Message-ID: <50C73C60.8060405@parallels.com>
Date:	Tue, 11 Dec 2012 18:00:00 +0400
From:	Stanislav Kinsbursky <skinsbursky@...allels.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	<linux-nfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<devel@...nvz.org>
Subject: Re: [PATCH 2/6] nfsd: swap fs root in NFSd kthreads

11.12.2012 00:28, J. Bruce Fields пишет:
> On Thu, Dec 06, 2012 at 06:34:47PM +0300, Stanislav Kinsbursky wrote:
>> NFSd does lookup. Lookup is done starting from current->fs->root.
>> NFSd is a kthread, cloned by kthreadd, and thus have global (but luckely
>> unshared) root.
>> So we have to swap root to those, which process, started NFSd, has. Because
>> that process can be in a container with it's own root.
>
> This doesn't sound right to me.
>
> Which lookups exactly do you see being done relative to
> current->fs->root ?
>

Ok, you are right. I was mistaken here.
This is not a exactly lookup, but d_path() problem in svc_export_request().
I.e. without root swapping, d_path() will give not local export path (like "/export")
but something like this "/root/containers_root/export".

> --b.
>
>>
>> Signed-off-by: Stanislav Kinsbursky <skinsbursky@...allels.com>
>> ---
>>   fs/nfsd/netns.h  |    1 +
>>   fs/nfsd/nfssvc.c |   33 ++++++++++++++++++++++++++++++++-
>>   2 files changed, 33 insertions(+), 1 deletions(-)
>>
>> diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
>> index abfc97c..5c423c6 100644
>> --- a/fs/nfsd/netns.h
>> +++ b/fs/nfsd/netns.h
>> @@ -101,6 +101,7 @@ struct nfsd_net {
>>   	struct timeval nfssvc_boot;
>>
>>   	struct svc_serv *nfsd_serv;
>> +	struct path	root;
>>   };
>>
>>   extern int nfsd_net_id;
>> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
>> index cee62ab..177bb60 100644
>> --- a/fs/nfsd/nfssvc.c
>> +++ b/fs/nfsd/nfssvc.c
>> @@ -392,6 +392,7 @@ int nfsd_create_serv(struct net *net)
>>
>>   	set_max_drc();
>>   	do_gettimeofday(&nn->nfssvc_boot);		/* record boot time */
>> +	get_fs_root(current->fs, &nn->root);
>>   	return 0;
>>   }
>>
>> @@ -426,8 +427,10 @@ void nfsd_destroy(struct net *net)
>>   	if (destroy)
>>   		svc_shutdown_net(nn->nfsd_serv, net);
>>   	svc_destroy(nn->nfsd_serv);
>> -	if (destroy)
>> +	if (destroy) {
>> +		path_put(&nn->root);
>>   		nn->nfsd_serv = NULL;
>> +	}
>>   }
>>
>>   int nfsd_set_nrthreads(int n, int *nthreads, struct net *net)
>> @@ -533,6 +536,25 @@ out:
>>   	return error;
>>   }
>>
>> +/*
>> + * This function is actually slightly modified set_fs_root()<F9>
>> + */
>> +static void nfsd_swap_root(struct net *net)
>> +{
>> +	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
>> +	struct fs_struct *fs = current->fs;
>> +	struct path old_root;
>> +
>> +	path_get(&nn->root);
>> +	spin_lock(&fs->lock);
>> +	write_seqcount_begin(&fs->seq);
>> +	old_root = fs->root;
>> +	fs->root = nn->root;
>> +	write_seqcount_end(&fs->seq);
>> +	spin_unlock(&fs->lock);
>> +	if (old_root.dentry)
>> +		path_put(&old_root);
>> +}
>>
>>   /*
>>    * This is the NFS server kernel thread
>> @@ -559,6 +581,15 @@ nfsd(void *vrqstp)
>>   	current->fs->umask = 0;
>>
>>   	/*
>> +	 * We have to swap NFSd kthread's fs->root.
>> +	 * Why so? Because NFSd can be started in container, which has it's own
>> +	 * root.
>> +	 * And so what? NFSd lookup files, and lookup start from
>> +	 * current->fs->root.
>> +	 */
>> +	nfsd_swap_root(net);
>> +
>> +	/*
>>   	 * thread is spawned with all signals set to SIG_IGN, re-enable
>>   	 * the ones that will bring down the thread
>>   	 */
>>


-- 
Best regards,
Stanislav Kinsbursky
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