Subject: Re: [PATCH] SUNRPC: set desired file system root before connecting local transports

From: Stanislav Kinsbursky <[email-masked]>
Date: Wed, 07 Mar 2012 12:34:11 +0400

07.03.2012 04:19, Myklebust, Trond пишет:
> On Wed, 2012-02-29 at 18:59 +0400, Stanislav Kinsbursky wrote:
>> Today, there is a problem in connecting of local SUNRPC thansports. These
>> transports uses UNIX sockets and connection itself is done by rpciod workqueue.
>> But UNIX sockets lookup is done in context of process file system root. I.e.
>> all local thunsports are connecting in rpciod context.
>> This works nice until we will try to mount NFS from process with other root -
>> for example in container. This container can have it's own (nested) root and
>> rcpbind process, listening on it's own unix sockets. But NFS mount attempt in
>> this container will register new service (Lockd for example) in global rpcbind
>> - not containers's one.
>> This patch solves the problem by switching rpciod kernel thread's file system
>> root to right one (stored on transport) while connecting of local transports.
>>
>> Signed-off-by: Stanislav Kinsbursky<[email-masked]>
>>
>> ---
>>   fs/fs_struct.c        |    1 +
>>   net/sunrpc/xprtsock.c |   32 ++++++++++++++++++++++++++++++--
>>   2 files changed, 31 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/fs_struct.c b/fs/fs_struct.c
>> index 78b519c..0f984c3 100644
>> --- a/fs/fs_struct.c
>> +++ b/fs/fs_struct.c
>> @@ -36,6 +36,7 @@ void set_fs_root(struct fs_struct *fs, struct path *path)
>>   	if (old_root.dentry)
>>   		path_put_longterm(&old_root);
>>   }
>> +EXPORT_SYMBOL_GPL(set_fs_root);
>>
>>   /*
>>    * Replace the fs->{pwdmnt,pwd} with {mnt,dentry}. Put the old values.
>> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
>> index 4c8281d..c94c181 100644
>> --- a/net/sunrpc/xprtsock.c
>> +++ b/net/sunrpc/xprtsock.c
>> @@ -37,6 +37,7 @@
>>   #include<linux/sunrpc/svcsock.h>
>>   #include<linux/sunrpc/xprtsock.h>
>>   #include<linux/file.h>
>> +#include<linux/fs_struct.h>
>>   #ifdef CONFIG_SUNRPC_BACKCHANNEL
>>   #include<linux/sunrpc/bc_xprt.h>
>>   #endif
>> @@ -255,6 +256,11 @@ struct sock_xprt {
>>   	void			(*old_state_change)(struct sock *);
>>   	void			(*old_write_space)(struct sock *);
>>   	void			(*old_error_report)(struct sock *);
>> +
>> +	/*
>> +	 * Saved transport creator root. Required for local transports only.
>> +	 */
>> +	struct path		root;
>>   };
>>
>>   /*
>> @@ -1891,6 +1897,7 @@ static void xs_local_setup_socket(struct work_struct *work)
>>   	struct rpc_xprt *xprt =&transport->xprt;
>>   	struct socket *sock;
>>   	int status = -EIO;
>> +	struct path root;
>>
>>   	if (xprt->shutdown)
>>   		goto out;
>> @@ -1908,7 +1915,14 @@ static void xs_local_setup_socket(struct work_struct *work)
>>   	dprintk("RPC:       worker connecting xprt %p via AF_LOCAL to %s\n",
>>   			xprt, xprt->address_strings[RPC_DISPLAY_ADDR]);
>>
>> +	get_fs_root(current->fs,&root);
>> +	set_fs_root(current->fs,&transport->root);
>> +
>>   	status = xs_local_finish_connecting(xprt, sock);
>> +
>> +	set_fs_root(current->fs,&root);
>> +	path_put(&root);
>> +
>>   	switch (status) {
>>   	case 0:
>
> Hi Stanislav,
>
> What happens here if the mount namespace of the process that originally
> created the sock_xprt no longer exists? Should we care about that case?
>

Hi, Trond.
Looks like this is not a problem, because process fs->root->mnt usage counter 
was increased on transport creation.
IOW, transport holds current root and thus mount namespace can't disappear.

-- 
Best regards,
Stanislav Kinsbursky