Re: [PATCH] nfsd: enable UMH client tracker in container

From: Stanislav Kinsbursky
Date: Thu Apr 25 2013 - 03:18:25 EST

24.04.2013 00:00, J. Bruce Fields ÐÐÑÐÑ:
On Fri, Apr 19, 2013 at 07:01:50AM -0700, Jeff Layton wrote:
On Tue, 16 Apr 2013 15:17:44 +0400
Stanislav Kinsbursky <skinsbursky@xxxxxxxxxxxxx> wrote:

This patch adds support for UserModeHelper tracker in a container.
The reason for this is that the only containerised tracker ("nfsdcld") is
going to be removed in 3.10 kernel, thus at least one more tracker have to be
containerised to replace the deprecated one.
UMH tracker looks more preferable comparing to legacy since it's the latest
To make UMH tracker work in a container, we have to make sure, that it
executes right binary (i.e., this binary have to be taken from the container
But, UMH is a kernel thread, which works in global root environment by design
(kernel thread's root is inherited from kthreadd, which in turn inherited it's
root from global init). So, the root have to be swapped to the container's
one before binary execution.

This patch passes "init" callback and private "data" to UMH interface, which
are used to swap root for spawned kernel thread.

Note: container's root can be stored on stack, because UMH calls are

Signed-off-by: Stanislav Kinsbursky <skinsbursky@xxxxxxxxxxxxx>
fs/nfsd/nfs4recover.c | 29 ++++++++++++++++++++++-------
1 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/fs/nfsd/nfs4recover.c b/fs/nfsd/nfs4recover.c
index 899ca26..15f8de6 100644
--- a/fs/nfsd/nfs4recover.c
+++ b/fs/nfsd/nfs4recover.c
@@ -38,6 +38,7 @@
#include <linux/crypto.h>
#include <linux/sched.h>
#include <linux/fs.h>
+#include <linux/fs_struct.h>
#include <linux/module.h>
#include <net/net_namespace.h>
#include <linux/sunrpc/rpc_pipe_fs.h>
@@ -1122,12 +1123,28 @@ nfsd4_cltrack_legacy_recdir(const struct xdr_netobj *name)
return result;

+static int nfsd_swap_root(struct subprocess_info *info, struct cred *new)
+ struct path *root = info->data;
+ struct fs_struct *fs = current->fs;
+ struct path current_root;
+ spin_lock(&fs->lock);
+ current_root = fs->root;
+ fs->root = *root;
+ spin_unlock(&fs->lock);
+ if (current_root.dentry)
+ path_put(&current_root);
+ return 0;
static int
nfsd4_umh_cltrack_upcall(char *cmd, char *arg, char *legacy)
char *envp[2];
char *argv[4];
int ret;
+ struct path root;

if (unlikely(!cltrack_prog[0])) {
dprintk("%s: cltrack_prog is disabled\n", __func__);
@@ -1146,7 +1163,11 @@ nfsd4_umh_cltrack_upcall(char *cmd, char *arg, char *legacy)
argv[2] = arg;
argv[3] = NULL;

- ret = call_usermodehelper(argv[0], argv, envp, UMH_WAIT_PROC);
+ get_fs_root(current->fs, &root);
+ ret = call_usermodehelper_fns(argv[0], argv, envp, UMH_WAIT_PROC,
+ nfsd_swap_root, NULL, &root);
* Disable the upcall mechanism if we're getting an ENOENT or EACCES
* error. The admin can re-enable it on the fly by using sysfs
@@ -1185,12 +1206,6 @@ bin_to_hex_dup(const unsigned char *src, int srclen)
static int
nfsd4_umh_cltrack_init(struct net __attribute__((unused)) *net)
- /* XXX: The usermode helper s not working in container yet. */
- if (net != &init_net) {
- WARN(1, KERN_ERR "NFSD: attempt to initialize umh client "
- "tracking in a container!\n");
- return -EINVAL;
- }
return nfsd4_umh_cltrack_upcall("init", NULL, NULL);

I think this looks correct and like it'll work, but I wonder whether
we'd be best served by making this part of the UMH code itself?

IOW, add some fields to the struct subprocess_info to hold the new
root, and then do what you're doing in nfsd_swap_root before calling
the "init" function?

I imagine we'll eventually need do something similar for at least some
of the callers of call_usermodehelper so it would make sense to me to
not replicate copies of nfsd_swap_root all over the place.


Hmm. Yes, me too.
Ok, I'll prepare patch for the UMH itself.
And resend.
Thanks, guys.


Best regards,
Stanislav Kinsbursky
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