Re: [PATCH 2/3] lib/kobject_uevent.c: add uevent forwarding function

[Michael J Coss]

On 9/8/2015 11:55 PM, Greg KH wrote:
> On Tue, Sep 08, 2015 at 10:10:29PM -0400, Michael J. Coss wrote:
>> Adds capability to allow userspace programs to forward a given event to
>> a specific network namespace as determined by the provided pid.  In
>> addition, support for a per-namespace kobject_sequence counter was
>> added.  Sysfs was modified to return the correct event counter based on
>> the current network namespace.
>>
>> Signed-off-by: Michael J. Coss <michael.coss@xxxxxxxxxxxxxxxxxx>
>> ---
>>  include/linux/kobject.h     |  3 ++
>>  include/net/net_namespace.h |  3 ++
>>  kernel/ksysfs.c             | 12 ++++++
>>  lib/kobject_uevent.c        | 90 +++++++++++++++++++++++++++++++++++++++++++++
>>  4 files changed, 108 insertions(+)
>>
>> diff --git a/include/linux/kobject.h b/include/linux/kobject.h
>> index 637f670..d1bb509 100644
>> --- a/include/linux/kobject.h
>> +++ b/include/linux/kobject.h
>> @@ -215,6 +215,9 @@ extern struct kobject *firmware_kobj;
>>  int kobject_uevent(struct kobject *kobj, enum kobject_action action);
>>  int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
>>  			char *envp[]);
>> +#if defined(CONFIG_UDEVNS) || defined(CONFIG_UDEVNS_MODULE)
>> +int kobject_uevent_forward(char *buf, size_t len, pid_t pid);
>> +#endif
>>  
>>  __printf(2, 3)
>>  int add_uevent_var(struct kobj_uevent_env *env, const char *format, ...);
>> diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
>> index e951453..a4013e5 100644
>> --- a/include/net/net_namespace.h
>> +++ b/include/net/net_namespace.h
>> @@ -134,6 +134,9 @@ struct net {
>>  #if IS_ENABLED(CONFIG_MPLS)
>>  	struct netns_mpls	mpls;
>>  #endif
>> +#if defined(CONFIG_UDEVNS) || defined(CONFIG_UDEVNS_MODULE)
>> +	u64				kevent_seqnum;
>> +#endif
>>  	struct sock		*diag_nlsk;
>>  	atomic_t		fnhe_genid;
>>  };
>> diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
>> index 6683cce..4bc15fd 100644
>> --- a/kernel/ksysfs.c
>> +++ b/kernel/ksysfs.c
>> @@ -21,6 +21,9 @@
>>  #include <linux/compiler.h>
>>  
>>  #include <linux/rcupdate.h>	/* rcu_expedited */
>> +#if defined(CONFIG_UDEVNS) || defined(CONFIG_UDEVNS_MODULE)
>> +#include <net/net_namespace.h>
>> +#endif
> #if isn't needed here, right?
>
>>  
>>  #define KERNEL_ATTR_RO(_name) \
>>  static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
>> @@ -33,6 +36,15 @@ static struct kobj_attribute _name##_attr = \
>>  static ssize_t uevent_seqnum_show(struct kobject *kobj,
>>  				  struct kobj_attribute *attr, char *buf)
>>  {
>> +#if defined(CONFIG_UDEVNS) || defined(CONFIG_UDEVNS_MODULE)
>> +	pid_t p = task_pid_vnr(current);
>> +	struct net *n = get_net_ns_by_pid(p);
>> +
>> +	if (n != ERR_PTR(-ESRCH)) {
>> +		if (!net_eq(n, &init_net))
>> +			return sprintf(buf, "%llu\n", n->kevent_seqnum);
>> +	}
>> +#endif
>>  	return sprintf(buf, "%llu\n", (unsigned long long)uevent_seqnum);
>>  }
>>  KERNEL_ATTR_RO(uevent_seqnum);
>> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
>> index d791e33..7589745 100644
>> --- a/lib/kobject_uevent.c
>> +++ b/lib/kobject_uevent.c
>> @@ -379,6 +379,96 @@ int kobject_uevent(struct kobject *kobj, enum kobject_action action)
>>  }
>>  EXPORT_SYMBOL_GPL(kobject_uevent);
>>  
>> +#if defined(CONFIG_UDEVNS) || defined(CONFIG_UDEVNS_MODULE)
>> +/**
>> + * kobject_uevent_forward - forward event to specified network namespace
>> + *
>> + * @buf: event buffer
>> + * @len: event length
>> + * @pid: pid of network namespace
>> + *
>> + * Returns 0 if kobject_uevent_forward() is completed with success or the
>> + * corresponding error when it fails.
>> + */
>> +int kobject_uevent_forward(char *buf, size_t len, pid_t pid)
>> +{
>> +	int retval = 0;
>> +#if defined(CONFIG_NET)
>> +	struct uevent_sock *ue_sk;
>> +	struct net *pns;
>> +	char *p;
>> +	u64 num;
>> +
>> +	/* grab the network namespace of the provided pid */
>> +	pns = get_net_ns_by_pid(pid);
>> +	if (pns == ERR_PTR(-ESRCH))
>> +		return -ESRCH;
>> +
>> +	/* find sequence number in buffer */
>> +	p = buf;
>> +	num = 0;
>> +	while (p < (buf + len)) {
>> +		if (strncmp(p, "SEQNUM=", 7) == 0) {
>> +			int r;
>> +
>> +			p += 7;
>> +			r = kstrtoull(p, 10, &num);
>> +			if (r) {
>> +				put_net(pns);
>> +				return r;
>> +			}
>> +			break;
>> +		}
>> +		p += (strlen(p) + 1);
>> +	}
> Ok, that's crazy, replacing an existing sequence number with a sequence
> number of the namespace?  An interesting hack, yes, but something we
> want to maintain for the next 20+ years, no.  Surely there's a better
> way to do this?
>
> But again, I'm not sold on this whole idea anyway.
>
> greg k-h
>
Re: the #if
The #if is only needed because the new udevns code references a
structure defined in that header.  Obviously it could be included
without consequences but I #if it to show it was part of the forwarding
code.

Re: sequence #
I wanted it as transparent as possible, without this the udevd running
inside the container has issues with the values of the sequence numbers
seen, by making it tied to the namespace, udevd could run unchanged. 
Our goal was to minimize the changes to a linux distro and still be able
to run a full desktop inside a container.  But even in absence of our
use case, the first question is should uevents be broadcasts to every
network namespace?  I don't think that broadcasting is the correct
action.  And follow on questions are what if anything should be done
with regards to uevents and containers.

-- 
---Michael J Coss

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