Re: [RFC v5 1/1] ns: add binfmt_misc to the user namespace

[Laurent Vivier]

Le 09/10/2018 Ã 19:01, Kirill Tkhai a ÃcritÂ:
> On 09.10.2018 19:45, Laurent Vivier wrote:
>> Le 09/10/2018 Ã 18:15, Kirill Tkhai a ÃcritÂ:
>>> On 09.10.2018 13:37, Laurent Vivier wrote:
>>>> This patch allows to have a different binfmt_misc configuration
>>>> for each new user namespace. By default, the binfmt_misc configuration
>>>> is the one of the previous level, but if the binfmt_misc filesystem is
>>>> mounted in the new namespace a new empty binfmt instance is created and
>>>> used in this namespace.
>>>>
>>>> For instance, using "unshare" we can start a chroot of an another
>>>> architecture and configure the binfmt_misc interpreter without being root
>>>> to run the binaries in this chroot.
>>>>
>>>> Signed-off-by: Laurent Vivier <laurent@xxxxxxxxx>
>>>> ---
>>>>  fs/binfmt_misc.c               | 106 ++++++++++++++++++++++++---------
>>>>  include/linux/user_namespace.h |  13 ++++
>>>>  kernel/user.c                  |  13 ++++
>>>>  kernel/user_namespace.c        |   3 +
>>>>  4 files changed, 107 insertions(+), 28 deletions(-)
>>>>
>>>> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
>>>> index aa4a7a23ff99..1e0029d097d9 100644
>>>> --- a/fs/binfmt_misc.c
>>>> +++ b/fs/binfmt_misc.c
>> ...
>>>> @@ -80,18 +74,32 @@ static int entry_count;
>>>>   */
>>>>  #define MAX_REGISTER_LENGTH 1920
>>>>  
>>>> +static struct binfmt_namespace *binfmt_ns(struct user_namespace *ns)
>>>> +{
>>>> +	struct binfmt_namespace *b_ns;
>>>> +
>>>> +	while (ns) {
>>>> +		b_ns = READ_ONCE(ns->binfmt_ns);
>>>> +		if (b_ns)
>>>> +			return b_ns;
>>>> +		ns = ns->parent;
>>>> +	}
>>>> +	WARN_ON_ONCE(1);
>>>> +	return NULL;
>>>> +}
>>>> +
>> ...
>>>> @@ -823,12 +847,34 @@ static const struct super_operations s_ops = {
>>>>  static int bm_fill_super(struct super_block *sb, void *data, int silent)
>>>>  {
>>>>  	int err;
>>>> +	struct user_namespace *ns = sb->s_user_ns;
>>>>  	static const struct tree_descr bm_files[] = {
>>>>  		[2] = {"status", &bm_status_operations, S_IWUSR|S_IRUGO},
>>>>  		[3] = {"register", &bm_register_operations, S_IWUSR},
>>>>  		/* last one */ {""}
>>>>  	};
>>>>  
>>>> +	/* create a new binfmt namespace
>>>> +	 * if we are not in the first user namespace
>>>> +	 * but the binfmt namespace is the first one
>>>> +	 */
>>>> +	if (READ_ONCE(ns->binfmt_ns) == NULL) {
>>>> +		struct binfmt_namespace *new_ns;
>>>> +
>>>> +		new_ns = kmalloc(sizeof(struct binfmt_namespace),
>>>> +				 GFP_KERNEL);
>>>> +		if (new_ns == NULL)
>>>> +			return -ENOMEM;
>>>> +		INIT_LIST_HEAD(&new_ns->entries);
>>>> +		new_ns->enabled = 1;
>>>> +		rwlock_init(&new_ns->entries_lock);
>>>> +		new_ns->bm_mnt = NULL;
>>>> +		new_ns->entry_count = 0;
>>>> +		/* ensure new_ns is completely initialized before sharing it */
>>>> +		smp_wmb();
>>>
>>> (I haven't dived into patch logic, here just small barrier remark from quick sight).
>>> smp_wmb() has no sense without paired smp_rmb() on the read side. Possible,
>>> you want something like below in read hunk:
>>>
>>> +		b_ns = READ_ONCE(ns->binfmt_ns);
>>> +		if (b_ns) {
>>> +			smp_rmb();
>>> +			return b_ns;
>>> +		}
>>>
>>>
>>
>> The write barrier is here to ensure the structure is fully written
>> before we set the pointer.
>>
>> I don't understand how read barrier can change something at this level,
>> IMHO the couple WRITE_ONCE()/READ_ONCE() should be enough to ensure we
>> have correctly initialized the pointer and the structure when we read
>> the pointer back.
>>
>> I think the pointer itself is the "barrier" to access the memory
>> modified before.
> 
> smp_rmb() guarantees you see stores in the order you want. If you have:
> 
> [cpu0]					[cpu1]
> new_ns->entry_count = 0; 
> smp_wmb();
> WRITE_ONCE(ns->binfmt_ns, new_ns); 	b_ns = READ_ONCE(ns->binfmt_ns);
> 					smp_rmb();
> 					<access b_ns->entry_count>
> 
> smp_rmb() guarantees you see true entry_count on the cpu1. Without
> smp_rmb() you may see old value of new_ns->entry_count.
> 					
> See Documentation/memory-barriers.txt

Yes, I tried to read this document several times...

What I understand from example line 1077 (7696f9910a9a
Documentation/memory-barriers.txt) is we only need a data dependency
barrier, and as explained by Jann it comes with the READ_ONCE() (and is
only needed for alpha).

Thanks,
Laurent