Subject: Re: [RFC v3 03/22] bpf,landlock: Add a new arraymap type to deal with (Landlock) handles

From: Mickaël Salaün <[email-masked]>
Date: Thu, 15 Sep 2016 23:51:36 +0200

On 15/09/2016 01:28, Alexei Starovoitov wrote:
> On Thu, Sep 15, 2016 at 01:22:49AM +0200, Mickaël Salaün wrote:
>>
>> On 14/09/2016 20:51, Alexei Starovoitov wrote:
>>> On Wed, Sep 14, 2016 at 09:23:56AM +0200, Mickaël Salaün wrote:
>>>> This new arraymap looks like a set and brings new properties:
>>>> * strong typing of entries: the eBPF functions get the array type of
>>>>   elements instead of CONST_PTR_TO_MAP (e.g.
>>>>   CONST_PTR_TO_LANDLOCK_HANDLE_FS);
>>>> * force sequential filling (i.e. replace or append-only update), which
>>>>   allow quick browsing of all entries.
>>>>
>>>> This strong typing is useful to statically check if the content of a map
>>>> can be passed to an eBPF function. For example, Landlock use it to store
>>>> and manage kernel objects (e.g. struct file) instead of dealing with
>>>> userland raw data. This improve efficiency and ensure that an eBPF
>>>> program can only call functions with the right high-level arguments.
>>>>
>>>> The enum bpf_map_handle_type list low-level types (e.g.
>>>> BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD) which are identified when
>>>> updating a map entry (handle). This handle types are used to infer a
>>>> high-level arraymap type which are listed in enum bpf_map_array_type
>>>> (e.g. BPF_MAP_ARRAY_TYPE_LANDLOCK_FS).
>>>>
>>>> For now, this new arraymap is only used by Landlock LSM (cf. next
>>>> commits) but it could be useful for other needs.
>>>>
>>>> Changes since v2:
>>>> * add a RLIMIT_NOFILE-based limit to the maximum number of arraymap
>>>>   handle entries (suggested by Andy Lutomirski)
>>>> * remove useless checks
>>>>
>>>> Changes since v1:
>>>> * arraymap of handles replace custom checker groups
>>>> * simpler userland API
>>>>
>>>> Signed-off-by: Mickaël Salaün <[email-masked]>
>>>> Cc: Alexei Starovoitov <[email-masked]>
>>>> Cc: Andy Lutomirski <[email-masked]>
>>>> Cc: Daniel Borkmann <[email-masked]>
>>>> Cc: David S. Miller <[email-masked]>
>>>> Cc: Kees Cook <[email-masked]>
>>>> Link: https://lkml.kernel.org/r/[email-masked]
>>>> ---
>>>>  include/linux/bpf.h      |  14 ++++
>>>>  include/uapi/linux/bpf.h |  18 +++++
>>>>  kernel/bpf/arraymap.c    | 203 +++++++++++++++++++++++++++++++++++++++++++++++
>>>>  kernel/bpf/verifier.c    |  12 ++-
>>>>  4 files changed, 246 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>>>> index fa9a988400d9..eae4ce4542c1 100644
>>>> --- a/include/linux/bpf.h
>>>> +++ b/include/linux/bpf.h
>>>> @@ -13,6 +13,10 @@
>>>>  #include <linux/percpu.h>
>>>>  #include <linux/err.h>
>>>>  
>>>> +#ifdef CONFIG_SECURITY_LANDLOCK
>>>> +#include <linux/fs.h> /* struct file */
>>>> +#endif /* CONFIG_SECURITY_LANDLOCK */
>>>> +
>>>>  struct perf_event;
>>>>  struct bpf_map;
>>>>  
>>>> @@ -38,6 +42,7 @@ struct bpf_map_ops {
>>>>  struct bpf_map {
>>>>  	atomic_t refcnt;
>>>>  	enum bpf_map_type map_type;
>>>> +	enum bpf_map_array_type map_array_type;
>>>>  	u32 key_size;
>>>>  	u32 value_size;
>>>>  	u32 max_entries;
>>>> @@ -187,6 +192,9 @@ struct bpf_array {
>>>>  	 */
>>>>  	enum bpf_prog_type owner_prog_type;
>>>>  	bool owner_jited;
>>>> +#ifdef CONFIG_SECURITY_LANDLOCK
>>>> +	u32 n_entries;	/* number of entries in a handle array */
>>>> +#endif /* CONFIG_SECURITY_LANDLOCK */
>>>>  	union {
>>>>  		char value[0] __aligned(8);
>>>>  		void *ptrs[0] __aligned(8);
>>>> @@ -194,6 +202,12 @@ struct bpf_array {
>>>>  	};
>>>>  };
>>>>  
>>>> +#ifdef CONFIG_SECURITY_LANDLOCK
>>>> +struct map_landlock_handle {
>>>> +	u32 type; /* enum bpf_map_handle_type */
>>>> +};
>>>> +#endif /* CONFIG_SECURITY_LANDLOCK */
>>>> +
>>>>  #define MAX_TAIL_CALL_CNT 32
>>>>  
>>>>  struct bpf_event_entry {
>>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>>>> index 7cd36166f9b7..b68de57f7ab8 100644
>>>> --- a/include/uapi/linux/bpf.h
>>>> +++ b/include/uapi/linux/bpf.h
>>>> @@ -87,6 +87,15 @@ enum bpf_map_type {
>>>>  	BPF_MAP_TYPE_PERCPU_ARRAY,
>>>>  	BPF_MAP_TYPE_STACK_TRACE,P_TYPE_CGROUP_ARRAY
>>>>  	BPF_MAP_TYPE_CGROUP_ARRAY,
>>>> +	BPF_MAP_TYPE_LANDLOCK_ARRAY,
>>>> +};
>>>> +
>>>> +enum bpf_map_array_type {
>>>> +	BPF_MAP_ARRAY_TYPE_UNSPEC,
>>>> +};
>>>> +
>>>> +enum bpf_map_handle_type {
>>>> +	BPF_MAP_HANDLE_TYPE_UNSPEC,
>>>>  };
>>>
>>> missing something. why it has to be special to have it's own
>>> fd array implementation?
>>> Please take a look how BPF_MAP_TYPE_PERF_EVENT_ARRAY, 
>>> BPF_MAP_TYPE_CGROUP_ARRAY and BPF_MAP_TYPE_PROG_ARRAY are done.
>>> The all store objects into array map that user space passes via FD.
>>> I think the same model should apply here.
>>
>> The idea is to have multiple way for userland to describe a resource
>> (e.g. an open file descriptor, a path or a glob pattern). The kernel
>> representation could then be a "struct path *" or dedicated types (e.g.
>> custom glob).
> 
> hmm. I think user space api should only deal with FD. Everything
> else is user space job to encapsulate/hide.

How would you create a FD referring to a glob, a user or port ranges for
example ?

> 
>> Another interesting point (that could replace
>> check_map_func_compatibility()) is that BPF_MAP_TYPE_LANDLOCK_ARRAY
>> translate to dedicated (abstract) types (instead of CONST_PTR_TO_MAP)
>> thanks to bpf_reg_type_from_map(). This is useful to abstract userland
>> (map) interface with kernel object(s) dealing with that type.
> 
> I probably missing something. If user space interface is FD,
> to the kernel they're different object types. Nothing else.

Yes but what if there is more than one way to express a resource (cf.
previous comment). A FD can refer to an *existing file* but a glob
pattern could match a bunch of files (existing or not). This was a
concern for Kees Cook and James Morris [1].

[1]
https://lkml.kernel.org/r/[email-masked]

> 
>> A third point is that BPF_MAP_TYPE_LANDLOCK_ARRAY is a kind of set. It
>> is optimized to quickly walk through all the elements in a sequential way.
> 
> why set is any faster to walk vs array?

It is an array with only sequential entries (i.e. no hole in the array).