Re: [PATCH bpf-next V9 1/3] bpf: new helper to obtain namespace data from current task

[Yonghong Song <yhs@fb.com>]

On 8/20/19 8:10 AM, Carlos Antonio Neira Bustos wrote:
> Hi Yonghong,
> 
> Thanks for taking the time to review this.
> 
> 
>>> + *
>>> + *		**-EINVAL** if *size_of_pidns* is not valid or unable to get ns, pid
>>> + *		or tgid of the current task.
>>> + *
>>> + *		**-ECHILD** if /proc/self/ns/pid does not exists.
>>> + *
>>> + *		**-ENOTDIR** if /proc/self/ns does not exists.
>>
>> Let us remove ECHILD and ENOTDIR and replace it with ENOENT as I
>> described below.
>>
>> Please *do verify* what happens when namespaces or pid_ns are not
>> configured.
>>
> 
> 
> I have tested kernel configurations without namespace support and with
> namespace support but without pid namespaces, the helper returns -EINVAL
> on both cases, now it should return -ENOENT.

Indeed. -ENOENT is better.

> 
> 
>>> +struct bpf_pidns_info {
>>> +	__u32 dev;
>>
>> Please add a comment for dev for how device major and minor number are
>> derived. User space gets device major and minor number, they need to
>> compare to the corresponding major/minor numbers returned by this helper.
>>
>>> +	__u32 nsid;
>>> +	__u32 tgid;
>>> +	__u32 pid;
>>> +};
>>
> 
> What do you think of this comment ?
> 
> struct bpf_pidns_info {
> 	__u32 dev;	/* major/minor numbers from /proc/self/ns/pid.
> 			 * User space gets device major and minor numbers from
> 			 * the same device that need to be compared against the
> 			 * major/minor numbers returned by this helper.
> 			 */
> 	__u32 nsid;
> 	__u32 tgid;
> 	__u32 pid;
> };
> 

To be more specific, I like a comment similar to below in uapi bpf.h

struct bpf_cgroup_dev_ctx {
         /* access_type encoded as (BPF_DEVCG_ACC_* << 16) | 
BPF_DEVCG_DEV_* */
         __u32 access_type;
         __u32 major;
         __u32 minor;
};

Some like:
	/* dev encoded as (major << 8 | (minor & 0xff)) */

>>
>> Please put an empty line. As a general rule for readability,
>> put an empty line if control flow is interrupted, e.g., by
>> "return", "break" or "continue". At least this is what
>> I saw most in bpf mailing list.
>>
> I'll fix it in version 10.
> 
>>> +	len = strlen(pidns_path) + 1;
>>> +	memcpy((char *)tmp->name, pidns_path, len);
>>> +	tmp->uptr = NULL;
>>> +	tmp->aname = NULL;
>>> +	tmp->refcnt = 1;
>>> +	ret = filename_lookup(AT_FDCWD, tmp, 0, &kp, NULL);
>> Adding below to free kmem cache memory
>> 	kmem_cache_free(names_cachep, fname);
>>
> 
> I think we don't need to call kmem_cache_free as filename_lookup
> calls putname that calls kmem_cache_free.

Oh, right. Thanks for checking this.

> 
> 
> Thanks a lot for your help.
> 
> Bests
> 
>> In the above, we checked task_active_pid_ns().
>> If not returning NULL, we have a valid pid ns. So the above
>> filename_lookup should not go wrong. We can still keep
>> the error checking though.
>>
>>> +	if (ret) {
>>> +		memset((void *)pidns_info, 0, (size_t) size);
>>> +		return ret;
>>
>>
> 
> I think we could get rid of this.
> 
>