Subject: Re: [RFC v2 08/10] landlock: Handle file system comparisons

From: Mickaël Salaün <[email-masked]>
Date: Thu, 25 Aug 2016 16:10:45 +0200

On 25/08/2016 13:12, Andy Lutomirski wrote:
> On Thu, Aug 25, 2016 at 3:32 AM, Mickaël Salaün <[email-masked]> wrote:
>> Add eBPF functions to compare file system access with a Landlock file
>> system handle:
>> * bpf_landlock_cmp_fs_prop_with_struct_file(prop, map, map_op, file)
>>   This function allows to compare the dentry, inode, device or mount
>>   point of the currently accessed file, with a reference handle.
>> * bpf_landlock_cmp_fs_beneath_with_struct_file(opt, map, map_op, file)
>>   This function allows an eBPF program to check if the current accessed
>>   file is the same or in the hierarchy of a reference handle.
>>
>> The goal of file system handle is to abstract kernel objects such as a
>> struct file or a struct inode. Userland can create this kind of handle
>> thanks to the BPF_MAP_UPDATE_ELEM command. The element is a struct
>> landlock_handle containing the handle type (e.g.
>> BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD) and a file descriptor. This could
>> also be any descriptions able to match a struct file or a struct inode
>> (e.g. path or glob string).
> 
> This needs Eric's opinion.
> 
> Also, where do all the struct file *'s get stashed?  Are they
> preserved in the arraymap?  What prevents reference cycles or absurdly
> large numbers of struct files getting pinned?

Yes, the struct file are kept in the arraymap and dropped when there is
no more reference on them. Currently, the limitations are the maximum
number of open file descriptors referring to an arraymap and the maximum
number of eBPF Landlock programs loaded in a process
(LANDLOCK_PROG_LIST_MAX_PAGES in kernel/seccomp.c).

What kind of reference cycles have you in mind?

It probably needs another limit for kernel object references as well.
What is the best option here? Add another static limitation or use an
existing one?

 Mickaël