[PATCH v3 fanotify 2/2] samples/fanotify: Add a sample fanotify fiter

[Song Liu]

> On Nov 23, 2024, at 9:07 PM, Amir Goldstein <amir73il at gmail.com> wrote:
> 
> On Sat, Nov 23, 2024 at 12:00 AM Song Liu <song at kernel.org> wrote:

[...]

>> +}
>> +
>> +static void sample_filter_free(struct fanotify_filter_hook *filter_hook)
>> +{
>> +       struct fan_filter_sample_data *data = filter_hook->data;
>> +
>> +       path_put(&data->subtree_path);
>> +       kfree(data);
>> +}
>> +
> 
> Hi Song,
> 
> This example looks fine but it raises a question.
> This filter will keep the mount of subtree_path busy until the group is closed
> or the filter is detached.
> This is probably fine for many services that keep the mount busy anyway.
> 
> But what if this wasn't the intention?
> What if an Anti-malware engine that watches all mounts wanted to use that
> for configuring some ignore/block subtree filters?
> 
> One way would be to use a is_subtree() variant that looks for a
> subtree root inode
> number and then verifies it with a subtree root fid.
> A production subtree filter will need to use a variant of is_subtree()
> anyway that
> looks for a set of subtree root inodes, because doing a loop of is_subtree() for
> multiple paths is a no go.

Maybe some cache mechanism will be sufficient (and maybe also the
best we can do) in this case? 

Thanks,
Song

> 
> Don't need to change anything in the example, unless other people
> think that we do need to set a better example to begin with...