Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

[Will Drewry <wad@chromium.org>]

On Thu, Jan 12, 2012 at 8:50 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> On 01/11, Will Drewry wrote:
>>
>> This patch adds support for seccomp mode 2.  This mode enables dynamic
>> enforcement of system call filtering policy in the kernel as specified
>> by a userland task.  The policy is expressed in terms of a BPF program,
>> as is used for userland-exposed socket filtering.  Instead of network
>> data, the BPF program is evaluated over struct user_regs_struct at the
>> time of the system call (as retrieved using regviews).
>
> Cool ;)
>
> I didn't really read this patch yet, just one nit.
>
>> +#define seccomp_filter_init_task(_tsk) do { \
>> +     (_tsk)->seccomp.filter = NULL; \
>> +} while (0);
>
> Cosmetic and subjective, but imho it would be better to add inline
> functions instead of define's.

Refactoring it a bit to make that possible.  Since seccomp fork/init/free
never needs access to the whole task_structs, I'll just pass in what's
needed (and avoid the sched.h inclusion recursion).

Comments on the next round will most definitely be appreciated!

>> @@ -166,6 +167,7 @@ void free_task(struct task_struct *tsk)
>>       free_thread_info(tsk->stack);
>>       rt_mutex_debug_task_free(tsk);
>>       ftrace_graph_exit_task(tsk);
>> +     seccomp_filter_free_task(tsk);
>>       free_task_struct(tsk);
>>  }
>>  EXPORT_SYMBOL(free_task);
>> @@ -1209,6 +1211,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>>       /* Perform scheduler related setup. Assign this task to a CPU. */
>>       sched_fork(p);
>>
>> +     seccomp_filter_init_task(p);
>
> This doesn't look right or I missed something. something seccomp_filter_init_task()
> should be called right after dup_task_struct(), at least before copy process can
> fail.
>
> Otherwise copy_process()->free_fork()->seccomp_filter_free_task() can put
> current->seccomp.filter copied by arch_dup_task_struct().

Ah - makes sense!  I moved it under dup_task_struct before any goto's
to bad_fork_free.

>> +struct seccomp_filter {
>> +     struct kref usage;
>> +     struct pid *creator;
>
> Why? seccomp_filter->creator is never used, no?

Removing it. It is from a related patch I'm experimenting with (adding
optional tracehook support), but it has no bearing here.

Thanks - new patch revision incoming!
will