Hi,
On 11/10/2023 3:55 AM, Paul E. McKenney wrote:
> On Thu, Nov 09, 2023 at 07:55:50AM -0800, Alexei Starovoitov wrote:
>> On Wed, Nov 8, 2023 at 11:26 PM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote:
>>> Hi,
>>>
>>> On 11/9/2023 2:36 PM, Martin KaFai Lau wrote:
>>>> On 11/7/23 6:06 AM, Hou Tao wrote:
>>>>> From: Hou Tao <houtao1@xxxxxxxxxx>
>>>>>
>>>>> bpf_map_of_map_fd_get_ptr() will convert the map fd to the pointer
>>>>> saved in map-in-map. bpf_map_of_map_fd_put_ptr() will release the
>>>>> pointer saved in map-in-map. These two helpers will be used by the
>>>>> following patches to fix the use-after-free problems for map-in-map.
>>>>>
>>>>> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
>>>>> ---
>>>>> kernel/bpf/map_in_map.c | 51 +++++++++++++++++++++++++++++++++++++++++
>>>>> kernel/bpf/map_in_map.h | 11 +++++++--
>>>>> 2 files changed, 60 insertions(+), 2 deletions(-)
>>>>>
>>>>>
>>> SNIP
>>>>> +void bpf_map_of_map_fd_put_ptr(void *ptr, bool need_defer)
>>>>> +{
>>>>> + struct bpf_inner_map_element *element = ptr;
>>>>> +
>>>>> + /* Do bpf_map_put() after a RCU grace period and a tasks trace
>>>>> + * RCU grace period, so it is certain that the bpf program which is
>>>>> + * manipulating the map now has exited when bpf_map_put() is
>>>>> called.
>>>>> + */
>>>>> + if (need_defer)
>>>> "need_defer" should only happen from the syscall cmd? Instead of
>>>> adding rcu_head to each element, how about
>>>> "synchronize_rcu_mult(call_rcu, call_rcu_tasks)" here?
>>> No. I have tried the method before, but it didn't work due to dead-lock
>>> (will mention that in commit message in v2). The reason is that bpf
>>> syscall program may also do map update through sys_bpf helper. Because
>>> bpf syscall program is running with sleep-able context and has
>>> rcu_read_lock_trace being held, so call synchronize_rcu_mult(call_rcu,
>>> call_rcu_tasks) will lead to dead-lock.
>> Dead-lock? why?
>>
>> I think it's legal to do call_rcu_tasks_trace() while inside RCU CS
>> or RCU tasks trace CS.
> Just confirming that this is the case. If invoking call_rcu_tasks_trace()
> within under either rcu_read_lock() or rcu_read_lock_trace() deadlocks,
> then there is a bug that needs fixing. ;-)
The case for dead-lock is that calling synchronize_rcu_mult(call_rcu,
call_rcu_tasks_trace) within under rcu_read_lock_trace() and I think it
is expected. The case that calling call_rcu_tasks_trace() with
rcu_read_lock_trace() being held is OK.
>
> Thanx, Paul
>
> .
