Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> writes:
> On 2022-03-03 14:59:47 [+0100], Toke Høiland-Jørgensen wrote:
>> Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> writes:
>>
>> > Since the commit mentioned below __xdp_reg_mem_model() can return a NULL
>> > pointer. This pointer is dereferenced in trace_mem_connect() which leads
>> > to segfault. It can be reproduced with enabled trace events during ifup.
>> >
>> > Only assign the arguments in the trace-event macro if `xa' is set.
>> > Otherwise set the parameters to 0.
>> >
>> > Fixes: 4a48ef70b93b8 ("xdp: Allow registering memory model without rxq reference")
>> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
>>
>> Hmm, so before the commit you mention, the tracepoint wasn't triggered
>> at all in the code path that now sets xdp_alloc is NULL. So I'm
>> wondering if we should just do the same here? Is the trace event useful
>> in all cases?
>
> Correct. It says:
> | ip-1230 [003] ..... 3.053473: mem_connect: mem_id=0 mem_type=PAGE_SHARED allocator=0000000000000000 ifindex=2
>
>> Alternatively, if we keep it, I think the mem.id and mem.type should be
>> available from rxq->mem, right?
>
> Yes, if these are the same things. In my case they are also 0:
>
> | ip-1245 [000] ..... 3.045684: mem_connect: mem_id=0 mem_type=PAGE_SHARED allocator=0000000000000000 ifindex=2
> | ifconfig-1332 [003] ..... 21.030879: mem_connect: mem_id=0 mem_type=PAGE_SHARED allocator=0000000000000000 ifindex=3
>
> So depends on what makes sense that tp can be skipped for xa == NULL or
> remain with
> __entry->mem_id = rxq->mem.id;
> __entry->mem_type = rxq->mem.type;
> __entry->allocator = xa ? xa->allocator : NULL;
>
> if it makes sense.
Right, looking at the code again, the id is only assigned in the path
that doesn't return NULL from __xdp_reg_mem_model().
Given that the trace points were put in specifically to be able to pair
connect/disconnect using the IDs, I don't think there's any use to
creating the events if there's no ID, so I think we should fix it by
skipping the trace event entirely if xdp_alloc is NULL.
-Toke
