Subject: Re: [PATCH net] xdp: xdp_mem_allocator can be NULL in trace_mem_connect().

From: Sebastian Andrzej Siewior <[email-masked]>
Date: Thu, 3 Mar 2022 15:12:33 +0100

On 2022-03-03 14:59:47 [+0100], Toke Høiland-Jørgensen wrote:
> Sebastian Andrzej Siewior <[email-masked]> 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 <[email-masked]>
> 
> 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.

> -Toke

Sebastian