netdev - Re: [PATCH net] xdp: xdp_mem_allocator can be NULL in trace_mem

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Date:   Thu, 03 Mar 2022 14:59:47 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Ingo Molnar <mingo@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yonghong Song <yhs@...com>
Subject: Re: [PATCH net] xdp: xdp_mem_allocator can be NULL in
 trace_mem_connect().

Sebastian Andrzej Siewior <bigeasy@...utronix.de> 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@...utronix.de>

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?

Alternatively, if we keep it, I think the mem.id and mem.type should be
available from rxq->mem, right?

-Toke