On 05 Mar 11:27, Ananyev, Konstantin wrote:
>
>
> >
> > On Thu, Mar 05, 2020 at 01:33:49AM +0800, ZY Qiu wrote:
> > > When compiling with -O0,
> > > the compiler does not optimize two memory accesses into one.
> > > Leads to accessing a null pointer when queue post Rx burst callback
> > > removal while traffic is running.
> > > See rte_eth_tx_burst function.
> > >
> > > Signed-off-by: ZY Qiu <tgw_t...@tencent.com>
> > > ---
> > > lib/librte_ethdev/rte_ethdev.h | 6 ++----
> > > 1 file changed, 2 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/lib/librte_ethdev/rte_ethdev.h
> > > b/lib/librte_ethdev/rte_ethdev.h
> > > index d1a593ad1..35eb580ff 100644
> > > --- a/lib/librte_ethdev/rte_ethdev.h
> > > +++ b/lib/librte_ethdev/rte_ethdev.h
> > > @@ -4388,10 +4388,8 @@ rte_eth_rx_burst(uint16_t port_id, uint16_t
> > > queue_id,
> > > rx_pkts, nb_pkts);
> > >
> > > #ifdef RTE_ETHDEV_RXTX_CALLBACKS
> > > - if (unlikely(dev->post_rx_burst_cbs[queue_id] != NULL)) {
> > > - struct rte_eth_rxtx_callback *cb =
> > > - dev->post_rx_burst_cbs[queue_id];
> > > -
> > > + struct rte_eth_rxtx_callback *cb = dev->post_rx_burst_cbs[queue_id];
> > > + if (unlikely(cb != NULL)) {
> > > do {
> > > nb_rx = cb->fn.rx(port_id, queue_id, rx_pkts, nb_rx,
> > > nb_pkts, cb->param);
> > > --
> > > 2.17.1
> > While I don't have an issue with this fix, can you explain as to why this
> > is a
> > problem that needs to be fixed? Normally TOCTOU issues are flagged and
> > fixed for external resources e.g. files, that can be modified between check
> > and use, but this is just referencing internal data in the program itself,
> > so I'm wondering what the risk is? From a security viewpoint if an attacker
> > can modify the function pointers in our code, is it not already "game over"
> > for keeping the running program safe?
> >
>
> Right now RX/TX cb functions are not protected by any sync mechanism.
> So while dataplane thread can do RX/TX control threads supposed to
> be able to add/remove callbacks.
> I am agree with Stephen here, we probably need either (volatile *)
> or compiler_barrier() here.
>
>
For my opinion,
the key question here is if the abstract layer code has to be thread safe
or application
developer look after thread safe of key data structure ?
1. Single thread case :
Current code has no issue even compiler behavior is different with
-O0 or O3.
-O3 merge 2 loads into 1, -O0 still use 2 loads.
2. Multiple thread case:
As Konstantin said, there is no sync primitive to protect cb
pointer at all.
Because of X86 64bit memory access is atomic, then, -O3 and -O0
will lead to totally different result.
I don’t think that's a fix because a Fix cannot depend on
specific Arch is strong memory order or weak memory order.
Volatile or memory barrier may not fix this with a general style for
multi-threads.
I will suggest add comment to clarify the scenario and let developer make
decision.
Regards
