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Date: Sun, 7 Aug 2022 23:17:54 +0300 From: Kirill Tkhai <tkhai@...ru> To: Eric Dumazet <edumazet@...gle.com> Cc: Paolo Abeni <pabeni@...hat.com>, "David S. Miller" <davem@...emloft.net>, Linux Kernel Network Developers <netdev@...r.kernel.org> Subject: Re: [PATCH] net: skb content must be visible for lockless skb_peek() and its variations On 01.08.2022 21:45, Kirill Tkhai wrote: > On 01.08.2022 10:39, Eric Dumazet wrote: >> On Mon, Aug 1, 2022 at 9:00 AM Kirill Tkhai <tkhai@...ru> wrote: >>> >>> On 01.08.2022 09:52, Paolo Abeni wrote: >>>> On Sun, 2022-07-31 at 23:39 +0300, Kirill Tkhai wrote: >>>>> From: Kirill Tkhai <tkhai@...ru> >>>>> >>>>> Currently, there are no barriers, and skb->xxx update may become invisible on cpu2. >>>>> In the below example var2 may point to intial_val0 instead of expected var1: >>>>> >>>>> [cpu1] [cpu2] >>>>> skb->xxx = initial_val0; >>>>> ... >>>>> skb->xxx = var1; skb = READ_ONCE(prev_skb->next); >>>>> <no barrier> <no barrier> >>>>> WRITE_ONCE(prev_skb->next, skb); var2 = skb->xxx; >>>>> >>>>> This patch adds barriers and fixes the problem. Note, that __skb_peek() is not patched, >>>>> since it's a lowlevel function, and a caller has to understand the things it does (and >>>>> also __skb_peek() is used under queue lock in some places). >>>>> >>>>> Signed-off-by: Kirill Tkhai <tkhai@...ru> >>>>> --- >>>>> Hi, David, Eric and other developers, >>>>> >>>>> picking unix sockets code I found this problem, >>>> >>>> Could you please report exactly how/where the problem maifests (e.g. >>>> the involved call paths/time sequence)? >>> >>> I didn't get why call paths in the patch description are not enough for you. Please, explain >>> what you want. >>> >>>>> and for me it looks like it exists. If there >>>>> are arguments that everything is OK and it's expected, please, explain. >>>> >>>> I don't see why such barriers are needed for the locked peek/tail >>>> variants, as the spin_lock pair implies a full memory barrier. >>> >>> This is for lockless skb_peek() calls and the patch is called in that way :). For locked skb_peek() >>> this is not needed. I'm not sure we need separate skb_peek() and skb_peek_lockless(). Do we? >> >> We prefer explicit _lockless variants to document the precise points >> they are needed. >> >> A new helper (and its initial usage) will clearly point to the problem >> you saw in af_unix. > > The problem is: > > unix_stream_sendmsg() unix_stream_read_generic() > skb->len = size; skb = skb_peek(); > skb_queue_tail(skb); unix_skb_len(skb); <- here we read wrong len Oh, there are unix_state_lock(). Please, ignore this patch...
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