Messages in this thread | | | Subject | Re: [PATCH net-next v4] skb_expand_head() adjust skb->truesize incorrectly | From | Vasily Averin <> | Date | Thu, 2 Sep 2021 10:33:09 +0300 |
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On 9/2/21 10:13 AM, Vasily Averin wrote: > On 9/2/21 7:48 AM, Eric Dumazet wrote: >> On 9/1/21 9:32 PM, Eric Dumazet wrote: >>> I think you missed netem case, in particular >>> skb_orphan_partial() which I already pointed out. >>> >>> You can setup a stack of virtual devices (tunnels), >>> with a qdisc on them, before ip6_xmit() is finally called... >>> >>> Socket might have been closed already. >>> >>> To test your patch, you could force a skb_orphan_partial() at the beginning >>> of skb_expand_head() (extending code coverage) >> >> To clarify : >> >> It is ok to 'downgrade' an skb->destructor having a ref on sk->sk_wmem_alloc to >> something owning a ref on sk->refcnt. >> >> But the opposite operation (ref on sk->sk_refcnt --> ref on sk->sk_wmem_alloc) is not safe. > > Could you please explain in more details, since I stil have a completely opposite point of view? > > Every sk referenced in skb have sk_wmem_alloc > 9 > It is assigned to 1 in sk_alloc and decremented right before last __sk_free(), > inside both sk_free() sock_wfree() and __sock_wfree() > > So it is safe to adjust skb->sk->sk_wmem_alloc, > because alive skb keeps reference to alive sk and last one keeps sk_wmem_alloc > 0 > > So any destructor used sk->sk_refcnt will already have sk_wmem_alloc > 0, > because last sock_put() calls sk_free(). > > However now I'm not sure in reversed direction. > skb_set_owner_w() check !sk_fullsock(sk) and call sock_hold(sk); > If sk->sk_refcnt can be 0 here (i.e. after execution of old destructor inside skb_orphan) > -- it can be trigger pointed problem: > "refcount_add() will trigger a warning (panic under KASAN)". > > Could you please explain where I'm wrong?
To clarify: I'm agree it is unsafe to call on alive skb: skb_orphan(skb) adjust(skb_>sk->sk_wmem_alloc)
becasue 2 reasone: 1) old destructor can decrease sk_vmem_alloc to zero and free sk 2) becasue old destructor if !sk_fullsock(sk) can call sock_out and release last sk->sk_refcnt reference. in this case sock_hold() will trigger warning.
1) can be handled, we can adjust(sk_wmem_alloc) before skb_orphan() but I badly understand how to handle 2nd case.
Thank you, Vasily Averin
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