netdev - Re: [PATCH v3 bpf-next 1/6] bpf: Create BPF_PROG_CGROUP_INET_EGRESS_RUN

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Date:   Tue, 28 May 2019 18:54:54 +0000
From:   Lawrence Brakmo <brakmo@...com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>
CC:     Martin Lau <kafai@...com>, Alexei Starovoitov <ast@...com>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 1/6] bpf: Create
 BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY

On 5/28/19, 6:43 AM, "netdev-owner@...r.kernel.org on behalf of Eric Dumazet" <netdev-owner@...r.kernel.org on behalf of eric.dumazet@...il.com> wrote:

    
    
    On 5/27/19 8:49 PM, brakmo wrote:
    > Create new macro BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY() to be used by
    > __cgroup_bpf_run_filter_skb for EGRESS BPF progs so BPF programs can
    > request cwr for TCP packets.
    > 
    
    ...
    
    > +#define BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY(array, ctx, func)		\
    > +	({						\
    > +		struct bpf_prog_array_item *_item;	\
    > +		struct bpf_prog *_prog;			\
    > +		struct bpf_prog_array *_array;		\
    > +		u32 ret;				\
    > +		u32 _ret = 1;				\
    > +		u32 _cn = 0;				\
    > +		preempt_disable();			\
    > +		rcu_read_lock();			\
    > +		_array = rcu_dereference(array);	\
    
    Why _array can not be NULL here ?

I replaced the call chain 
  BPF_CGROUP_RUN_PROG_INET_EGRESS()
    __cgroup_bpf_run_filter_skb()
      BPF_PROG_RUN_ARRAY()
        __BPF_PROG_RUN_ARRAY( , , false)

With
  BPF_CGROUP_RUN_PROG_INET_EGRESS()
    __cgroup_bpf_run_filter_skb()
      BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY()

BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY() is a instantiation of
__BPF_PROG_RUN_ARRAY() with the argument "check_non_null" = false
Hence I removed the test "if (unlikely(check_non_null && !_array))"
since it always fails. I have assumed the existing code is correct.

    
    > +		_item = &_array->items[0];		\
    > +		while ((_prog = READ_ONCE(_item->prog))) {		\
    > +			bpf_cgroup_storage_set(_item->cgroup_storage);	\
    > +			ret = func(_prog, ctx);		\
    > +			_ret &= (ret & 1);		\
    > +			_cn |= (ret & 2);		\
    > +			_item++;			\
    > +		}					\
    > +		rcu_read_unlock();			\
    > +		preempt_enable_no_resched();	
    
    Why are you using preempt_enable_no_resched() here ?

Because that is what __BPF_PROG_RUN_ARRAY() calls and the macro
BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY() is an instantiation of it
(with minor changes in the return value).
    
    	\
    > +		if (_ret)				\
    > +			_ret = (_cn ? NET_XMIT_CN : NET_XMIT_SUCCESS);	\
    > +		else					\
    > +			_ret = (_cn ? NET_XMIT_DROP : -EPERM);		\
    > +		_ret;					\
    > +	})
    > +
    >  #define BPF_PROG_RUN_ARRAY(array, ctx, func)		\
    >  	__BPF_PROG_RUN_ARRAY(array, ctx, func, false)
    >  
    >