Subject: Re: [PATCH bpf-next v2 1/5] bpf: Support injecting chain calls into BPF programs on load

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 08 Oct 2019 11:02:50 +0200

Alexei Starovoitov <[email-masked]> writes:

> On Mon, Oct 07, 2019 at 12:11:31PM +0200, Toke Høiland-Jørgensen wrote:
>> Alexei Starovoitov <[email-masked]> writes:
>> 
>> > On Fri, Oct 04, 2019 at 07:22:41PM +0200, Toke Høiland-Jørgensen wrote:
>> >> From: Toke Høiland-Jørgensen <[email-masked]>
>> >> 
>> >> This adds support for injecting chain call logic into eBPF programs before
>> >> they return. The code injection is controlled by a flag at program load
>> >> time; if the flag is set, the verifier will add code to every BPF_EXIT
>> >> instruction that first does a lookup into a chain call structure to see if
>> >> it should call into another program before returning. The actual calls
>> >> reuse the tail call infrastructure.
>> >> 
>> >> Ideally, it shouldn't be necessary to set the flag on program load time,
>> >> but rather inject the calls when a chain call program is first loaded.
>> >> However, rewriting the program reallocates the bpf_prog struct, which is
>> >> obviously not possible after the program has been attached to something.
>> >> 
>> >> One way around this could be a sysctl to force the flag one (for enforcing
>> >> system-wide support). Another could be to have the chain call support
>> >> itself built into the interpreter and JIT, which could conceivably be
>> >> re-run each time we attach a new chain call program. This would also allow
>> >> the JIT to inject direct calls to the next program instead of using the
>> >> tail call infrastructure, which presumably would be a performance win. The
>> >> drawback is, of course, that it would require modifying all the JITs.
>> >> 
>> >> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>> > ...
>> >>  
>> >> +static int bpf_inject_chain_calls(struct bpf_verifier_env *env)
>> >> +{
>> >> +	struct bpf_prog *prog = env->prog;
>> >> +	struct bpf_insn *insn = prog->insnsi;
>> >> +	int i, cnt, delta = 0, ret = -ENOMEM;
>> >> +	const int insn_cnt = prog->len;
>> >> +	struct bpf_array *prog_array;
>> >> +	struct bpf_prog *new_prog;
>> >> +	size_t array_size;
>> >> +
>> >> +	struct bpf_insn call_next[] = {
>> >> +		BPF_LD_IMM64(BPF_REG_2, 0),
>> >> +		/* Save real return value for later */
>> >> +		BPF_MOV64_REG(BPF_REG_6, BPF_REG_0),
>> >> +		/* First try tail call with index ret+1 */
>> >> +		BPF_MOV64_REG(BPF_REG_3, BPF_REG_0),
>> >> +		BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, 1),
>> >> +		BPF_RAW_INSN(BPF_JMP | BPF_TAIL_CALL, 0, 0, 0, 0),
>> >> +		/* If that doesn't work, try with index 0 (wildcard) */
>> >> +		BPF_MOV64_IMM(BPF_REG_3, 0),
>> >> +		BPF_RAW_INSN(BPF_JMP | BPF_TAIL_CALL, 0, 0, 0, 0),
>> >> +		/* Restore saved return value and exit */
>> >> +		BPF_MOV64_REG(BPF_REG_0, BPF_REG_6),
>> >> +		BPF_EXIT_INSN()
>> >> +	};
>> >
>> > How did you test it?
>> > With the only test from patch 5?
>> > +int xdp_drop_prog(struct xdp_md *ctx)
>> > +{
>> > +       return XDP_DROP;
>> > +}
>> >
>> > Please try different program with more than one instruction.
>> > And then look at above asm and think how it can be changed to
>> > get valid R1 all the way to each bpf_exit insn.
>> > Do you see amount of headaches this approach has?
>> 
>> Ah yes, that's a good point. It seems that I totally overlooked that
>> issue, somehow...
>> 
>> > The way you explained the use case of XDP-based firewall plus XDP-based
>> > IPS/IDS it's about "knows nothing" admin that has to deal with more than
>> > one XDP application on an unfamiliar server.
>> > This is the case of debugging.
>> 
>> This is not about debugging. The primary use case is about deploying
>> multiple, independently developed, XDP-enabled applications on the same
>> server.
>> 
>> Basically, we want the admin to be able to do:
>> 
>> # yum install MyIDS
>> # yum install MyXDPFirewall
>> 
>> and then have both of those *just work* in XDP mode, on the same
>> interface.
>> 
>> I originally started solving this in an XDP-specific way (v1 of this
>> patch set), but the reactions to that was pretty unanimous that this
>> could be useful as a general eBPF feature. Do you agree with this?
>
> Chaining in general is useful, but
> yum install ids
> yum install firewall
> is not.
>
> Say, xdp doesn't exist. Such ids and firewall will be using iptables.
> And they will collide and conflict all over it.

Yeah, and conventions have evolved to handle these conflicts (for
iptables: create your own chain and put all your rules there; for
connmark, masking off different bits so each application can have its
own mark, etc). As I explained in my reply to Daniel, this is the
minimum kernel infrastructure required to enable userspace applications
to play nice with each other...

-Toke