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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:

> On Fri, Oct 04, 2019 at 07:22:41PM +0200, Toke Høiland-Jørgensen wrote:
>> From: Toke Høiland-Jørgensen <toke@redhat.com>
>> 
>> 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 <toke@redhat.com>
> ...
>>  
>> +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?

-Toke