Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:
> On Fri, Oct 04, 2019 at 07:22:41PM +0200, Toke Høiland-Jørgensen wrote:
>> From: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
> ...
>>
>> +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
