Yonghong Song wrote: > > > On 11/9/22 1:52 PM, John Fastabend wrote: > > Allow xdp progs to read the net_device structure. Its useful to extract > > info from the dev itself. Currently, our tracing tooling uses kprobes > > to capture statistics and information about running net devices. We use > > kprobes instead of other hooks tc/xdp because we need to collect > > information about the interface not exposed through the xdp_md structures. > > This has some down sides that we want to avoid by moving these into the > > XDP hook itself. First, placing the kprobes in a generic function in > > the kernel is after XDP so we miss redirects and such done by the > > XDP networking program. And its needless overhead because we are > > already paying the cost for calling the XDP program, calling yet > > another prog is a waste. Better to do everything in one hook from > > performance side. > > > > Of course we could one-off each one of these fields, but that would > > explode the xdp_md struct and then require writing convert_ctx_access > > writers for each field. By using BTF we avoid writing field specific > > convertion logic, BTF just knows how to read the fields, we don't > > have to add many fields to xdp_md, and I don't have to get every > > field we will use in the future correct. > > > > For reference current examples in our code base use the ifindex, > > ifname, qdisc stats, net_ns fields, among others. With this > > patch we can now do the following, > > > > dev = ctx->rx_dev; > > net = dev->nd_net.net; > > > > uid.ifindex = dev->ifindex; > > memcpy(uid.ifname, dev->ifname, NAME); > > if (net) > > uid.inum = net->ns.inum; > > > > to report the name, index and ns.inum which identifies an > > interface in our system. > > In > https://lore.kernel.org/bpf/ad15b398-9069-4a0e-48cb-4bb651ec3088@xxxxxxxx/ > Namhyung Kim wanted to access new perf data with a helper. > I proposed a helper bpf_get_kern_ctx() which will get > the kernel ctx struct from which the actual perf data > can be retrieved. The interface looks like > void *bpf_get_kern_ctx(void *) > the input parameter needs to be a PTR_TO_CTX and > the verifer is able to return the corresponding kernel > ctx struct based on program type. > > The following is really hacked demonstration with > some of change coming from my bpf_rcu_read_lock() > patch set https://lore.kernel.org/bpf/20221109211944.3213817-1-yhs@xxxxxx/ > > I modified your test to utilize the > bpf_get_kern_ctx() helper in your test_xdp_md.c. > > With this single helper, we can cover the above perf > data use case and your use case and maybe others > to avoid new UAPI changes. hmm I like the idea of just accessing the xdp_buff directly instead of adding more fields. I'm less convinced of the kfunc approach. What about a terminating field *self in the xdp_md. Then we can use existing convert_ctx_access to make it BPF inlined and no verifier changes needed. Something like this quickly typed up and not compiled, but I think shows what I'm thinking. diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h index 94659f6b3395..10ebd90d6677 100644 --- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -6123,6 +6123,10 @@ struct xdp_md { __u32 rx_queue_index; /* rxq->queue_index */ __u32 egress_ifindex; /* txq->dev->ifindex */ + /* Last xdp_md entry, for new types add directly to xdp_buff and use + * BTF access. Reading this gives BTF access to xdp_buff. + */ + __bpf_md_ptr(struct xdp_buff *, self); }; /* DEVMAP map-value layout diff --git a/net/core/filter.c b/net/core/filter.c index bb0136e7a8e4..547e9576a918 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -9808,6 +9808,11 @@ static u32 xdp_convert_ctx_access(enum bpf_access_type type, *insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->dst_reg, offsetof(struct net_device, ifindex)); break; + case offsetof(struct xdp_md, self): + *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(struct xdp_buff, self), + si->dst_reg, si->src_reg, + offsetof(struct xdp_buff, 0)); + break; } return insn - insn_buf; Actually even that single insn conversion is a bit unnessary because should be enough to just change the type to the correct BTF_ID in the verifier and omit any instructions. But it wwould be a bit confusing for C side. Might be a good use for passing 'cast' info through to the verifier as an annotation so it could just do the BTF_ID cast for us without any insns.
