On Mon, Jul 27, 2020 at 09:59:14AM +0200, Eelco Chaudron wrote: > > > On 26 Jul 2020, at 14:24, Jiri Olsa wrote: > > > On Tue, Jun 09, 2020 at 10:52:34AM +0200, Eelco Chaudron wrote: > > > > SNIP > > > > > > > libbpf: failed to load object 'test_xdp_bpf2bpf' > > > > > libbpf: failed to load BPF skeleton 'test_xdp_bpf2bpf': -4007 > > > > > test_xdp_fentry_ext:FAIL:__load ftrace skeleton failed > > > > > #91 xdp_fentry_ext:FAIL > > > > > Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED > > > > > > > > > > Any idea what could be the case here? The same fentry/fexit attach > > > > > code works fine in the xdp_bpf2bpf.c tests case. > > > > > > <SNIP> > > > > > > > > I think this is not supported now. That is, you cannot attach a > > > > fentry > > > > trace > > > > to the EXT program. The current implementation for fentry > > > > program simply > > > > trying to find and match the signature of freplace program which by > > > > default > > > > is a pointer to void. > > > > > > > > It is doable in that in kernel we could recognize to-be-attached > > > > program > > > > is > > > > a freplace and further trace down to find the real signature. The > > > > related > > > > kernel function is btf_get_prog_ctx_type(). You can try to > > > > implement by > > > > yourself > > > > or I can have a patch for this once bpf-next opens. > > > > > > I’m not familiar with this area of the code, so if you could prepare > > > a patch > > > that would nice. > > > You can also send it to me before bpf-next opens and I can verify > > > it, and > > > clean up the self-test so it can be included as well. > > > > > > > hi, > > it seems that you cannot exten fentry/fexit programs, > > but it's possible to attach fentry/fexit to ext program. > > > > /* Program extensions can extend all program types > > * except fentry/fexit. The reason is the following. > > * The fentry/fexit programs are used for performance > > * analysis, stats and can be attached to any program > > * type except themselves. When extension program is > > * replacing XDP function it is necessary to allow > > * performance analysis of all functions. Both original > > * XDP program and its program extension. Hence > > * attaching fentry/fexit to BPF_PROG_TYPE_EXT is > > * allowed. If extending of fentry/fexit was allowed it > > * would be possible to create long call chain > > * fentry->extension->fentry->extension beyond > > * reasonable stack size. Hence extending fentry is not > > * allowed. > > */ > > > > I changed fexit_bpf2bpf.c test just to do a quick check > > and it seems to work: > > Hi Jiri this is exactly what I’m trying, however when you do this where the > first argument is a pointer to some context data which you are accessing > it’s failing in the verifier. > This is a link to the original email, which has a test patch attached that > will show the failure when trying to load/attach the fentry function and > access the context: > > https://lore.kernel.org/bpf/159162546868.10791.12432342618156330247.stgit@ebuild/ ok, I tried to trace ext program with __sk_buff argument and I can see the issue as well.. can't acess the skb argument patch below fixes it for me, I can access the skb pointer and its data via probe read, like: SEC("fexit/new_get_skb_ifindex") int BPF_PROG(fexit_new_get_skb_ifindex, int val, struct __sk_buff *skb, int var, int ret) { __u32 data; int err; bpf_printk("EXIT skb %p", skb); bpf_probe_read_kernel(&data, sizeof(data), &skb->data); bpf_printk("EXIT ret %d, data %p", err, data); return 0; } I think it should fix the xdp_md acess as well jirka --- diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index ee36b7f60936..2145329f7b1b 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -3828,6 +3828,10 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type, } info->reg_type = PTR_TO_BTF_ID; + + if (tgt_prog && tgt_prog->type == BPF_PROG_TYPE_EXT) + tgt_prog = tgt_prog->aux->linked_prog; + if (tgt_prog) { ret = btf_translate_to_vmlinux(log, btf, t, tgt_prog->type, arg); if (ret > 0) {
