On 27 Jul 2020, at 16:53, Jiri Olsa wrote:
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
Excellent patch ;) It works with xdp_md as well, and even better it does
not require the bpf_probe_read_kernel(), so the test_xdp_bpf2bpf.c code
just works.
Are you planning to send the patch upstream?
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) {
