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/
//Eelco
# echo > /sys/kernel/debug/tracing/trace
# ./test_progs -t fexit_bpf2bpf
#25 fexit_bpf2bpf:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
# cat /sys/kernel/debug/tracing/trace | tail -2
<...>-75365 [012] d... 313932.416780: bpf_trace_printk:
ENTRY val 123
<...>-75365 [012] d... 313932.416784: bpf_trace_printk:
EXIT val 123, ret 1
< SNIP>
