Re: fentry/fexit attach to EXT type XDP program does not work

["Eelco Chaudron" <echaudro@redhat.com>]

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>