From: Jiri Olsa <jolsa@redhat.com>
To: Eelco Chaudron <echaudro@redhat.com>
Cc: Yonghong Song <yhs@fb.com>,
bpf@vger.kernel.org, netdev@vger.kernel.org, ast@kernel.org,
daniel@iogearbox.net, kafai@fb.com, songliubraving@fb.com,
andriin@fb.com, toke@redhat.com
Subject: Re: fentry/fexit attach to EXT type XDP program does not work
Date: Wed, 29 Jul 2020 10:09:05 +0200 [thread overview]
Message-ID: <20200729080905.GG1319041@krava> (raw)
In-Reply-To: <95AF8533-2C7D-4038-AD39-4C81DBF25551@redhat.com>
On Wed, Jul 29, 2020 at 08:23:56AM +0200, Eelco Chaudron wrote:
SNIP
> > > > > 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.
great ;-) will check on xdp_md
>
> Are you planning to send the patch upstream?
yep, I'll add some test for that and send it
thanks,
jirka
prev parent reply other threads:[~2020-07-29 8:15 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-08 14:11 fentry/fexit attach to EXT type XDP program does not work Eelco Chaudron
2020-06-08 16:58 ` Yonghong Song
2020-06-09 8:52 ` Eelco Chaudron
2020-07-26 12:24 ` Jiri Olsa
2020-07-27 7:59 ` Eelco Chaudron
2020-07-27 14:53 ` Jiri Olsa
2020-07-29 6:23 ` Eelco Chaudron
2020-07-29 8:09 ` Jiri Olsa [this message]
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