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From: Rafael David Tinoco <rafaeldtinoco@ubuntu.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>,
	Vamsi Kodavanty <vamsi@araalinetworks.com>,
	bpf <bpf@vger.kernel.org>
Subject: Re: [BPF CO-RE clarification] Use CO-RE on older kernel versions.
Date: Wed, 17 Mar 2021 11:31:38 -0300	[thread overview]
Message-ID: <B4B873B5-464D-46D7-A0DC-841B08FA34AD@ubuntu.com> (raw)
In-Reply-To: <CFD47A17-D20D-49FB-A357-5476C8EE02AF@ubuntu.com>


>>
>>>> From what I see all the CO-RE relocations applied successfully (even
>>>> though all the offsets stayed the same, so presumably you compiled
>>>> your BPF program with vmlinux.h from the exact same kernel you are
>>>> running it on?). Are you sure that vmlinux image you are providing
>>>> corresponds to the actual kernel you are running on?
>
> FOUND the cause of the issue…
>

...

>
> bpf_check():
>
> if (log->len_total < 128 || log->len_total > UINT_MAX >> 8 || !log->level  
> || !log->ubuf)
>
> and a simple change in libbpf (mitigation of course):
>
>   attr.log_buf = 0;
>   attr.log_level = 0;
>   attr.log_size = 0;
>
> before
>
> fd = sys_bpf_prog_load(&attr, sizeof(attr));

With instrumented kernel… no changes to libbpf or code:

attr.log_buf = (nil)
attr.log_level = 0
attr.log_size = 0
load_attr.log_buf = 0x7fd1c0a92010
load_attr.log_level = 0
load_attr.log_size = 16777215
libbpf: load bpf program failed: Invalid argument
libbpf: failed to load program 'ip_set_create'
libbpf: failed to load object 'mine_bpf'
libbpf: failed to load BPF skeleton 'mine_bpf': -22
failed to load BPF object: -22

[   27.857858] MINE: BPF_PROG_TYPE_KPROBE KERNEL VERSION ISSUE
[   27.857993] MINE: LINUX_VERSION_CODE: 266002
[   27.858131] MINE: YOUR VERSION: 265984

2 problems here:

0) attr.kern_version

- looks like for some reason attr.kern_version is different from
the one running

- bypassing kernel BPF_KPROBE version check, I get:

1) load_attr has log_buf and log_size but not log_level for some reason.

- this triggers an issue in bpf_check() within kernel IF I bypass
the BPF_KPROBE version check (kerne 4.15).

NOW, If I hard-code attr.kern_version in bpf.c to:

attr.kern_version = 266002;
fd = sys_bpf_prog_load(&attr, sizeof(attr));

then

attr.log_buf = (nil)
attr.log_level = 0
attr.log_size = 0
load_attr.log_buf = (nil)
load_attr.log_level = 0
load_attr.log_size = 0
Tracing... Hit Ctrl-C to end.

I don’t have the 2 problems, as log_level is zeroed, together with
log_buf and log_size.

Any clue on why this happens ?

Thank you!

-rafaeldtinoco


  reply	other threads:[~2021-03-17 14:32 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-06 18:02 [BPF CO-RE clarification] Use CO-RE on older kernel versions Vamsi Kodavanty
2021-01-06 23:55 ` Andrii Nakryiko
2021-01-07 18:12   ` Vamsi Kodavanty
2021-01-07 18:52     ` Andrii Nakryiko
2021-01-07 22:45       ` Vamsi Kodavanty
2021-01-07 23:32         ` Andrii Nakryiko
2021-01-08  0:16           ` Vamsi Kodavanty
2021-01-08  1:31             ` Vamsi Kodavanty
2021-03-03 18:14               ` Rafael David Tinoco
2021-03-04  7:05                 ` Andrii Nakryiko
2021-03-04 13:10                   ` Arnaldo Carvalho de Melo
2021-03-05  6:32                   ` Rafael David Tinoco
     [not found]                     ` <67E3C788-2835-4793-8A9C-51C5D807C294@ubuntu.com>
2021-03-10  6:00                       ` Fwd: " Rafael David Tinoco
2021-03-10 19:19                       ` Andrii Nakryiko
2021-03-10 22:45                         ` Rafael David Tinoco
2021-03-12 18:36                           ` Andrii Nakryiko
2021-03-17  4:39                             ` Rafael David Tinoco
2021-03-17 14:31                               ` Rafael David Tinoco [this message]
2021-03-19  4:36                                 ` Andrii Nakryiko
2021-03-19  4:42                                   ` Rafael David Tinoco

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