netdev - Re: [PATCH v7 bpf-next 7/7] selftests: bpf: add dummy prog for bpf2bpf with tailcall

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Message-ID: <5bf5a63c-7607-a24d-7e14-e41caa84bfc3@iogearbox.net>
Date:   Tue, 15 Sep 2020 17:03:40 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     ast@...nel.org, bpf@...r.kernel.org, netdev@...r.kernel.org,
        bjorn.topel@...el.com, magnus.karlsson@...el.com
Subject: Re: [PATCH v7 bpf-next 7/7] selftests: bpf: add dummy prog for
 bpf2bpf with tailcall

On 9/15/20 6:39 AM, Alexei Starovoitov wrote:
> On Fri, Sep 11, 2020 at 08:59:27PM +0200, Maciej Fijalkowski wrote:
>> On Thu, Sep 03, 2020 at 12:51:14PM -0700, Alexei Starovoitov wrote:
>>> On Wed, Sep 02, 2020 at 10:08:15PM +0200, Maciej Fijalkowski wrote:
[...]
>>> Could you add few more tests to exercise the new feature more thoroughly?
>>> Something like tailcall3.c that checks 32 limit, but doing tail_call from subprog.
>>> And another test that consume non-trival amount of stack in each function.
>>> Adding 'volatile char arr[128] = {};' would do the trick.
>>
>> Yet another prolonged silence from my side, but not without a reason -
>> this request opened up a Pandora's box.
> 
> Great catch and thanks to our development practices! As a community we should
> remember this lesson and request selftests more often than not.

+1, speaking of pandora ... ;-) I recently noticed that we also have the legacy
ld_abs/ld_ind instructions. Right now check_ld_abs() gates them by bailing out
if env->subprog_cnt > 1, but that doesn't solve everything given the prog itself
may not have bpf2bpf calls, but it could get tail-called out of a subprog. We
need to reject such cases (& add selftests for it), otherwise this would be a
verifier bypass given they may implicitly exit the program (and then mismatch
the return type that the verifier was expecting).

Best,
Daniel