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

On Tue, Sep 15, 2020 at 8:03 AM Daniel Borkmann [off-list ref] wrote:

On 9/15/20 6:39 AM, Alexei Starovoitov wrote:

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On Fri, Sep 11, 2020 at 08:59:27PM +0200, Maciej Fijalkowski wrote:

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On Thu, Sep 03, 2020 at 12:51:14PM -0700, Alexei Starovoitov wrote:

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On Wed, Sep 02, 2020 at 10:08:15PM +0200, Maciej Fijalkowski wrote:

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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).

Good point. I think it's easier to allow ld_abs though.
The comment in check_ld_abs() is obsolete after gen_ld_abs() was added.
The verifier needs to check that subprog that is doing ld_abs or tail_call
has 'int' return type and check_reference_leak() doesn't error before
ld_abs and before bpf_tail_call.
In that sense doing bpf_tail_call from subprog has the same issues as ld_abs
(reference leaks and int return requirement)