netdev - Re: [PATCH v3 bpf-next 14/18] bpf: Compare BTF types of functions arguments with actual types

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Message-ID: <a95217de-16b2-4150-51a1-513f190e2079@fb.com>
Date:   Fri, 8 Nov 2019 17:57:09 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Song Liu <songliubraving@...com>,
        Alexei Starovoitov <ast@...nel.org>
CC:     David Miller <davem@...emloft.net>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "x86@...nel.org" <x86@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 14/18] bpf: Compare BTF types of functions
 arguments with actual types

On 11/8/19 9:32 AM, Song Liu wrote:
> 
> 
>> On Nov 8, 2019, at 9:28 AM, Song Liu <songliubraving@...com> wrote:
>>
>>
>>
>>> On Nov 7, 2019, at 10:40 PM, Alexei Starovoitov <ast@...nel.org> wrote:
>>>
>>> Make the verifier check that BTF types of function arguments match actual types
>>> passed into top-level BPF program and into BPF-to-BPF calls. If types match
>>> such BPF programs and sub-programs will have full support of BPF trampoline. If
>>> types mismatch the trampoline has to be conservative. It has to save/restore
>>> all 5 program arguments and assume 64-bit scalars. If FENTRY/FEXIT program is
>>> attached to this program in the future such FENTRY/FEXIT program will be able
>>> to follow pointers only via bpf_probe_read_kernel().
>>>
>>> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>>
>> Acked-by: Song Liu <songliubraving@...com>
> 
> One nit though: maybe use "reliable" instead of "unreliable"
> 
> +struct bpf_func_info_aux {
> +	bool reliable;
> +};
> +
> 
> +	bool func_proto_reliable;
> 
> So the default value 0, is not reliable.

I don't see how this can work.
Once particular func proto was found unreliable the verifier won't keep 
checking. If we start with 'bool reliable = false'
how do you see the whole mechanism working ?
Say the first time the verifier analyzed the subroutine and everything
matches. Can it do reliable = true ? No. It has to check all other
callsites. Then it would need another variable and extra pass ?