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

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Message-ID: <7E549AA1-07A8-456E-8372-41242143582D@fb.com>
Date:   Fri, 8 Nov 2019 17:59:38 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <ast@...com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        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 Nov 8, 2019, at 9:57 AM, Alexei Starovoitov <ast@...com> wrote:
> 
> 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 ?

I see. I missed the multiple call sites part. 

Thanks for the explanation. 
Song