netdev - Re: [PATCH bpf-next v4 02/13] bpf: btf: Add BTF_KIND

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Message-ID: <70814462-13a0-f902-121e-8705f27e8f23@fb.com>
Date:   Fri, 9 Nov 2018 00:40:56 +0000
From:   Yonghong Song <yhs@...com>
To:     Edward Cree <ecree@...arflare.com>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Kernel Team <Kernel-team@...com>, Martin Lau <kafai@...com>
Subject: Re: [PATCH bpf-next v4 02/13] bpf: btf: Add BTF_KIND_FUNC



On 11/8/18 12:52 PM, Edward Cree wrote:
> On 08/11/18 20:36, Yonghong Song wrote:
>> This patch adds BTF_KIND_FUNC support to the type section.
>> BTF_KIND_FUNC is used to specify the signature of a
>> defined subprogram or the pointee of a function pointer.
>>
>> In BTF, the function type related data structures are
>>    struct bpf_param {
>>      __u32 name_off; /* parameter name */
>>      __u32 type; /* parameter type */
>>    };
>>    struct bpf_type {
>>      __u32 name_off; /* function name */
>>      __u32 info; /* BTF_KIND_FUNC and num of parameters (#vlen) */
>>      __u32 type; /* return type */
>>    }
>> The data layout of the function type:
>>    struct bpf_type
>>    #vlen number of bpf_param's
>>
>> For a defined subprogram with valid function body,
>>    . function name and all parameter names except the vararg
>>      must be valid C identifier.
> Given that there's an intention to support other frontends besides
>   C, what's the reason for this restriction?

This (C) is the typical usage today. If later on other frontend
generates bpf programs with more relaxed symbol name requirement,
we can certainly relax the rule.

> 
>> For the pointee of a function pointer,
>>    . function name and all parameter names will
>> have name_off = 0 to indicate a non-existing name.
> Why can't function pointer parameters have names?

Currently, both bcc and llvm does not retain function pointer
arguments in dwarf. For LLVM, the IR generation for function pointer
type discards the argument name. So I did the checking because
llvm does not generate it.

We can relax the restrictions later if the compiler starts
to keep argument names in the IR.

> E.g. imagine something like struct net_device_ops.  All those
>   function pointers have named parameters and that's relevant info
>   when debugging.
> 
> -Ed
>