netdev - Re: [PATCH net-next 6/8] tools: bpftool: print all relevant byte opcodes for "load double word"

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Message-ID: <CAH3iqT6Dj5x7dnz5_k3r-EQai=5ebn=JHEKMN+jJDm6aGQdqdQ@mail.gmail.com>
Date:   Fri, 20 Oct 2017 17:41:55 +0100
From:   Quentin Monnet <quentin.monnet@...ronome.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     David Laight <David.Laight@...lab.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "oss-drivers@...ronome.com" <oss-drivers@...ronome.com>
Subject: Re: [PATCH net-next 6/8] tools: bpftool: print all relevant byte
 opcodes for "load double word"

On 20 October 2017 at 17:12, Daniel Borkmann <daniel@...earbox.net> wrote:
> On 10/20/2017 05:50 PM, Quentin Monnet wrote:
> [...]
>>
>> The remaining four bytes are taken from the "immediate" field of the
>> second
>> instruction, which leaves the first four fields (offset, source and
>> destination
>> registers, and in particular opcode) unused. As far as I know, these
>> fields
>> remain at zero, and this makes it the only “instruction” to have a null
>> code
>> (although I am not sure this is a strict requirement, because I did not
>> find
>> the code in the verifier that would reject a program having a non-null
>> opcode
>> right after a "load double word immediate" instruction).
>
>
> It's in replace_map_fd_with_map_ptr(), invalid insns for the 2nd part
> are rejected there, they have to be otherwise it's not extendable anymore
> from abi pov; check also 'test1* ld_imm64' in the verifier test cases.
>
> Cheers,
> Daniel

Indeed, thanks Daniel!
Quentin