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

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Message-ID: <59EA2077.7000901@iogearbox.net>
Date:   Fri, 20 Oct 2017 18:12:39 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Quentin Monnet <quentin.monnet@...ronome.com>
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 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