netdev - Re: [PATCH stable 4.9 1/8] x86: bpf_jit: small optimization in emit_bpf_tail

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CANn89iKbXNBJcteAtZpkM=u5dwcTGALNWc9rektdz1BHVuGb+g@mail.gmail.com>
Date:   Mon, 29 Jan 2018 06:04:30 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Willy Tarreau <w@....eu>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH stable 4.9 1/8] x86: bpf_jit: small optimization in emit_bpf_tail_call()

On Sun, Jan 28, 2018 at 10:39 PM, Willy Tarreau <w@....eu> wrote:
> Hi,
>
> [ replaced stable@ and greg@ by netdev@ as my question below is not
>   relevant to stable ]
>
> On Mon, Jan 29, 2018 at 02:48:54AM +0100, Daniel Borkmann wrote:
>> From: Eric Dumazet <edumazet@...gle.com>
>>
>> [ upstream commit 84ccac6e7854ebbfb56d2fc6d5bef9be49bb304c ]
>>
>> Saves 4 bytes replacing following instructions :
>>
>> lea rax, [rsi + rdx * 8 + offsetof(...)]
>> mov rax, qword ptr [rax]
>> cmp rax, 0
>>
>> by :
>>
>> mov rax, [rsi + rdx * 8 + offsetof(...)]
>> test rax, rax
>
> I've just noticed this on stable@. If these 4 bytes matter, why not use
> cmpq with an immediate value instead, which saves 2 extra bytes ? :
>
>   - the mov above is 11 bytes total :
>
>    0:   48 8b 84 d6 78 56 34    mov    0x12345678(%rsi,%rdx,8),%rax
>    7:   12
>    8:   48 85 c0                test   %rax,%rax
>
>   - the equivalent cmp is only 9 bytes :
>
>    0:   48 83 bc d6 78 56 34    cmpq   $0x0,0x12345678(%rsi,%rdx,8)
>    7:   12 00
>
> And as a bonus, it doesn't even clobber rax.
>
> Just my two cents,


Hi Willy

Please look more closely at following instructions.

We need the value later, not only testing it being zero :)