netdev - Re: [PATCH] net: initialize msg.msg

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Message-ID: <CAG_fn=VyD6eNP6k-4q16ydy4h7VKWxQY9Pa9We_28GWQezVZUw@mail.gmail.com>
Date:   Tue, 7 Mar 2017 16:24:04 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Dmitriy Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: initialize msg.msg_flags in recvfrom

On Tue, Mar 7, 2017 at 3:26 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2017-03-07 at 14:58 +0100, Alexander Potapenko wrote:
>> KMSAN (KernelMemorySanitizer, a new error detection tool) reports use
>> of uninitialized memory in put_cmsg()):
>
> I would prefer that you do not put the stack trace in the changelog,
> same for the reproducer since this has little value in understanding the
> impact.
Understood.
Should be ok to put the report/reproducer below the triple dash, right?

> It looks like a false positive, but you do not say.
Ah, now I see.
Irrespective of the value of (MSG_CMSG_COMPAT & msg->msg_flags) the
code will return 0 either directly from put_cmsg(), or from
put_cmsg_compat().
I wouldn't call this a false positive, as KMSAN can't possibly know
that both branches taken depending on the uninitialized condition are
safe.
But I can imagine this to be less of an problem to the code owners who
do know that :)

> recvmsg() does not care about msg.msg_flags, only KMSAN.
>
> (The important part is that msg.msg_control and msg.msg_controllen are
> 0)
>
> Fine to avoid the false positive, but better be explicit in the
> changelog and says there is no visible effect for this bug.
Ok, I'll change the description.
> If there is a visible effect, please state so instead of technical
> details.
>
> We try to reduce S/N in the changelogs ;)
>
> Thanks a lot !
>
>



-- 
Alexander Potapenko
Software Engineer

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