linux-kernel - Re: [PATCH] infiniband: avoid overflow warning

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Message-ID: <CAK8P3a04DR6NT5jXJSmcw_cDjuKQ-y6f1SQz0cD9Bp=OaoaAhQ@mail.gmail.com>
Date:   Mon, 31 Jul 2017 21:19:22 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "parav@...lanox.com" <parav@...lanox.com>,
        "monis@...lanox.com" <monis@...lanox.com>,
        "Michal.Kalderon@...ium.com" <Michal.Kalderon@...ium.com>,
        "sean.hefty@...el.com" <sean.hefty@...el.com>,
        "danielmicay@...il.com" <danielmicay@...il.com>,
        "Ariel.Elior@...ium.com" <Ariel.Elior@...ium.com>,
        "hal.rosenstock@...il.com" <hal.rosenstock@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "dledford@...hat.com" <dledford@...hat.com>,
        "noaos@...lanox.com" <noaos@...lanox.com>
Subject: Re: [PATCH] infiniband: avoid overflow warning

On Mon, Jul 31, 2017 at 6:17 PM, Bart Van Assche <Bart.VanAssche@....com> wrote:
> On Mon, 2017-07-31 at 18:04 +0200, Arnd Bergmann wrote:
>> On Mon, Jul 31, 2017 at 5:32 PM, Bart Van Assche <Bart.VanAssche@....com> wrote:
>> > So inetaddr_event() assigns AF_INET so .sin_family and gcc warns about code
>> > that is only executed if .sin_family == AF_INET6? Since this warning is the
>> > result of incorrect interprocedural analysis by gcc, shouldn't this be
>> > reported as a bug to the gcc authors?
>>
>> I think the interprocedural analysis here is just a little worse than it could
>> be, but it's not actually correct.  It's not gcc that prints the warning (if
>> it did, then I'd agree it would be a gcc bug) but the warning is triggered
>> intentionally by the fortified version of memcpy in include/linux/string.h.
>>
>> The problem as I understand it is that gcc cannot guarantee that it
>> tracks the value of addr->sa_family at  least as far as the size of the
>> stack object, and it has no strict reason to do so, so the inlined
>> rdma_ip2gid() will still contain both cases.
>
> Hello Arnd,
>
> Had you already considered to uninline the rdma_ip2gid() function?

Not really, that would prevent the normal optimization from happening,
so that would be worse than uninlining addr_event() as I tried.

It would of course get rid of the warning, so if you think that's a better
solution, I won't complain.

       Arnd