linux-kernel - Re: [PATCH] tcp: fix compiler array bounds warning on selective

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Message-ID: <CAErSpo4KOe5zBYNXDhdEFUMdhCBhs00CCvYHbW3071ZUdN6s6A@mail.gmail.com>
Date:	Fri, 11 Apr 2014 10:12:43 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	David Laight <David.Laight@...lab.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Florian Fainelli <f.fainelli@...il.com>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	James Morris <jmorris@...ei.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH] tcp: fix compiler array bounds warning on selective_acks[]

On Fri, Apr 11, 2014 at 2:59 AM, David Laight <David.Laight@...lab.com> wrote:
> Bjorn Helgaas
>> With -Werror=array-bounds, gcc v4.8.x warns that in tcp_sack_remove(), a
>> selective_acks[] "array subscript is above array bounds".
>>
>> I don't understand how gcc figures this out, or why we don't see similar
>> problems many other places, but this is the only fix I can figure out.
> ...
>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>> index 65cf90e063d5..65133b108236 100644
>> --- a/net/ipv4/tcp_input.c
>> +++ b/net/ipv4/tcp_input.c
>> @@ -4047,7 +4047,8 @@ static void tcp_sack_remove(struct tcp_sock *tp)
>>
>>                       /* Zap this SACK, by moving forward any other SACKS. */
>>                       for (i = this_sack+1; i < num_sacks; i++)
>> -                             tp->selective_acks[i-1] = tp->selective_acks[i];
>> +                             if (i < ARRAY_SIZE(tp->selective_acks))
>> +                                     tp->selective_acks[i-1] = tp->selective_acks[i];
>>                       num_sacks--;
>>                       continue;
>>               }
>
> You really shouldn't add that test every time around the loop.
>
> Try changing the loop so the assignment is:
>    tp->selective_acks[i] = tp->selective_acks[i + 1];
>
> or the loop test to:
>         i <= num_sacks - 1;
>
> Or beat up the gcc developers :-)

You're right, that *is* ugly.  And it seems that gcc-v4.9.x doesn't
complain about this, and that's good enough for my purposes, so I
withdraw this patch.

Thanks and sorry for the noise,
  Bjorn
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