linux-kernel - Re: xt_hashlimig build error (was Re: [RFC 01/17] x86/asm/64: Remove the restore_c_regs_and

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Message-ID: <69b38985-e094-ddcc-6f7e-d6e5cc2c657e@akamai.com>
Date:   Thu, 7 Sep 2017 14:25:04 -0400
From:   Vishwanath Pai <vpai@...mai.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Igor Lubashev <ilubashe@...mai.com>,
        Josh Hunt <johunt@...mai.com>,
        Pablo Neira Ayuso <pablo@...filter.org>
Cc:     Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Pablo Neira Ayuso <pablo@...filter.org>
Subject: Re: xt_hashlimig build error (was Re: [RFC 01/17] x86/asm/64: Remove
 the restore_c_regs_and_iret label)

On 09/07/2017 01:51 PM, Linus Torvalds wrote:
> On Thu, Sep 7, 2017 at 3:29 AM, Ingo Molnar <mingo@...nel.org> wrote:
>>
>> not the best of kernels, 32-bit allyesconfig doesn't even appear to build:
>>
>>   net/netfilter/xt_hashlimit.o: In function `hashlimit_mt_common.isra.6':
>>   xt_hashlimit.c:(.text+0x1146): undefined reference to `__udivdi3'
> 
> I think this is due to commit bea74641e378 ("netfilter: xt_hashlimit:
> add rate match mode").
> 
> It adds a 64-bit divide in user2rate_bytes() afaik, and to make things
> worse it seems to be a really stupid one too.
> 
> Although I guess "worse" is not bad when the stupidity of it should
> mean that it's easy to avoid the 64-bit issue.
> 
> Oddly, user2rate() that actually *does* need a 64-bit divide, seems to
> do it right and use "div64_u64()" to do so.
> 
> But user2rate_bytes() could easily avoid any 64-bit issues, since it
> divides the max 32-bit (unsigned) number with a 64-bit unsigned
> number.
> 
> It would be easy to just say
> 
>  - "if high 32 bits are set, result is 0"
> 
>  - else do a 32-bit divide
> 
> or just use "div64_u64()" in that code too.
> 
> But honestly, that math is odd in other ways too (is that "r-1"
> _supposed_ to underflow to -1 for large 'user' counts?), so somebody
> needs to look at that logic.
> 
> And there might be some other 64-bit divide I missed, so please,
> netfilter people, check the 32-bit build.
> 
>                   Linus
> 

Sorry about the build failure, we have already queued up a fix for this:
https://patchwork.ozlabs.org/patch/810772/

I agree, this could've been easily avoided, I happened to overlook this
particular line. There are other places in xt_hashlimit where we use
64bit division and I believe we have already covered those cases using
div64_u64.

- Vishwanath