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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 7 Sep 2017 10:51:21 -0700
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Subject: xt_hashlimig build error (was Re: [RFC 01/17] x86/asm/64: Remove the
 restore_c_regs_and_iret label)
To: Ingo Molnar <mingo@kernel.org>, Igor Lubashev <ilubashe@akamai.com>,
        Josh Hunt <johunt@akamai.com>, Vishwanath Pai <vpai@akamai.com>,
        Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Juergen Gross <jgross@suse.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "David S. Miller" <davem@davemloft.net>
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On Thu, Sep 7, 2017 at 3:29 AM, Ingo Molnar <mingo@kernel.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