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: <6667f710-68f3-b97e-b0eb-d9879476831e@akamai.com>
Date:   Thu, 7 Sep 2017 16:16:47 -0400
From:   Vishwanath Pai <vpai@...mai.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Igor Lubashev <ilubashe@...mai.com>,
        Josh Hunt <johunt@...mai.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        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>,
        Arnd Bergmann <arnd@...db.de>
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 02:43 PM, Linus Torvalds wrote:
> Note: that patch has *exactly* the issue I was talking about above.
> 
> Doing that
> 
>     if (user > 0xFFFFFFFFULL)
>         return 0;
> 
> is different from the old code, which used to result in a zero in the
> divide, and then
> 
>     r = (r - 1) << 4;
> 
> would cause it to return a large value.
> 
> So the patch in question doesn't just fix the build error, it
> completely changes the semantics of the function too.
> 
> I *think* the new behavior is likely what you want, but these kinds of
> things should be _described_.
> 
> Also, even with the patch, we have garbage:
> 
>     0xFFFFFFFFULL / (u32)user
> 
> why is that sub-expression pointlessly doing a 64-bit divide with a
> 32-bit number? The compiler is hopefully smart enough to point things
> out, but that "ULL" really is _wrong_ there, and could cause a stupid
> compiler to still do a 64-bit divide (although hopefully the simpler
> version that is 64/32).
> 
> So please clarify both the correct behavior _and_ the actual typing of
> the divide, ok?
> 
>                  Linus

The value of 'user' is sent from userspace, which is the return value of
this function:

static uint64_t bytes_to_cost(uint32_t bytes)
{
        uint32_t r = bytes >> XT_HASHLIMIT_BYTE_SHIFT;
        return UINT32_MAX / (r+1);
}

What user2rate_bytes() is trying to do is the opposite of above. The
size of 'user' is 64bit for a different reason altogether, but in this
case it is guaranteed to be always < U32_MAX. And hence using 64bit
divide is completely pointless (which I now realize).

Writing U32INT_MAX as 0xFFFFFFFFULL was a mistake on my part. I could
have avoided all of this by using built-in constants instead of trying
to define them myself. I will rewrite the function as below and send out
another patch:

static u64 user2rate_bytes(u64 user)
{
        u64 r;

        r = user ? U32_MAX / (u32) user : U32_MAX;
        r = (r - 1) << XT_HASHLIMIT_BYTE_SHIFT;
        return r;
}

-Vishwanath