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SubjectRE: xt_hashlimig build error (was Re: [RFC 01/17] x86/asm/64: Remove the restore_c_regs_and_iret label)
Date
Since user is u64, it is best to have a predictable return value for all possible values of user.  So maybe:

static u64 user2rate_bytes(u64 user)
{
u64 r;

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


-----Original Message-----
From: Vishwanath Pai [mailto:vpai@akamai.com]
Sent: Thursday, September 07, 2017 4:17 PM
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>; Lubashev, Igor <ilubashe@akamai.com>; Hunt, Joshua <johunt@akamai.com>; Pablo Neira Ayuso <pablo@netfilter.org>; 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>; Arnd Bergmann <arnd@arndb.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
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