linux-kernel - Re: [PATCH net-next v5 02/20] zinc: introduce minimal cryptography library

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Date:   Thu, 20 Sep 2018 20:23:43 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Eric Biggers <ebiggers@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Samuel Neves <sneves@....uc.pt>,
        Andrew Lutomirski <luto@...nel.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>
Subject: Re: [PATCH net-next v5 02/20] zinc: introduce minimal cryptography library


> On Sep 20, 2018, at 8:12 PM, Andrew Lunn <andrew@...n.ch> wrote:
> 
>> On Fri, Sep 21, 2018 at 02:11:43AM +0200, Jason A. Donenfeld wrote:
>> Hey Arnd,
>> 
>>> On Thu, Sep 20, 2018 at 6:02 PM Arnd Bergmann <arnd@...db.de> wrote:
>>> Right, if you hit a stack requirement like this, it's usually the compiler
>>> doing something bad, not just using too much stack but also generating
>>> rather slow object code in the process. It's better to fix the bug by
>>> optimizing the code to not spill registers to the stack.
>>> 
>>> In the long run, I'd like to reduce the stack frame size further, so
>>> best assume that anything over 1024 bytes (on 32-bit) or 1280 bytes
>>> (on 64-bit) is a bug in the code, and stay below that.
>>> 
>>> For prototyping, you can just mark the broken functions individually
>>> by setting the warning limit for a specific function that is known to
>>> be misoptimized by the compiler (with a comment about which compiler
>>> and architectures are affected), but not override the limit for the
>>> entire file.
>> 
>> Thanks for the explanation. Fortunately in my case, the issues were
>> trivially fixable to get it under 1024/1280. (By the way, why does
>> 64-bit have a slightly larger stack frame? To account for 32 pointers
>> taking double the space or something?) That will be rectified in v6.
> 
> Hi Jason
> 
> Do you any stack usage information?
> 
> A VPN can be at the bottom of some deep stack calls. Swap on NFS over
> the VPN? If you have one frame of 1K, you might be O.K. But if you
> have a few of these, i can see there might be issues of overflowing
> the stack.
> 
>  

At the risk on suggesting something awful: on x86_64, since we turn preemption off for simd, it wouldn’t be *completely* insane to do the crypto on the irq stack. It would look like:

kernel_fpu_call(func, arg);

And this helper would disable preemption, enable FPU, switch to the irq stack, call func(arg), disable FPU, enable preemption, and return. And we can have large IRQ stacks.

I refuse to touch this with a ten-foot pole until the lazy FPU restore patches land.

All that being said, why are these frames so large?  It sounds like something may be spilling that ought not to.