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From: Andrew Lutomirski <luto@mit.edu>
Date: Thu, 16 Feb 2012 18:16:36 -0800
Message-ID: <CAObL_7GUTbc=Ew9mQw7CoWfdAfRWSWUrY3w+UnrYrqWEiHxtcQ@mail.gmail.com>
Subject: Re: [PATCH v8 3/8] seccomp: add system call filtering using BPF
To: Indan Zupancic <indan@nul.nu>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Will Drewry <wad@chromium.org>,
        linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
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On Thu, Feb 16, 2012 at 6:00 PM, Indan Zupancic <indan@nul.nu> wrote:
> On Fri, February 17, 2012 02:33, H. Peter Anvin wrote:
>> On 02/16/2012 04:48 PM, Indan Zupancic wrote:
>>> On Thu, February 16, 2012 22:17, H. Peter Anvin wrote:
>>>
>>> I would go for something like:
>>>
>>> struct seccomp_data {
>>> ? ? ?int nr;
>>> ? ? ?__u32 arg_low[6];
>>> ? ? ?__u32 arg_high[6];
>>> ? ? ?__u32 instruction_pointer_low;
>>> ? ? ?__u32 instruction_pointer_high;
>>> ? ? ?__u32 __reserved[3];
>>> };
>>>
>>
>> Uh, that is the absolutely WORST way to do it - not only are you
>> creating two fields, they're not even adjacent.
>
> You want:
>
> struct seccomp_data {
> ? ? ? ?int nr;
> ? ? ? ?__u32 __reserved[3];
> ? ? ? ?__u64 arg[6];
> ? ? ? ?__u64 instruction_pointer;
> };
>
> And I agree it looks a lot nicer.
>
> You can pretend a 64-bit arg will be one field, but it won't be. It will
> be always two fields no matter what. Making them adjacent is only good
> because seccomp_data won't have to change if 64-bit support is ever added
> to BPF.
>
> It looks nicer, but it only makes it harder to know the right offset for
> the fields for the 32-bit only BPF programs. You can try to hide reality,
> but that won't change it.
>
>>> (Not sure what use the IP is because that doesn't tell anything about how
>>> the system call instruction was reached.)
>>>
>>> The only way to avoid splitting args is to add 64-bit support to BPF.
>>> That is probably the best way forwards, but would require breaking the
>>> BPF ABI by either adding a 64-bit version directly or adding extra
>>> instructions.
>>
>> Or the compiler or whatever generates the BPF code just is going to have
>> to generate two instructions -- just like we always have to handle
>> [u]int64_t on 32-bit platforms. ?There is no difference here.
>
> Except that if you don't hide the platform differences your compiler
> or whatever needs to generate different instructions depending on the
> endianess, while it could always generate the same instructions instead.
>
> My impression is that you want to push all extra complexity into the
> compiler or whatever instead of making the ABI cross-platform, because
> it looks nicer. I don't care that much, but I think you're just pushing
> the ugliness around instead of getting rid of it.

Is there really no syscall that cares about endianness?

Even if it ends up working, forcing syscall arguments to have a
particular endianness seems like a bad decision, especially if anyone
ever wants to make a 64-bit BPF implementation.  (Or if any
architecture adds 128-bit syscall arguments to a future syscall
namespace or whatever it's called.  x86-64 has 128-bit xmm
registers...)

--Andy
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