linux-kernel - Re: [RFC 4/4] x86/arch_prctl: add ARCH_{GET,SET}_TASK

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Message-ID: <CAJwJo6ZW_sm4ZwYamOQQh5sHggYnJ52MVkrGR==N1=ZggN0RAQ@mail.gmail.com>
Date:   Sat, 31 Dec 2016 14:10:19 +0300
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Dmitry Safonov <dsafonov@...tuozzo.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        X86 ML <x86@...nel.org>
Subject: Re: [RFC 4/4] x86/arch_prctl: add ARCH_{GET,SET}_TASK_SIZE

Hi Andy,
thanks for reviewing!

2016-12-31 5:02 GMT+03:00 Andy Lutomirski <luto@...capital.net>:
> On Fri, Dec 30, 2016 at 7:56 AM, Dmitry Safonov <dsafonov@...tuozzo.com> wrote:
>> Add arch_prctl getters/setters for size of virtual address space of task.
>> This adds ability to change task's virtual address space limit.
>> I need this for correctly restore virtual address space limits in CRIU.
>> Currently, on x86 there are three task sizes: 3GB for some old 32 bit java
>> apps, 4Gb for ordinary 32-bit compatible apps and 47-bits for native
>> x86_64 processes.
>> 32-bit applications are restored by CRIU with the help of 64-bit clone()-d
>> child, and on restore we need to place correct address space limitations
>> back - otherwise 32-bit restored application may mmap() address over
>> 4Gb space and as this address will not fit into 4-byte pointer, it
>> will silently reuse/corrupt the pointer that has the same lower 4-bytes.
>>
>
> I agree we need something like this, but this particular justification
> is a bit bogus.  If 32-bit mmap() returns an address above 2^32, then
> I think it's a straight-up bug.  The address space limit shouldn't
> have anything to do with  it -- the kernel *knows* that it's a
> "compat" syscall.

Yep, I guess, I didn't realize that the real wrong thing is that
compat syscall returns address above 4Gb and not the address
space limits here. Thanks, will look into that.

-- 
             Dmitry