Re: [RFC 4/4] x86/arch_prctl: add ARCH_{GET,SET}_TASK_SIZE

From	Andy Lutomirski <luto@amacapital.net>
Newsgroups	linux.kernel
Subject	Re: [RFC 4/4] x86/arch_prctl: add ARCH_{GET,SET}_TASK_SIZE
Date	2016-12-31 03:10 +0100
Message-ID	<sUhho-57Y-1@gated-at.bofh.it> (permalink)
References	<sUgOl-4AL-3@gated-at.bofh.it> <sUhho-57Y-3@gated-at.bofh.it>
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On Fri, Dec 30, 2016 at 7:56 AM, Dmitry Safonov <dsafonov@virtuozzo.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.

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Re: [RFC 4/4] x86/arch_prctl: add ARCH_{GET,SET}_TASK_SIZE Andy Lutomirski <luto@amacapital.net> - 2016-12-31 03:10 +0100
  Re: [RFC 4/4] x86/arch_prctl: add ARCH_{GET,SET}_TASK_SIZE Dmitry Safonov <0x7f454c46@gmail.com> - 2016-12-31 12:20 +0100

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