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

From: Dmitry Safonov
Date: Sat Dec 31 2016 - 06:10:51 EST

Hi Andy,
thanks for reviewing!

2016-12-31 5:02 GMT+03:00 Andy Lutomirski <luto@xxxxxxxxxxxxxx>:
> On Fri, Dec 30, 2016 at 7:56 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> 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.