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

[Andy Lutomirski]

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.