Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

Christian Brauner
2020-07-30T15:27:15Z

On Thu, Jul 30, 2020 at 04:22:50PM +0100, Matthew Wilcox wrote:
> On Mon, Jul 27, 2020 at 10:11:22AM -0700, Anthony Yznaga wrote:
> > This patchset adds support for preserving an anonymous memory range across
> > exec(3) using a new madvise MADV_DOEXEC argument. The primary benefit for
> > sharing memory in this manner, as opposed to re-attaching to a named shared
> > memory segment, is to ensure it is mapped at the same virtual address in
> > the new process as it was in the old one. An intended use for this is to
> > preserve guest memory for guests using vfio while qemu exec's an updated
> > version of itself. By ensuring the memory is preserved at a fixed address,
> > vfio mappings and their associated kernel data structures can remain valid.
> > In addition, for the qemu use case, qemu instances that back guest RAM with
> > anonymous memory can be updated.
>
> I just realised that something else I'm working on might be a suitable
> alternative to this. Apologies for not realising it sooner.
>
> http://www.wil.cx/~willy/linux/sileby.html

Just skimming: make it O_CLOEXEC by default. ;)

>
> To use this, you'd mshare() the anonymous memory range, essentially
> detaching the VMA from the current process's mm_struct and reparenting
> it to this new mm_struct, which has an fd referencing it.
>
> Then you call exec(), and the exec'ed task gets to call mmap() on that
> new fd to attach the memory range to its own address space.
>
> Presto!