linux-kernel - Re: [RFC PATCH 1/2] mm: restrictedmem: Allow userspace to specify mount_path for memfd

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Date:   Fri, 24 Feb 2023 12:36:00 +0300
From:   kirill.shutemov@...ux.intel.com
To:     Ackerley Tng <ackerleytng@...gle.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>, kvm@...r.kernel.org,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
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Subject: Re: [RFC PATCH 1/2] mm: restrictedmem: Allow userspace to specify
 mount_path for memfd_restricted

On Thu, Feb 23, 2023 at 12:55:16AM +0000, Ackerley Tng wrote:
> 
> "Kirill A. Shutemov" <kirill@...temov.name> writes:
> 
> > On Thu, Feb 16, 2023 at 12:41:16AM +0000, Ackerley Tng wrote:
> > > By default, the backing shmem file for a restrictedmem fd is created
> > > on shmem's kernel space mount.
> 
> > > With this patch, an optional tmpfs mount can be specified, which will
> > > be used as the mountpoint for backing the shmem file associated with a
> > > restrictedmem fd.
> 
> > > This change is modeled after how sys_open() can create an unnamed
> > > temporary file in a given directory with O_TMPFILE.
> 
> > > This will help restrictedmem fds inherit the properties of the
> > > provided tmpfs mounts, for example, hugepage allocation hints, NUMA
> > > binding hints, etc.
> 
> > > Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
> > > ---
> > >   include/linux/syscalls.h           |  2 +-
> > >   include/uapi/linux/restrictedmem.h |  8 ++++
> > >   mm/restrictedmem.c                 | 63 +++++++++++++++++++++++++++---
> > >   3 files changed, 66 insertions(+), 7 deletions(-)
> > >   create mode 100644 include/uapi/linux/restrictedmem.h
> 
> > > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> > > index f9e9e0c820c5..4b8efe9a8680 100644
> > > --- a/include/linux/syscalls.h
> > > +++ b/include/linux/syscalls.h
> > > @@ -1056,7 +1056,7 @@ asmlinkage long sys_memfd_secret(unsigned int
> > > flags);
> > >   asmlinkage long sys_set_mempolicy_home_node(unsigned long start,
> > > unsigned long len,
> > >   					    unsigned long home_node,
> > >   					    unsigned long flags);
> > > -asmlinkage long sys_memfd_restricted(unsigned int flags);
> > > +asmlinkage long sys_memfd_restricted(unsigned int flags, const char
> > > __user *mount_path);
> 
> > >   /*
> > >    * Architecture-specific system calls
> 
> > I'm not sure what the right practice now: do we provide string that
> > contains mount path or fd that represents the filesystem (returned from
> > fsmount(2) or open_tree(2)).
> 
> > fd seems more flexible: it allows to specify unbind mounts.
> 
> I tried out the suggestion of passing fds to memfd_restricted() instead
> of strings.
> 
> One benefit I see of using fds is interface uniformity: it feels more
> aligned with other syscalls like fsopen(), fsconfig(), and fsmount() in
> terms of using and passing around fds.
> 
> Other than being able to use a mount without a path attached to the
> mount, are there any other benefits of using fds over using the path string?

It would be nice if anyone from fs folks comment on this.

> Should I post the patches that allows specifying a mount using fds?
> Should I post them as a separate RFC, or as a new revision to this RFC?

Let's first decide what the right direction is.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov