Re: [RFC PATCH 1/2] mm: restrictedmem: Allow userspace to specify mount_path for memfd_restricted

[Ackerley Tng]

"Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >   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?

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?