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

From: Kirill A. Shutemov
Date: Thu Feb 16 2023 - 05:23:11 EST


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.

--
Kiryl Shutsemau / Kirill A. Shutemov