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

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Date:   Thu, 16 Feb 2023 13:01:50 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [RFC PATCH 1/2] mm: restrictedmem: Allow userspace to specify
 mount_path for memfd_restricted

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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov