Re: [PATCH 04/12] bpf: use __anon_inode_getfd

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, May 8, 2020 at 8:39 AM Christoph Hellwig <hch@lst.de> wrote:
>
> Use __anon_inode_getfd instead of opencoding the logic using
> get_unused_fd_flags + anon_inode_getfile.  Also switch the
> bpf_link_new_file calling conventions to match __anon_inode_getfd.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  include/linux/bpf.h  |  2 +-
>  kernel/bpf/cgroup.c  |  6 +++---
>  kernel/bpf/syscall.c | 31 +++++++++----------------------
>  3 files changed, 13 insertions(+), 26 deletions(-)
>

[...]

> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 64783da342020..cb2364e17423c 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -2307,23 +2307,10 @@ int bpf_link_new_fd(struct bpf_link *link)
>   * complicated and expensive operations and should be delayed until all the fd
>   * reservation and anon_inode creation succeeds.
>   */

The comment above explains the reason why we do want to split getting
fd, getting file, and installing fd later. I'd like to keep it this
way. Also, this code was refactored in bpf-next by [0] (it still uses
get_unused_fd_flag + anon_inode_getfile + fd_install, by design).

  [0] https://patchwork.ozlabs.org/project/netdev/patch/20200429001614.1544-3-andriin@fb.com/

> -struct file *bpf_link_new_file(struct bpf_link *link, int *reserved_fd)
> +int bpf_link_new_file(struct bpf_link *link, struct file **file)
>  {
> -       struct file *file;
> -       int fd;
> -
> -       fd = get_unused_fd_flags(O_CLOEXEC);
> -       if (fd < 0)
> -               return ERR_PTR(fd);
> -
> -       file = anon_inode_getfile("bpf_link", &bpf_link_fops, link, O_CLOEXEC);
> -       if (IS_ERR(file)) {
> -               put_unused_fd(fd);
> -               return file;
> -       }
> -
> -       *reserved_fd = fd;
> -       return file;
> +       return __anon_inode_getfd("bpf_link", &bpf_link_fops, link, O_CLOEXEC,
> +                       file);
>  }
>

[...]