linux-kernel - Re: [PATCH 08/17] proc/fd: In proc_fd_link use fcheck

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Date:   Tue, 18 Aug 2020 12:36:16 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        criu@...nvz.org, bpf@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Jann Horn <jann@...jh.net>, Kees Cook <keescook@...omium.org>,
        Daniel P. Berrangé <berrange@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Matthew Wilcox <willy@...ian.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Matthew Wilcox <matthew@....cx>,
        Trond Myklebust <trond.myklebust@....uio.no>,
        Chris Wright <chrisw@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH 08/17] proc/fd: In proc_fd_link use fcheck_task

On Mon, Aug 17, 2020 at 05:04:16PM -0500, Eric W. Biederman wrote:
> When discussing[1] exec and posix file locks it was realized that none
> of the callers of get_files_struct fundamentally needed to call
> get_files_struct, and that by switching them to helper functions
> instead it will both simplify their code and remove unnecessary
> increments of files_struct.count.  Those unnecessary increments can
> result in exec unnecessarily unsharing files_struct which breaking
> posix locks, and it can result in fget_light having to fallback to
> fget reducing system performance.
> 
> Using fcheck_task instead of get_files_struct simplifies proc_fd_link by
> removing unnecessary locking, and reference counting.
> 
> [1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
> Suggested-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---

Acked-by: Christian Brauner <christian.brauner@...ntu.com>

>  fs/proc/fd.c | 14 ++++----------
>  1 file changed, 4 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/proc/fd.c b/fs/proc/fd.c
> index 4048a87c51ee..abfdcb21cc79 100644
> --- a/fs/proc/fd.c
> +++ b/fs/proc/fd.c
> @@ -141,29 +141,23 @@ static const struct dentry_operations tid_fd_dentry_operations = {
>  
>  static int proc_fd_link(struct dentry *dentry, struct path *path)
>  {
> -	struct files_struct *files = NULL;
>  	struct task_struct *task;
>  	int ret = -ENOENT;
>  
>  	task = get_proc_task(d_inode(dentry));
>  	if (task) {
> -		files = get_files_struct(task);
> -		put_task_struct(task);
> -	}
> -
> -	if (files) {
>  		unsigned int fd = proc_fd(d_inode(dentry));
>  		struct file *fd_file;
>  
> -		spin_lock(&files->file_lock);
> -		fd_file = fcheck_files(files, fd);
> +		rcu_read_lock();
> +		fd_file = fcheck_task(task, fd);
>  		if (fd_file) {
>  			*path = fd_file->f_path;
>  			path_get(&fd_file->f_path);
>  			ret = 0;
>  		}
> -		spin_unlock(&files->file_lock);
> -		put_files_struct(files);
> +		rcu_read_unlock();
> +		put_task_struct(task);
>  	}
>  
>  	return ret;
> -- 
> 2.25.0
>