Commit 439be326 authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

proc/fd: In proc_fd_link use fget_task 

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.

Simplifying proc_fd_link is a little bit tricky.  It is necessary to
know that there is a reference to fd_f	 ile while path_get is running.
This reference can either be guaranteed to exist either by locking the
fdtable as the code currently does or by taking a reference on the
file in question.

Use fget_task to remove the need for get_files_struct and
to take a reference to file in question.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com


Suggested-by: default avatarOleg Nesterov <oleg@redhat.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-8-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-6-ebiederm@xmission.com


Signed-off-by: default avatarEric W. Biederman <ebiederm@xmission.com>
parent b48845af

fs/proc/fd.c

+3 −10
Original line number Diff line number Diff line
@@ -146,29 +146,22 @@ 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);

		fd_file = fget_task(task, fd);

		if (fd_file) {

			*path = fd_file->f_path;

			path_get(&fd_file->f_path);

			ret = 0;

			fput(fd_file);

		}

		spin_unlock(&files->file_lock);

		put_files_struct(files);

		put_task_struct(task);

	}



	return ret;