Commit 775e0656 authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

proc/fd: In fdinfo seq_show don't use get_files_struct 

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.

Instead hold task_lock for the duration that task->files needs to be
stable in seq_show.  The task_lock was already taken in
get_files_struct, and so skipping get_files_struct performs less work
overall, and avoids the problems with the files_struct reference
count.

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


Suggested-by: default avatarOleg Nesterov <oleg@redhat.com>
Acked-by: default avatarChristian Brauner <christian.brauner@ubuntu.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-12-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-17-ebiederm@xmission.com


Signed-off-by: default avatarEric W. Biederman <ebiederm@xmission.com>
parent 66ed5944

fs/proc/fd.c

+5 −4
Original line number Diff line number Diff line
@@ -28,9 +28,8 @@ static int seq_show(struct seq_file *m, void *v) 
	if (!task)

		return -ENOENT;



	files = get_files_struct(task);

	put_task_struct(task);



	task_lock(task);

	files = task->files;

	if (files) {

		unsigned int fd = proc_fd(m->private);
@@ -47,8 +46,9 @@ static int seq_show(struct seq_file *m, void *v) 
			ret = 0;

		}

		spin_unlock(&files->file_lock);

		put_files_struct(files);

	}

	task_unlock(task);

	put_task_struct(task);



	if (ret)

		return ret;
@@ -57,6 +57,7 @@ static int seq_show(struct seq_file *m, void *v) 
		   (long long)file->f_pos, f_flags,

		   real_mount(file->f_path.mnt)->mnt_id);



	/* show_fd_locks() never deferences files so a stale value is safe */

	show_fd_locks(m, file, files);

	if (seq_has_overflowed(m))

		goto out;