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: Oleg Nesterov <oleg@redhat.com>
Acked-by: Christian 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: Eric W. Biederman <ebiederm@xmission.com>

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 72c1525..cb51763 100644 (file)
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -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;