[PATCH 08/17] proc/fd: In proc_fd_link use fcheck_task

Eric W. Biederman
2020-08-17T22:10:26Z

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@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.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