proc/fd: In tid_fd_mode use task_lookup_fd_rcu

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 of manually coding finding the files struct for a task and
then calling files_lookup_fd_rcu, use the helper task_lookup_fd_rcu
that combines those to steps.   Making the code simpler and removing
the need to get a reference on a files_struct.

[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/20200817220425.9389-7-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-12-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 3dec44d7c5c5c499a90dc3423e328889ecb900e5..c1a984f3c4df78a24157eba3aab72f57f7a53a24 100644 (file)
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -83,18 +83,13 @@ static const struct file_operations proc_fdinfo_file_operations = {
 
 static bool tid_fd_mode(struct task_struct *task, unsigned fd, fmode_t *mode)
 {
-       struct files_struct *files = get_files_struct(task);
        struct file *file;
 
-       if (!files)
-               return false;
-
        rcu_read_lock();
-       file = files_lookup_fd_rcu(files, fd);
+       file = task_lookup_fd_rcu(task, fd);
        if (file)
                *mode = file->f_mode;
        rcu_read_unlock();
-       put_files_struct(files);
        return !!file;
 }