bpf: In bpf_task_fd_query use fget_task

Use the helper fget_task to simplify bpf_task_fd_query.

As well as simplifying the code this removes one unnecessary increment of
struct files_struct.  This unnecessary increment of files_struct.count can
result in exec unnecessarily unsharing files_struct and breaking posix
locks, and it can result in fget_light having to fallback to fget reducing
performance.

This simplification comes from the observation that none of the
callers of get_files_struct actually need to call get_files_struct
that was made when discussing[1] exec and posix file locks.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-5-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-5-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 8f50c9c..6d49c2e 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3878,7 +3878,6 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
        pid_t pid = attr->task_fd_query.pid;
        u32 fd = attr->task_fd_query.fd;
        const struct perf_event *event;
-       struct files_struct *files;
        struct task_struct *task;
        struct file *file;
        int err;
@@ -3896,23 +3895,11 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
        if (!task)
                return -ENOENT;
 
-       files = get_files_struct(task);
-       put_task_struct(task);
-       if (!files)
-               return -ENOENT;
-
        err = 0;
-       spin_lock(&files->file_lock);
-       file = fcheck_files(files, fd);
+       file = fget_task(task, fd);
+       put_task_struct(task);
        if (!file)
-               err = -EBADF;
-       else
-               get_file(file);
-       spin_unlock(&files->file_lock);
-       put_files_struct(files);
-
-       if (err)
-               goto out;
+               return -EBADF;
 
        if (file->f_op == &bpf_link_fops) {
                struct bpf_link *link = file->private_data;
@@ -3952,7 +3939,6 @@ out_not_supp:
        err = -ENOTSUPP;
 put_file:
        fput(file);
-out:
        return err;
 }