[v2,05/24] bpf: In bpf_task_fd_query use fget_task

Submitted by Eric W. Biederman on Nov. 20, 2020, 11:14 p.m.

Details

Message ID 20201120231441.29911-5-ebiederm@xmission.com
State New
Series "exec: Move unshare_files and guarantee files_struct.count is correct"
Headers show

Commit Message

Eric W. Biederman Nov. 20, 2020, 11:14 p.m.
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
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/bpf/syscall.c | 20 +++-----------------
 1 file changed, 3 insertions(+), 17 deletions(-)

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diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 8f50c9c19f1b..6d49c2e1634c 100644
--- 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 @@  static int bpf_task_fd_query(const union bpf_attr *attr,
 	err = -ENOTSUPP;
 put_file:
 	fput(file);
-out:
 	return err;
 }