[PATCH 05/17] bpf: In bpf_task_fd_query use fget_task

Eric W. Biederman
2020-08-17T22:10:25Z

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.

Use fget_task to simplify bpf_task_fd_query.

[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>
---
kernel/bpf/syscall.c | 20 +++-----------------
1 file changed, 3 insertions(+), 17 deletions(-)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 86299a292214..93657d5f6538 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3741,7 +3741,6 @@ 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;
@@ -3759,23 +3758,11 @@ 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;
@@ -3815,7 +3802,6 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
err = -ENOTSUPP;
put_file:
fput(file);
-out:
return err;
}

--
2.25.0