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Subject[PATCH v2 05/24] 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
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 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;
}

--
2.25.0
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