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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     linux-kernel@vger.kernel.org
Cc:     linux-fsdevel@vger.kernel.org, criu@openvz.org,
        bpf@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
Date:   Mon, 17 Aug 2020 17:04:13 -0500
Message-Id: <20200817220425.9389-5-ebiederm@xmission.com>
In-Reply-To: <87ft8l6ic3.fsf@x220.int.ebiederm.org>
References: <87ft8l6ic3.fsf@x220.int.ebiederm.org>
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Subject: [PATCH 05/17] bpf: In bpf_task_fd_query use fget_task
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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