[PATCH 5.10 111/770] bpf: In bpf_task_fd_query use fget_task

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5.10-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>

[ Upstream commit b48845af0152d790a54b8ab78cc2b7c07485fc98 ]

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@xxxxxxxxxx
Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
v1: https://lkml.kernel.org/r/20200817220425.9389-5-ebiederm@xxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20201120231441.29911-5-ebiederm@xxxxxxxxxxxx
Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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 e1bee8cd34044..fbe7f8e2b022c 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3929,7 +3929,6 @@ 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;
@@ -3949,23 +3948,11 @@ 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;
@@ -4005,7 +3992,6 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
 	err = -ENOTSUPP;
 put_file:
 	fput(file);
-out:
 	return err;
 }
 
-- 
2.43.0







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