linux-kernel - [PATCH v2 12/24] proc/fd: In tid_fd_mode use task_lookup_fd

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Date:   Fri, 20 Nov 2020 17:14:29 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, criu@...nvz.org,
        bpf@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Jann Horn <jann@...jh.net>, Kees Cook <keescook@...omium.org>,
        Daniel P . Berrangé <berrange@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Matthew Wilcox <willy@...radead.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Chris Wright <chrisw@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH v2 12/24] proc/fd: In tid_fd_mode use task_lookup_fd_rcu

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.

Instead of manually coding finding the files struct for a task and
then calling files_lookup_fd_rcu, use the helper task_lookup_fd_rcu
that combines those to steps.   Making the code simpler and removing
the need to get a reference on a files_struct.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@...hat.com>
Acked-by: Christian Brauner <christian.brauner@...ntu.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-7-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 fs/proc/fd.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 3dec44d7c5c5..c1a984f3c4df 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -83,18 +83,13 @@ static const struct file_operations proc_fdinfo_file_operations = {
 
 static bool tid_fd_mode(struct task_struct *task, unsigned fd, fmode_t *mode)
 {
-	struct files_struct *files = get_files_struct(task);
 	struct file *file;
 
-	if (!files)
-		return false;
-
 	rcu_read_lock();
-	file = files_lookup_fd_rcu(files, fd);
+	file = task_lookup_fd_rcu(task, fd);
 	if (file)
 		*mode = file->f_mode;
 	rcu_read_unlock();
-	put_files_struct(files);
 	return !!file;
 }
 
-- 
2.25.0