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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>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        Cyrill Gorcunov <gorcunov@gmail.com>,
        Jann Horn <jann@thejh.net>, Kees Cook <keescook@chromium.org>,
        =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= <berrange@redhat.com>,
        Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Matthew Wilcox <willy@debian.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Matthew Wilcox <matthew@wil.cx>,
        Trond Myklebust <trond.myklebust@fys.uio.no>,
        Chris Wright <chrisw@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        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 17:04:23 -0500
Message-Id: <20200817220425.9389-15-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 15/17] file: In f_dupfd read RLIMIT_NOFILE once.
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Simplify the code, and remove the chance of races by reading
RLIMIT_NOFILE only once in f_dupfd.

Pass the read value of RLIMIT_NOFILE into alloc_fd which is the other
location the rlimit was read in f_dupfd.  As f_dupfd is the only
caller of alloc_fd this changing alloc_fd is trivially safe.

Further this causes alloc_fd to take all of the same arguments as
__alloc_fd except for the files_struct argument.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 fs/file.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 1a755811669d..505b2e81ad3e 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -523,9 +523,9 @@ int __alloc_fd(struct files_struct *files,
 	return error;
 }
 
-static int alloc_fd(unsigned start, unsigned flags)
+static int alloc_fd(unsigned start, unsigned end, unsigned flags)
 {
-	return __alloc_fd(current->files, start, rlimit(RLIMIT_NOFILE), flags);
+	return __alloc_fd(current->files, start, end, flags);
 }
 
 int __get_unused_fd_flags(unsigned flags, unsigned long nofile)
@@ -1158,10 +1158,11 @@ SYSCALL_DEFINE1(dup, unsigned int, fildes)
 
 int f_dupfd(unsigned int from, struct file *file, unsigned flags)
 {
+	unsigned long nofile = rlimit(RLIMIT_NOFILE);
 	int err;
-	if (from >= rlimit(RLIMIT_NOFILE))
+	if (from >= nofile)
 		return -EINVAL;
-	err = alloc_fd(from, flags);
+	err = alloc_fd(from, nofile, flags);
 	if (err >= 0) {
 		get_file(file);
 		fd_install(err, file);
-- 
2.25.0