linux-kernel - /proc/pid/fd && anon_inode

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Message-ID: <20130824182939.GA23630@redhat.com>
Date:	Sat, 24 Aug 2013 20:29:39 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Willy Tarreau <w@....eu>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"security@...nel.org" <security@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Brad Spengler <spender@...ecurity.net>
Subject: /proc/pid/fd && anon_inode_fops

Sorry for off-topic, I am just curios.

On 08/22, Willy Tarreau wrote:
>
> It's not only that, it also supports sockets and pipes that you can access
> via /proc/pid/fd and not via a real symlink which would try to open eg
> "pipe:[23456]" instead of the real file.

But sock_no_open() disallows this, and for good reason I guess.

I am wondering, perhaps anon_inode should do the same? I do not
see any problem, but it looks pointless and misleading to allow
to open a file you can do nothing with.

Or is there any reason why, say, open("anon_inode:[perf_event]")
should succeed?

Thanks,

Oleg.

--- x/fs/anon_inodes.c
+++ x/fs/anon_inodes.c
@@ -24,7 +24,15 @@
 
 static struct vfsmount *anon_inode_mnt __read_mostly;
 static struct inode *anon_inode_inode;
-static const struct file_operations anon_inode_fops;
+
+static int anon_open(struct inode *inode, struct file *file)
+{
+	return -ENXIO;
+}
+
+static const struct file_operations anon_inode_fops = {
+	.open = anon_open,
+};
 
 /*
  * anon_inodefs_dname() is called from d_path().

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