linux-kernel - Re: [PATCH] Add pidfs filesystem

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Date:   Tue, 21 Feb 2017 15:57:47 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Alexey Gladkov <gladkov.alexey@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Vasiliy Kulikov <segoon@...nwall.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [PATCH] Add pidfs filesystem

On 02/18, Alexey Gladkov wrote:
>
> This patch allows to mount only the part of /proc related to pids
> without rest objects. Since this is an addon to /proc, flags applied to
> /proc have an effect on this pidfs filesystem.

I leave this to you and Eric, but imo it would be nice to avoid another
filesystem.

> Why not implement it as another flag to /proc ?
>
> The /proc flags is stored in the pid_namespace and are global for
> namespace. It means that if you add a flag to hide all except the pids,
> then it will act on all mounted instances of /proc.

But perhaps we can use mnt_flags? For example, lets abuse MNT_NODEV, see
the simple patch below. Not sure it is correct/complete, just to illustrate
the idea.

With this patch you can mount proc with -onodev and it will only show
pids/self/thread_self:

	# mkdir /tmp/D
	# mount -t proc -o nodev none /tmp/D
	# ls /tmp/D
	1   11	13  15	17  19	20  22	24  28	3   31	33  4  56  7  9     thread-self
	10  12	14  16	18  2	21  23	27  29	30  32	34  5  6   8  self
	# cat /tmp/D/meminfo
	cat: /tmp/D/meminfo: No such file or directory
	# ls /tmp/D/irq
	ls: cannot open directory /tmp/D/irq: No such file or directory

No?

Oleg.


--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -305,11 +305,22 @@ int proc_readdir_de(struct proc_dir_entry *de, struct file *file,
 
 int proc_readdir(struct file *file, struct dir_context *ctx)
 {
+	int mnt_flags = file->f_path.mnt->mnt_flags;
 	struct inode *inode = file_inode(file);
 
+	if (mnt_flags & MNT_NODEV)
+		return 1;
+
 	return proc_readdir_de(PDE(inode), file, ctx);
 }
 
+static int proc_dir_open(struct inode *inode, struct file *file)
+{
+	if (file->f_path.mnt->mnt_flags & MNT_NODEV)
+		return -ENOENT;
+	return 0;
+}
+
 /*
  * These are the generic /proc directory operations. They
  * use the in-memory "struct proc_dir_entry" tree to parse
@@ -319,6 +330,7 @@ static const struct file_operations proc_dir_operations = {
 	.llseek			= generic_file_llseek,
 	.read			= generic_read_dir,
 	.iterate_shared		= proc_readdir,
+	.open			= proc_dir_open,
 };
 
 /*
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -318,12 +318,16 @@ proc_reg_get_unmapped_area(struct file *file, unsigned long orig_addr,
 
 static int proc_reg_open(struct inode *inode, struct file *file)
 {
+	int mnt_flags = file->f_path.mnt->mnt_flags;
 	struct proc_dir_entry *pde = PDE(inode);
 	int rv = 0;
 	int (*open)(struct inode *, struct file *);
 	int (*release)(struct inode *, struct file *);
 	struct pde_opener *pdeo;
 
+	if (mnt_flags & MNT_NODEV)
+		return -ENOENT;
+
 	/*
 	 * Ensure that
 	 * 1) PDE's ->release hook will be called no matter what