Date:	Wed, 25 Jan 2012 11:52:28 +0100
From:	Ludwig Nussel <ludwig.nussel@...e.de>
To:	linux-kernel@...r.kernel.org
Cc:	Ludwig Nussel <ludwig.nussel@...e.de>,
	Randy Dunlap <rdunlap@...otime.net> (maintainer:DOCUMENTATION),
	Greg Kroah-Hartman <gregkh@...e.de> (supporter:DRIVER CORE, KOBJ...
	,commit_signer:2/4=50%),
	Alessandro Rubini <rubini@...dd.com> (commit_signer:1/4=25%),
	Giancarlo Asnaghi <giancarlo.asnaghi@...com> (commit_signer:1/4=25%),
	Akinobu Mita <akinobu.mita@...il.com> (commit_signer:1/4=25%),
	linux-doc@...r.kernel.org (open list:DOCUMENTATION)
Subject: [PATCH] debugfs: add mode, uid and gid options

Cautious admins may want to restrict access to debugfs. Currently a
manual chown/chmod e.g. in an init script is needed to achieve that.
Distributions that want to make the mount options configurable need
to add extra config files. By allowing to set the root inode's uid,
gid and mode via mount options no such hacks are needed anymore.
Instead configuration becomes straight forward via fstab.

Signed-off-by: Ludwig Nussel <ludwig.nussel@...e.de>
---
 Documentation/filesystems/debugfs.txt |    5 +-
 fs/debugfs/inode.c                    |  149 ++++++++++++++++++++++++++++++++-
 2 files changed, 152 insertions(+), 2 deletions(-)

compared to previous patch:
- adjug debugfs_show_options to new api
- mention mount options in docu

diff --git a/Documentation/filesystems/debugfs.txt b/Documentation/filesystems/debugfs.txt
index 6872c91..4e25758 100644
--- a/Documentation/filesystems/debugfs.txt
+++ b/Documentation/filesystems/debugfs.txt
@@ -14,7 +14,10 @@ Debugfs is typically mounted with a command like:
 
     mount -t debugfs none /sys/kernel/debug
 
-(Or an equivalent /etc/fstab line). 
+(Or an equivalent /etc/fstab line).
+The debugfs root directory is accessible by anyone by default. To
+restrict access to the tree the "uid", "gid" and "mode" mount
+options can be used.
 
 Note that the debugfs API is exported GPL-only to modules.
 
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 956d5dd..b80bc84 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -23,9 +23,13 @@
 #include <linux/debugfs.h>
 #include <linux/fsnotify.h>
 #include <linux/string.h>
+#include <linux/seq_file.h>
+#include <linux/parser.h>
 #include <linux/magic.h>
 #include <linux/slab.h>
 
+#define DEBUGFS_DEFAULT_MODE	0755
+
 static struct vfsmount *debugfs_mount;
 static int debugfs_mount_count;
 static bool debugfs_registered;
@@ -125,11 +129,154 @@ static inline int debugfs_positive(struct dentry *dentry)
 	return dentry->d_inode && !d_unhashed(dentry);
 }
 
+struct debugfs_mount_opts {
+	uid_t uid;
+	gid_t gid;
+	umode_t mode;
+};
+
+enum {
+	Opt_uid,
+	Opt_gid,
+	Opt_mode,
+	Opt_err
+};
+
+static const match_table_t tokens = {
+	{Opt_uid, "uid=%u"},
+	{Opt_gid, "gid=%u"},
+	{Opt_mode, "mode=%o"},
+	{Opt_err, NULL}
+};
+
+struct debugfs_fs_info {
+	struct debugfs_mount_opts mount_opts;
+};
+
+static int debugfs_parse_options(char *data, struct debugfs_mount_opts *opts)
+{
+	substring_t args[MAX_OPT_ARGS];
+	int option;
+	int token;
+	char *p;
+
+	opts->mode = DEBUGFS_DEFAULT_MODE;
+
+	while ((p = strsep(&data, ",")) != NULL) {
+		if (!*p)
+			continue;
+
+		token = match_token(p, tokens, args);
+		switch (token) {
+		case Opt_uid:
+			if (match_int(&args[0], &option))
+				return -EINVAL;
+			opts->uid = option;
+			break;
+		case Opt_gid:
+			if (match_octal(&args[0], &option))
+				return -EINVAL;
+			opts->gid = option;
+			break;
+		case Opt_mode:
+			if (match_octal(&args[0], &option))
+				return -EINVAL;
+			opts->mode = option & S_IALLUGO;
+			break;
+		/*
+		 * We might like to report bad mount options here;
+		 * but traditionally debugfs has ignored all mount options
+		 */
+		}
+	}
+
+	return 0;
+}
+
+static int debugfs_apply_options(struct super_block *sb)
+{
+	struct debugfs_fs_info *fsi = sb->s_fs_info;
+	struct inode *inode = sb->s_root->d_inode;
+	struct debugfs_mount_opts *opts = &fsi->mount_opts;
+
+	inode->i_mode &= ~S_IALLUGO;
+	inode->i_mode |= opts->mode;
+
+	inode->i_uid = opts->uid;
+	inode->i_gid = opts->gid;
+
+	return 0;
+}
+
+static int debugfs_remount(struct super_block *sb, int *flags, char *data)
+{
+	int err;
+	struct debugfs_fs_info *fsi = sb->s_fs_info;
+
+	err = debugfs_parse_options(data, &fsi->mount_opts);
+	if (err)
+		goto fail;
+
+	debugfs_apply_options(sb);
+
+fail:
+	return err;
+}
+
+static int debugfs_show_options(struct seq_file *m, struct dentry *root)
+{
+	struct debugfs_fs_info *fsi = root->d_sb->s_fs_info;
+	struct debugfs_mount_opts *opts = &fsi->mount_opts;
+
+	if (opts->uid != 0)
+		seq_printf(m, ",uid=%u", opts->uid);
+	if (opts->gid != 0)
+		seq_printf(m, ",gid=%u", opts->gid);
+	if (opts->mode != DEBUGFS_DEFAULT_MODE)
+		seq_printf(m, ",mode=%o", opts->mode);
+
+	return 0;
+}
+
+static const struct super_operations debugfs_super_operations = {
+	.statfs		= simple_statfs,
+	.remount_fs	= debugfs_remount,
+	.show_options	= debugfs_show_options,
+};
+
 static int debug_fill_super(struct super_block *sb, void *data, int silent)
 {
 	static struct tree_descr debug_files[] = {{""}};
+	struct debugfs_fs_info *fsi;
+	int err;
+
+	save_mount_options(sb, data);
+
+	fsi = kzalloc(sizeof(struct debugfs_fs_info), GFP_KERNEL);
+	sb->s_fs_info = fsi;
+	if (!fsi) {
+		err = -ENOMEM;
+		goto fail;
+	}
+
+	err = debugfs_parse_options(data, &fsi->mount_opts);
+	if (err)
+		goto fail;
+
+	err  =  simple_fill_super(sb, DEBUGFS_MAGIC, debug_files);
+	if (err)
+		goto fail;
+
+	sb->s_op = &debugfs_super_operations;
+
+	debugfs_apply_options(sb);
+
+	return 0;
 
-	return simple_fill_super(sb, DEBUGFS_MAGIC, debug_files);
+fail:
+	kfree(fsi);
+	sb->s_fs_info = NULL;
+	return err;
 }
 
 static struct dentry *debug_mount(struct file_system_type *fs_type,
-- 
1.7.7

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