Quoting Miklos Szeredi (miklos@szeredi.hu):
quoted text
> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> Add the following:
> 
>   /proc/sys/fs/types/${FS_TYPE}/usermount_safe
> 
> Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
> ---
> 
> Index: linux/fs/filesystems.c
> ===================================================================
> --- linux.orig/fs/filesystems.c	2008-01-16 13:24:52.000000000 +0100
> +++ linux/fs/filesystems.c	2008-01-16 13:25:09.000000000 +0100
> @@ -12,6 +12,7 @@
>  #include <linux/kmod.h>
>  #include <linux/init.h>
>  #include <linux/module.h>
> +#include <linux/sysctl.h>
>  #include <asm/uaccess.h>
> 
>  /*
> @@ -51,6 +52,57 @@ static struct file_system_type **find_fi
>  	return p;
>  }
> 
> +#define MAX_FILESYSTEM_VARS 1
> +
> +struct filesystem_sysctl_table {
> +	struct ctl_table_header *header;
> +	struct ctl_table table[MAX_FILESYSTEM_VARS + 1];
> +};
> +
> +/*
> + * Create /sys/fs/types/${FSNAME} directory with per fs-type tunables.
> + */
> +static int filesystem_sysctl_register(struct file_system_type *fs)
> +{
> +	struct filesystem_sysctl_table *t;
> +	struct ctl_path path[] = {
> +		{ .procname = "fs", .ctl_name = CTL_FS },
> +		{ .procname = "types", .ctl_name = CTL_UNNUMBERED },
> +		{ .procname = fs->name, .ctl_name = CTL_UNNUMBERED },
> +		{ }
> +	};
> +
> +	t = kzalloc(sizeof(*t), GFP_KERNEL);
> +	if (!t)
> +		return -ENOMEM;
> +
> +
> +	t->table[0].ctl_name = CTL_UNNUMBERED;
> +	t->table[0].procname = "usermount_safe";
> +	t->table[0].maxlen = sizeof(int);
> +	t->table[0].data = &fs->fs_safe;
> +	t->table[0].mode = 0644;
> +	t->table[0].proc_handler = &proc_dointvec;
> +
> +	t->header = register_sysctl_paths(path, t->table);
> +	if (!t->header) {
> +		kfree(t);
> +		return -ENOMEM;
> +	}
> +
> +	fs->sysctl_table = t;
> +
> +	return 0;
> +}
> +
> +static void filesystem_sysctl_unregister(struct file_system_type *fs)
> +{
> +	struct filesystem_sysctl_table *t = fs->sysctl_table;
> +
> +	unregister_sysctl_table(t->header);
> +	kfree(t);
> +}
> +
>  /**
>   *	register_filesystem - register a new filesystem
>   *	@fs: the file system structure
> @@ -80,6 +132,13 @@ int register_filesystem(struct file_syst
>  	else
>  		*p = fs;
>  	write_unlock(&file_systems_lock);
> +
> +	if (res == 0) {
> +		res = filesystem_sysctl_register(fs);

What do you think about doing this only if FS_SAFE is also set,
so for instance at first only FUSE would allow itself to be
made user-mountable?

A safe thing to do, or overly intrusive?

quoted text
> +		if (res != 0)
> +			unregister_filesystem(fs);
> +	}
> +
>  	return res;
>  }
> 
> @@ -108,6 +167,7 @@ int unregister_filesystem(struct file_sy
>  			*tmp = fs->next;
>  			fs->next = NULL;
>  			write_unlock(&file_systems_lock);
> +			filesystem_sysctl_unregister(fs);
>  			return 0;
>  		}
>  		tmp = &(*tmp)->next;
> Index: linux/include/linux/fs.h
> ===================================================================
> --- linux.orig/include/linux/fs.h	2008-01-16 13:25:09.000000000 +0100
> +++ linux/include/linux/fs.h	2008-01-16 13:25:09.000000000 +0100
> @@ -1437,6 +1437,7 @@ struct file_system_type {
>  	struct module *owner;
>  	struct file_system_type * next;
>  	struct list_head fs_supers;
> +	struct filesystem_sysctl_table *sysctl_table;
> 
>  	struct lock_class_key s_lock_key;
>  	struct lock_class_key s_umount_key;
> Index: linux/Documentation/filesystems/proc.txt
> ===================================================================
> --- linux.orig/Documentation/filesystems/proc.txt	2008-01-16 13:25:07.000000000 +0100
> +++ linux/Documentation/filesystems/proc.txt	2008-01-16 13:25:09.000000000 +0100
> @@ -43,6 +43,7 @@ Table of Contents
>    2.13	/proc/<pid>/oom_score - Display current oom-killer score
>    2.14	/proc/<pid>/io - Display the IO accounting fields
>    2.15	/proc/<pid>/coredump_filter - Core dump filtering settings
> +  2.16	/proc/sys/fs/types - File system type specific parameters
> 
>  ------------------------------------------------------------------------------
>  Preface
> @@ -2283,4 +2284,21 @@ For example:
>    $ echo 0x7 > /proc/self/coredump_filter
>    $ ./some_program
> 
> +2.16 /proc/sys/fs/types/ - File system type specific parameters
> +----------------------------------------------------------------
> +
> +There's a separate directory /proc/sys/fs/types/<type>/ for each
> +filesystem type, containing the following files:
> +
> +usermount_safe
> +--------------
> +
> +Setting this to non-zero will allow filesystems of this type to be
> +mounted by unprivileged users (note, that there are other
> +prerequisites as well).
> +
> +Care should be taken when enabling this, since most
> +filesystems haven't been designed with unprivileged mounting
> +in mind.
> +
>  ------------------------------------------------------------------------------
> 
> --
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