Re: [PATCH v3 1/1] vfs: Respect MS_RDONLY at bind mount creation

[Mateusz Guzik]

On Fri, Aug 01, 2014 at 11:55:12PM -0400, Richard Yao wrote:
> `mount -o bind,ro ...` suffers from a silent failure where the readonly
> flag is ignored. The bind mount will be created rw whenever the target
> is rw. Users typically workaround this by remounting readonly, but that
> does not work when you want to define readonly bind mounts in fstab.
> This is a major annoyance when dealing with recursive bind mounts
> because the userland mount command does not expose the option to
> recursively remount a subtree as readonly.
> 
> Signed-off-by: Richard Yao <ryao@xxxxxxxxxx>
> ---
>  fs/namespace.c | 15 +++++++++++----
>  fs/pnode.h     | 17 +++++++++--------
>  2 files changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 182bc41..6b3a566 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -921,6 +921,9 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
>  	if (flag & CL_MAKE_SHARED)
>  		set_mnt_shared(mnt);
>  
> +	if (flag & CL_MAKE_RDONLY)
> +		mnt_make_readonly(mnt);
> +

Is not this more heavyweight than necessary?

i.e. would not mnt->mnt.mnt_flags |= MNT_READONLY suffice?

Since this mount point is not attached anywhere there cannot be any
writers to synchronize against (and if there could be some, you would
have to check error code from mnt_make_readonly).

That said, apart from this thingy and cosmetic issues looks good to me,
but note I'm just some random guy commenting on a patch.

>  	/* stick the duplicate mount on the same expiry list
>  	 * as the original if that was on one */
>  	if (flag & CL_EXPIRE) {
> @@ -1827,11 +1830,13 @@ static bool has_locked_children(struct mount *mnt, struct dentry *dentry)
>   * do loopback mount.
>   */
>  static int do_loopback(struct path *path, const char *old_name,
> -				int recurse)
> +				unsigned long flags)
>  {
>  	struct path old_path;
>  	struct mount *mnt = NULL, *old, *parent;
>  	struct mountpoint *mp;
> +	int recurse = flags & MS_REC;
> +	int clflags = (flags & MS_RDONLY) ? CL_MAKE_RDONLY : 0;
>  	int err;
>  	if (!old_name || !*old_name)
>  		return -EINVAL;
> @@ -1862,9 +1867,10 @@ static int do_loopback(struct path *path, const char *old_name,
>  		goto out2;
>  
>  	if (recurse)
> -		mnt = copy_tree(old, old_path.dentry, CL_COPY_MNT_NS_FILE);
> +		mnt = copy_tree(old, old_path.dentry, CL_COPY_MNT_NS_FILE |
> +			clflags);
>  	else
> -		mnt = clone_mnt(old, old_path.dentry, 0);
> +		mnt = clone_mnt(old, old_path.dentry, clflags);
>  
>  	if (IS_ERR(mnt)) {
>  		err = PTR_ERR(mnt);
> @@ -2444,7 +2450,8 @@ long do_mount(const char *dev_name, const char *dir_name,
>  		retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
>  				    data_page);
>  	else if (flags & MS_BIND)
> -		retval = do_loopback(&path, dev_name, flags & MS_REC);
> +		retval = do_loopback(&path, dev_name, flags & (MS_REC |
> +							       MS_RDONLY));
>  	else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
>  		retval = do_change_type(&path, flags);
>  	else if (flags & MS_MOVE)
> diff --git a/fs/pnode.h b/fs/pnode.h
> index 4a24635..7d32196 100644
> --- a/fs/pnode.h
> +++ b/fs/pnode.h
> @@ -20,14 +20,15 @@
>  #define SET_MNT_MARK(m) ((m)->mnt.mnt_flags |= MNT_MARKED)
>  #define CLEAR_MNT_MARK(m) ((m)->mnt.mnt_flags &= ~MNT_MARKED)
>  
> -#define CL_EXPIRE    		0x01
> -#define CL_SLAVE     		0x02
> -#define CL_COPY_UNBINDABLE	0x04
> -#define CL_MAKE_SHARED 		0x08
> -#define CL_PRIVATE 		0x10
> -#define CL_SHARED_TO_SLAVE	0x20
> -#define CL_UNPRIVILEGED		0x40
> -#define CL_COPY_MNT_NS_FILE	0x80
> +#define CL_EXPIRE    		0x001
> +#define CL_SLAVE     		0x002
> +#define CL_COPY_UNBINDABLE	0x004
> +#define CL_MAKE_SHARED 		0x008
> +#define CL_PRIVATE 		0x010
> +#define CL_SHARED_TO_SLAVE	0x020
> +#define CL_UNPRIVILEGED		0x040
> +#define CL_COPY_MNT_NS_FILE	0x080
> +#define CL_MAKE_RDONLY 		0x100
>  
>  #define CL_COPY_ALL		(CL_COPY_UNBINDABLE | CL_COPY_MNT_NS_FILE)
>  
> -- 
> 1.8.5.5
> 

-- 
Mateusz Guzik
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