ROOT_DEV usage in hostfs

ROOT_DEV usage in hostfs

[Christoph Hellwig]

Could someone explain why hosts does thnings like:

	if((ino->i_sb->s_dev == ROOT_DEV) && (ino->i_uid == getuid()))
		ino->i_uid = 0;

(in fs/hostfs/hostfs_kern.c:read_name())

and

	if(attr->ia_valid & ATTR_UID){
		if((dentry->d_inode->i_sb->s_dev == ROOT_DEV) &&
		   (attr->ia_uid == 0))
			attr->ia_uid = getuid();
		attrs.ia_valid |= HOSTFS_ATTR_UID;
		attrs.ia_uid = attr->ia_uid;
	}
	if(attr->ia_valid & ATTR_GID){
		if((dentry->d_inode->i_sb->s_dev == ROOT_DEV) &&
		   (attr->ia_gid == 0))
			attr->ia_gid = getuid();
		attrs.ia_valid |= HOSTFS_ATTR_GID;
		attrs.ia_gid = attr->ia_gid;
	}

(hostfs_getattr)

A filesystems shouldn't have special casing for the rootfs, and
chowning files from root to the current user sounds like an extremly
bad idea to me aswell.

It's also the last thing preventing us from exporting ROOT_DEV

Re: ROOT_DEV usage in hostfs

[Christoph Hellwig]

> It's also the last thing preventing us from exporting ROOT_DEV

unexporting

Re: ROOT_DEV usage in hostfs

[Petr Baudis]

Dear diary, on Tue, Jan 25, 2005 at 10:10:39AM CET, I got a letter,
where Christoph Hellwig <hch@lst.de> told me, that...
> Could someone explain why hosts does thnings like:
> 
> 	if((ino->i_sb->s_dev == ROOT_DEV) && (ino->i_uid == getuid()))
> 		ino->i_uid = 0;
> 
> (in fs/hostfs/hostfs_kern.c:read_name())
> 
> and
> 
> 	if(attr->ia_valid & ATTR_UID){
> 		if((dentry->d_inode->i_sb->s_dev == ROOT_DEV) &&
> 		   (attr->ia_uid == 0))
> 			attr->ia_uid = getuid();
> 		attrs.ia_valid |= HOSTFS_ATTR_UID;
> 		attrs.ia_uid = attr->ia_uid;
> 	}
> 	if(attr->ia_valid & ATTR_GID){
> 		if((dentry->d_inode->i_sb->s_dev == ROOT_DEV) &&
> 		   (attr->ia_gid == 0))
> 			attr->ia_gid = getuid();
> 		attrs.ia_valid |= HOSTFS_ATTR_GID;
> 		attrs.ia_gid = attr->ia_gid;
> 	}
> 
> (hostfs_getattr)
> 
> A filesystems shouldn't have special casing for the rootfs, and
> chowning files from root to the current user sounds like an extremly
> bad idea to me aswell.
> 
> It's also the last thing preventing us from exporting ROOT_DEV

Hmm, I pretty much forgot it all, but it seems to do a trick that if you
run UML with a non-root UID (if that's actually possible), the files
owned by you on the host system are owned by root inside of the UML
(which does not seem to be so unreasonable, but should be probably
configurable if it's the case) - depends on what context getuid() runs
in, which I'm not sure of anymore neither.

The gid handling is clearly buggy. It should test that in read_name()
too and use getgid().

I don't know why it happens only on ROOT_DEV, perhaps some heuristic.
Anyway, this comes (almost) verbatim from the 2.4.19 version by Jeff
Dike, so he will hopefully come with a better explanation :-).

-- 
				Petr "Pasky" Baudis
Stuff: http://pasky.or.cz/
98% of the time I am right. Why worry about the other 3%.

Re: ROOT_DEV usage in hostfs

[Jeff Dike]

hch@lst.de said:
> Could someone explain why hosts does thnings like:
> 	if((ino->i_sb->s_dev == ROOT_DEV) && (ino->i_uid == getuid()))
> 		ino->i_uid = 0; 

Don't let that stop you unexporting ROOT_DEV.  It was a nasty kludge to make
booting from a host filesystem kind of work.  It would flip permissions around
in order to make everything in the root filesystem owned by root, when the
real ownership on the host is some normal user.

I have a better way of doing ownerships properly, so this was going to die
anyway.

				Jeff