On Mon, 2020-02-17 at 22:20 +0100, Christian Brauner wrote: > On Mon, Feb 17, 2020 at 01:06:08PM -0800, James Bottomley wrote: > > On Fri, 2020-02-14 at 19:35 +0100, Christian Brauner wrote: > > [...] > > > People not as familiar with user namespaces might not be aware > > > that fsid mappings already exist. Right now, fsid mappings are > > > always identical to id mappings. Specifically, the kernel will > > > lookup fsuids in the uid mappings and fsgids in the gid mappings > > > of the relevant user namespace. > > > > This isn't actually entirely true: today we have the superblock > > user namespace, which can be used for fsid remapping on filesystems > > that support it (currently f2fs and fuse). Since this is a single > > shift, > > Note that this states "the relevant" user namespace not the caller's > user namespace. And the point is true even for such filesystems. fuse > does call make_kuid(fc->user_ns, attr->uid) and hence looks up the > mapping in the id mappings.. This would be replaced by make_kfsuid(). > > > how is it going to play with s_user_ns? Do you have to understand > > the superblock mapping to use this shift, or are we simply using > > this to replace s_user_ns? > > I'm not sure what you mean by understand the superblock mapping. The > case is not different from the devpts patch in this series. So since devpts wasn't originally a s_user_ns consumer, I assume you're thinking that this patch series just replaces the whole of s_user_ns for fuse and f2fs and we can remove it? > Fuse needs to be changed to call make_kfsuid() since it is mountable > inside user namespaces at which point everthing just works. The fuse case is slightly more complicated because there are sound reasons to run the daemon in a separate user namespace regardless of where the end fuse mount is. James
