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. Fuse needs to be changed to call make_kfsuid() since it is mountable inside user namespaces at which point everthing just works.
