On 2019-11-13, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
On Tue, Nov 05, 2019 at 08:05:47PM +1100, Aleksa Sarai wrote:@@ -862,6 +870,8 @@ static int nd_jump_root(struct nameidata *nd) void nd_jump_link(struct path *path) { struct nameidata *nd = current->nameidata; + + nd->last_magiclink.same_mnt = (nd->path.mnt == path->mnt); path_put(&nd->path); nd->path = *path; @@ -1082,6 +1092,10 @@ const char *get_link(struct nameidata *nd) if (nd->flags & LOOKUP_MAGICLINK_JUMPED) { if (unlikely(nd->flags & LOOKUP_NO_MAGICLINKS)) return ERR_PTR(-ELOOP); + if (unlikely(nd->flags & LOOKUP_NO_XDEV)) { + if (!nd->last_magiclink.same_mnt) + return ERR_PTR(-EXDEV); + } }Ugh... Wouldn't it be better to take that logics (some equivalent thereof) into nd_jump_link()? Or just have nd_jump_link() return an error...
This could be done, but the reason for stashing it away in last_magiclink is because of the future magic-link re-opening patches which can't be implemented like that without putting the open_flags inside nameidata (which was decided to be too ugly a while ago). My point being that I could implement it this way for this series, but I'd have to implement something like last_magiclink when I end up re-posting the magic-link stuff in a few weeks. Looking at all the nd_jump_link() users, the other option is to just disallow magic-link crossings entirely for LOOKUP_NO_XDEV. The only thing allowing them permits is to resolve file descriptors that are pointing to the same procfs mount -- and it's unclear to me how useful that really is (apparmorfs and nsfs will always give -EXDEV because aafs_mnt and nsfs_mnt are internal kernel vfsmounts). -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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