namei: only return -ECHILD from follow_dotdot_rcu()
to #26323588 commit 2b98149c2377bff12be5dd3ce02ae0506e2dd613 upstream. It's over-zealous to return hard errors under RCU-walk here, given that a REF-walk will be triggered for all other cases handling ".." under RCU. The original purpose of this check was to ensure that if a rename occurs such that a directory is moved outside of the bind-mount which the resolution started in, it would be detected and blocked to avoid being able to mess with paths outside of the bind-mount. However, triggering a new REF-walk is just as effective a solution. Cc: "Eric W. Biederman" <ebiederm@xmission.com> Fixes: 397d425d ("vfs: Test for and handle paths that are unreachable from their mnt_root") Suggested-by: NAl Viro <viro@zeniv.linux.org.uk> Signed-off-by: NAleksa Sarai <cyphar@cyphar.com> Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk> Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com> Acked-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
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