On Saturday, December 14, 2013 05:32:56 PM Oleg Nesterov wrote: > On 12/14, Paul Moore wrote: > > ... I'm curious about the removal of the task lock; shouldn't we keep > > the task lock in place? > > Why? It protects nothing in this case, afaics. Unless of course it > protects cred->security somehow, but it doesn't look as if. > > Probably task_lock() is here because PTRACE_ATTACH used the same lock, > but this was changed by 4b105cbbaf7c0 in 2009 (ptrace_attach() still > takes it for __ptrace_may_access() but this is another story). > > However (iirc) PTRACE_DETACH never took this lock, so this was always > racy and task_lock() is simply misleading and confusing, at least > currently.
Okay, you convinced me. > So I think the patch is fine, but I decided to send v2 without pid_alive(). > If we are going to keep ptrace_parent(), it would be better to add the > comment into ptrace_parent() to explain that ->ptrace != 0 guarantees that > this task is not unhashed. > > IOW, I also changed my mind about this part > > The patch also checks pid_alive(p) before ptrace_parent(p) to > ensure that this task can't be dead even before rcu_read_lock(), > in this case its ->parent points to nowhere. This is not really > needed "in practice", task->ptrace must be already cleared in > this case but we should not rely on this. > > in the changelog. Seems reasonable to me. I'll let you work on the ptrace_parent() side of things and I'll go ahead and merge your v2 patch; it looks fine to me and regardless of what happens with ptrace_parent() we should dump the task_lock() and add a RCU lock in its place. Thanks for your help. -- paul moore www.paul-moore.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
