Re: [PATCH 2/8] uprobes: revamp uprobe refcounting and lifetime management

Jiri Olsa
2024-08-01T11:09:52Z

On Wed, Jul 31, 2024 at 02:42:50PM -0700, Andrii Nakryiko wrote:

SNIP

> static void put_uprobe(struct uprobe *uprobe)
> {
> - if (refcount_dec_and_test(&uprobe->ref)) {
> - /*
> - * If application munmap(exec_vma) before uprobe_unregister()
> - * gets called, we don't get a chance to remove uprobe from
> - * delayed_uprobe_list from remove_breakpoint(). Do it here.
> - */
> - mutex_lock(&delayed_uprobe_lock);
> - delayed_uprobe_remove(uprobe, NULL);
> - mutex_unlock(&delayed_uprobe_lock);
> - kfree(uprobe);
> - }
> + if (!refcount_dec_and_test(&uprobe->ref))
> + return;
> +
> + write_lock(&uprobes_treelock);
> +
> + if (uprobe_is_active(uprobe))
> + rb_erase(&uprobe->rb_node, &uprobes_tree);
> +
> + write_unlock(&uprobes_treelock);
> +
> + /*
> + * If application munmap(exec_vma) before uprobe_unregister()
> + * gets called, we don't get a chance to remove uprobe from
> + * delayed_uprobe_list from remove_breakpoint(). Do it here.
> + */
> + mutex_lock(&delayed_uprobe_lock);
> + delayed_uprobe_remove(uprobe, NULL);
> + mutex_unlock(&delayed_uprobe_lock);

we should do kfree(uprobe) in here, right?

I think this is fixed later on when uprobe_free_rcu is introduced

SNIP

> @@ -1159,27 +1180,16 @@ struct uprobe *uprobe_register(struct inode *inode,
> if (!IS_ALIGNED(ref_ctr_offset, sizeof(short)))
> return ERR_PTR(-EINVAL);
>
> - retry:
> uprobe = alloc_uprobe(inode, offset, ref_ctr_offset);
> if (IS_ERR(uprobe))
> return uprobe;
>
> - /*
> - * We can race with uprobe_unregister()->delete_uprobe().
> - * Check uprobe_is_active() and retry if it is false.
> - */
> down_write(&uprobe->register_rwsem);
> - ret = -EAGAIN;
> - if (likely(uprobe_is_active(uprobe))) {
> - consumer_add(uprobe, uc);
> - ret = register_for_each_vma(uprobe, uc);
> - }
> + consumer_add(uprobe, uc);
> + ret = register_for_each_vma(uprobe, uc);
> up_write(&uprobe->register_rwsem);
> - put_uprobe(uprobe);
>
> if (ret) {
> - if (unlikely(ret == -EAGAIN))
> - goto retry;

nice, I like getting rid of this.. so far lgtm ;-)

jirka

> uprobe_unregister(uprobe, uc);
> return ERR_PTR(ret);
> }
> @@ -1286,15 +1296,19 @@ static void build_probe_list(struct inode *inode,
> u = rb_entry(t, struct uprobe, rb_node);
> if (u->inode != inode || u->offset < min)
> break;
> + u = try_get_uprobe(u);
> + if (!u) /* uprobe already went away, safe to ignore */
> + continue;
> list_add(&u->pending_list, head);
> - get_uprobe(u);
> }
> for (t = n; (t = rb_next(t)); ) {
> u = rb_entry(t, struct uprobe, rb_node);
> if (u->inode != inode || u->offset > max)
> break;
> + u = try_get_uprobe(u);
> + if (!u) /* uprobe already went away, safe to ignore */
> + continue;
> list_add(&u->pending_list, head);
> - get_uprobe(u);
> }
> }
> read_unlock(&uprobes_treelock);
> @@ -1752,6 +1766,12 @@ static int dup_utask(struct task_struct *t, struct uprobe_task *o_utask)
> return -ENOMEM;
>
> *n = *o;
> + /*
> + * uprobe's refcnt has to be positive at this point, kept by
> + * utask->return_instances items; return_instances can't be
> + * removed right now, as task is blocked due to duping; so
> + * get_uprobe() is safe to use here.
> + */
> get_uprobe(n->uprobe);
> n->next = NULL;
>
> @@ -1894,7 +1914,10 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
> }
> orig_ret_vaddr = utask->return_instances->orig_ret_vaddr;
> }
> -
> + /*
> + * uprobe's refcnt is positive, held by caller, so it's safe to
> + * unconditionally bump it one more time here
> + */
> ri->uprobe = get_uprobe(uprobe);
> ri->func = instruction_pointer(regs);
> ri->stack = user_stack_pointer(regs);
> --
> 2.43.0
>