On Fri, Jul 5, 2024 at 8:38 AM Oleg Nesterov [off-list ref] wrote:

Tried to read this patch, but I fail to understand it. It looks
obvioulsy wrong to me, see below.

I tend to agree with the comments from Peter, but lets ignore them
for the moment.

On 07/01, Andrii Nakryiko wrote:

quoted

 static void put_uprobe(struct uprobe *uprobe)
 {
-     if (refcount_dec_and_test(&uprobe->ref)) {
+     s64 v;
+
+     /*
+      * here uprobe instance is guaranteed to be alive, so we use Tasks
+      * Trace RCU to guarantee that uprobe won't be freed from under us, if
+      * we end up being a losing "destructor" inside uprobe_treelock'ed
+      * section double-checking uprobe->ref value below.
+      * Note call_rcu_tasks_trace() + uprobe_free_rcu below.
+      */
+     rcu_read_lock_trace();
+
+     v = atomic64_add_return(UPROBE_REFCNT_PUT, &uprobe->ref);
+
+     if (unlikely((u32)v == 0)) {

I must have missed something, but how can this ever happen?

Suppose uprobe_register(inode) is called the 1st time. To simplify, suppose
that this binary is not used, so _register() doesn't install breakpoints/etc.

IIUC, with this change (u32)uprobe->ref == 1 when uprobe_register() succeeds.

Now suppose that uprobe_unregister() is called right after that. It does

        uprobe = find_uprobe(inode, offset);

this increments the counter, (u32)uprobe->ref == 2

        __uprobe_unregister(...);

this wont't change the counter,

        put_uprobe(uprobe);

this drops the reference added by find_uprobe(), (u32)uprobe->ref == 1.

Where should the "final" put_uprobe() come from?

IIUC, this patch lacks another put_uprobe() after consumer_del(), no?

Argh, this is an artifact of splitting the overall change into
separate patches. The final version of uprobe_unregister() doesn't do
find_uprobe(), we just get it from uprobe_consumer->uprobe pointer
without any tree lookup.

Oleg.