Re: [PATCH v4 4/8] uprobes: travers uprobe's consumer list locklessly under SRCU protection

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On Thu, Aug 29, 2024 at 4:10 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Thu, Aug 29, 2024 at 11:37:37AM -0700, Andrii Nakryiko wrote:
> > uprobe->register_rwsem is one of a few big bottlenecks to scalability of
> > uprobes, so we need to get rid of it to improve uprobe performance and
> > multi-CPU scalability.
> >
> > First, we turn uprobe's consumer list to a typical doubly-linked list
> > and utilize existing RCU-aware helpers for traversing such lists, as
> > well as adding and removing elements from it.
> >
> > For entry uprobes we already have SRCU protection active since before
> > uprobe lookup. For uretprobe we keep refcount, guaranteeing that uprobe
> > won't go away from under us, but we add SRCU protection around consumer
> > list traversal.
> >
> > Lastly, to keep handler_chain()'s UPROBE_HANDLER_REMOVE handling simple,
> > we remember whether any removal was requested during handler calls, but
> > then we double-check the decision under a proper register_rwsem using
> > consumers' filter callbacks. Handler removal is very rare, so this extra
> > lock won't hurt performance, overall, but we also avoid the need for any
> > extra protection (e.g., seqcount locks).
> >
> > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > ---
> >  include/linux/uprobes.h |   2 +-
> >  kernel/events/uprobes.c | 104 +++++++++++++++++++++++-----------------
> >  2 files changed, 62 insertions(+), 44 deletions(-)
> >
> > diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> > index 9cf0dce62e4c..29c935b0d504 100644
> > --- a/include/linux/uprobes.h
> > +++ b/include/linux/uprobes.h
> > @@ -35,7 +35,7 @@ struct uprobe_consumer {
> >                               struct pt_regs *regs);
> >       bool (*filter)(struct uprobe_consumer *self, struct mm_struct *mm);
> >
> > -     struct uprobe_consumer *next;
> > +     struct list_head cons_node;
> >  };
> >
> >  #ifdef CONFIG_UPROBES
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index 8bdcdc6901b2..97e58d160647 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -59,7 +59,7 @@ struct uprobe {
> >       struct rw_semaphore     register_rwsem;
> >       struct rw_semaphore     consumer_rwsem;
> >       struct list_head        pending_list;
> > -     struct uprobe_consumer  *consumers;
> > +     struct list_head        consumers;
> >       struct inode            *inode;         /* Also hold a ref to inode */
> >       struct rcu_head         rcu;
> >       loff_t                  offset;
> > @@ -783,6 +783,7 @@ static struct uprobe *alloc_uprobe(struct inode *inode, loff_t offset,
> >       uprobe->inode = inode;
> >       uprobe->offset = offset;
> >       uprobe->ref_ctr_offset = ref_ctr_offset;
> > +     INIT_LIST_HEAD(&uprobe->consumers);
> >       init_rwsem(&uprobe->register_rwsem);
> >       init_rwsem(&uprobe->consumer_rwsem);
> >       RB_CLEAR_NODE(&uprobe->rb_node);
> > @@ -808,32 +809,19 @@ static struct uprobe *alloc_uprobe(struct inode *inode, loff_t offset,
> >  static void consumer_add(struct uprobe *uprobe, struct uprobe_consumer *uc)
> >  {
> >       down_write(&uprobe->consumer_rwsem);
> > -     uc->next = uprobe->consumers;
> > -     uprobe->consumers = uc;
> > +     list_add_rcu(&uc->cons_node, &uprobe->consumers);
> >       up_write(&uprobe->consumer_rwsem);
> >  }
> >
> >  /*
> >   * For uprobe @uprobe, delete the consumer @uc.
> > - * Return true if the @uc is deleted successfully
> > - * or return false.
> > + * Should never be called with consumer that's not part of @uprobe->consumers.
> >   */
> > -static bool consumer_del(struct uprobe *uprobe, struct uprobe_consumer *uc)
> > +static void consumer_del(struct uprobe *uprobe, struct uprobe_consumer *uc)
> >  {
> > -     struct uprobe_consumer **con;
> > -     bool ret = false;
> > -
> >       down_write(&uprobe->consumer_rwsem);
> > -     for (con = &uprobe->consumers; *con; con = &(*con)->next) {
> > -             if (*con == uc) {
> > -                     *con = uc->next;
> > -                     ret = true;
> > -                     break;
> > -             }
> > -     }
> > +     list_del_rcu(&uc->cons_node);
> >       up_write(&uprobe->consumer_rwsem);
> > -
> > -     return ret;
> >  }
> >
> >  static int __copy_insn(struct address_space *mapping, struct file *filp,
> > @@ -929,7 +917,8 @@ static bool filter_chain(struct uprobe *uprobe, struct mm_struct *mm)
> >       bool ret = false;
> >
> >       down_read(&uprobe->consumer_rwsem);
> > -     for (uc = uprobe->consumers; uc; uc = uc->next) {
> > +     list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
> > +                              srcu_read_lock_held(&uprobes_srcu)) {
> >               ret = consumer_filter(uc, mm);
> >               if (ret)
> >                       break;
> > @@ -1125,18 +1114,29 @@ void uprobe_unregister(struct uprobe *uprobe, struct uprobe_consumer *uc)
> >       int err;
> >
> >       down_write(&uprobe->register_rwsem);
> > -     if (WARN_ON(!consumer_del(uprobe, uc))) {
> > -             err = -ENOENT;
> > -     } else {
> > -             err = register_for_each_vma(uprobe, NULL);
> > -             /* TODO : cant unregister? schedule a worker thread */
> > -             if (unlikely(err))
> > -                     uprobe_warn(current, "unregister, leaking uprobe");
> > -     }
> > +     consumer_del(uprobe, uc);
> > +     err = register_for_each_vma(uprobe, NULL);
> >       up_write(&uprobe->register_rwsem);
> >
> > -     if (!err)
> > -             put_uprobe(uprobe);
> > +     /* TODO : cant unregister? schedule a worker thread */
> > +     if (unlikely(err)) {
> > +             uprobe_warn(current, "unregister, leaking uprobe");
> > +             goto out_sync;
> > +     }
> > +
> > +     put_uprobe(uprobe);
> > +
> > +out_sync:
> > +     /*
> > +      * Now that handler_chain() and handle_uretprobe_chain() iterate over
> > +      * uprobe->consumers list under RCU protection without holding
> > +      * uprobe->register_rwsem, we need to wait for RCU grace period to
> > +      * make sure that we can't call into just unregistered
> > +      * uprobe_consumer's callbacks anymore. If we don't do that, fast and
> > +      * unlucky enough caller can free consumer's memory and cause
> > +      * handler_chain() or handle_uretprobe_chain() to do an use-after-free.
> > +      */
> > +     synchronize_srcu(&uprobes_srcu);
> >  }
> >  EXPORT_SYMBOL_GPL(uprobe_unregister);
> >
> > @@ -1214,13 +1214,20 @@ EXPORT_SYMBOL_GPL(uprobe_register);
> >  int uprobe_apply(struct uprobe *uprobe, struct uprobe_consumer *uc, bool add)
> >  {
> >       struct uprobe_consumer *con;
> > -     int ret = -ENOENT;
> > +     int ret = -ENOENT, srcu_idx;
> >
> >       down_write(&uprobe->register_rwsem);
> > -     for (con = uprobe->consumers; con && con != uc ; con = con->next)
> > -             ;
> > -     if (con)
> > -             ret = register_for_each_vma(uprobe, add ? uc : NULL);
> > +
> > +     srcu_idx = srcu_read_lock(&uprobes_srcu);
> > +     list_for_each_entry_srcu(con, &uprobe->consumers, cons_node,
> > +                              srcu_read_lock_held(&uprobes_srcu)) {
> > +             if (con == uc) {
> > +                     ret = register_for_each_vma(uprobe, add ? uc : NULL);
> > +                     break;
> > +             }
> > +     }
> > +     srcu_read_unlock(&uprobes_srcu, srcu_idx);
> > +
> >       up_write(&uprobe->register_rwsem);
> >
> >       return ret;
> > @@ -2085,10 +2092,12 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
> >       struct uprobe_consumer *uc;
> >       int remove = UPROBE_HANDLER_REMOVE;
> >       bool need_prep = false; /* prepare return uprobe, when needed */
> > +     bool has_consumers = false;
> >
> > -     down_read(&uprobe->register_rwsem);
> >       current->utask->auprobe = &uprobe->arch;
> > -     for (uc = uprobe->consumers; uc; uc = uc->next) {
> > +
> > +     list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
> > +                              srcu_read_lock_held(&uprobes_srcu)) {
> >               int rc = 0;
> >
> >               if (uc->handler) {
> > @@ -2101,17 +2110,24 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
> >                       need_prep = true;
> >
> >               remove &= rc;
> > +             has_consumers = true;
> >       }
> >       current->utask->auprobe = NULL;
> >
> >       if (need_prep && !remove)
> >               prepare_uretprobe(uprobe, regs); /* put bp at return */
> >
> > -     if (remove && uprobe->consumers) {
> > -             WARN_ON(!uprobe_is_active(uprobe));
> > -             unapply_uprobe(uprobe, current->mm);
> > +     if (remove && has_consumers) {
> > +             down_read(&uprobe->register_rwsem);
> > +
> > +             /* re-check that removal is still required, this time under lock */
> > +             if (!filter_chain(uprobe, current->mm)) {
>
> sorry for late question, but I do not follow this change..
>
> at this point we got 1 as handler's return value from all the uprobe's consumers,
> why do we need to call filter_chain in here.. IIUC this will likely skip over
> the removal?
>

Because we don't hold register_rwsem we are now racing with
registration. So while we can get all consumers at the time we were
iterating over the consumer list to request deletion, a parallel CPU
can add another consumer that needs this uprobe+PID combination. So if
we don't double-check, we are risking having a consumer that will not
be triggered for the desired process.

Does it make sense? Given removal is rare, it's ok to take lock if we
*suspect* removal, and then check authoritatively again under lock.

> with single uprobe_multi consumer:
>
>   handler_chain
>     uprobe_multi_link_handler
>       uprobe_prog_run
>         bpf_prog returns 1
>
>     remove = 1
>
>     if (remove && has_consumers) {
>
>       filter_chain - uprobe_multi_link_filter returns true.. so the uprobe stays?
>
> maybe I just need to write test for it ;-)
>
> thanks,
> jirka
>
>
> > +                     WARN_ON(!uprobe_is_active(uprobe));
> > +                     unapply_uprobe(uprobe, current->mm);
> > +             }
> > +
> > +             up_read(&uprobe->register_rwsem);
> >       }
> > -     up_read(&uprobe->register_rwsem);
> >  }
> >
> >  static void
> > @@ -2119,13 +2135,15 @@ handle_uretprobe_chain(struct return_instance *ri, struct pt_regs *regs)
> >  {
> >       struct uprobe *uprobe = ri->uprobe;
> >       struct uprobe_consumer *uc;
> > +     int srcu_idx;
> >
> > -     down_read(&uprobe->register_rwsem);
> > -     for (uc = uprobe->consumers; uc; uc = uc->next) {
> > +     srcu_idx = srcu_read_lock(&uprobes_srcu);
> > +     list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
> > +                              srcu_read_lock_held(&uprobes_srcu)) {
> >               if (uc->ret_handler)
> >                       uc->ret_handler(uc, ri->func, regs);
> >       }
> > -     up_read(&uprobe->register_rwsem);
> > +     srcu_read_unlock(&uprobes_srcu, srcu_idx);
> >  }
> >
> >  static struct return_instance *find_next_ret_chain(struct return_instance *ri)
> > --
> > 2.43.5
> >





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