Re: [PATCHv3 1/7] uprobe: Add support for session consumer

[Andrii Nakryiko]

On Mon, Sep 9, 2024 at 12:46 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> Adding support for uprobe consumer to be defined as session and have
> new behaviour for consumer's 'handler' and 'ret_handler' callbacks.
>
> The session means that 'handler' and 'ret_handler' callbacks are
> connected in a way that allows to:
>
>   - control execution of 'ret_handler' from 'handler' callback
>   - share data between 'handler' and 'ret_handler' callbacks
>
> The session is enabled by setting new 'session' bool field to true
> in uprobe_consumer object.
>
> We use return_consumer object to keep track of consumers with
> 'ret_handler'. This object also carries the shared data between
> 'handler' and and 'ret_handler' callbacks.

and and

>
> The control of 'ret_handler' callback execution is done via return
> value of the 'handler' callback. This patch adds new 'ret_handler'
> return value (2) which means to ignore ret_handler callback.
>
> Actions on 'handler' callback return values are now:
>
>   0 - execute ret_handler (if it's defined)
>   1 - remove uprobe
>   2 - do nothing (ignore ret_handler)
>
> The session concept fits to our common use case where we do filtering
> on entry uprobe and based on the result we decide to run the return
> uprobe (or not).
>
> It's also convenient to share the data between session callbacks.
>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---

Just minor things:

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

>  include/linux/uprobes.h                       |  17 ++-
>  kernel/events/uprobes.c                       | 132 ++++++++++++++----
>  kernel/trace/bpf_trace.c                      |   6 +-
>  kernel/trace/trace_uprobe.c                   |  12 +-
>  .../selftests/bpf/bpf_testmod/bpf_testmod.c   |   2 +-
>  5 files changed, 133 insertions(+), 36 deletions(-)
>

[...]

>  enum rp_check {
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 4b7e590dc428..9e971f86afdf 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -67,6 +67,8 @@ struct uprobe {
>         loff_t                  ref_ctr_offset;
>         unsigned long           flags;

we should shorten flags to unsigned int, we use one bit out of it

>
> +       unsigned int            consumers_cnt;
> +

and then this won't increase the size of the struct unnecessarily

>         /*
>          * The generic code assumes that it has two members of unknown type
>          * owned by the arch-specific code:
> @@ -826,8 +828,12 @@ static struct uprobe *alloc_uprobe(struct inode *inode, loff_t offset,
>

[...]

>         current->utask->auprobe = NULL;
>
> -       if (need_prep && !remove)
> -               prepare_uretprobe(uprobe, regs); /* put bp at return */
> +       if (ri && !remove)
> +               prepare_uretprobe(uprobe, regs, ri); /* put bp at return */
> +       else
> +               kfree(ri);

maybe `else if (ri) kfree(ri)` to avoid unnecessary calls to kfree
when we only have uprobes?

>
>         if (remove && has_consumers) {
>                 down_read(&uprobe->register_rwsem);
> @@ -2160,15 +2230,25 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
>  static void
>  handle_uretprobe_chain(struct return_instance *ri, struct pt_regs *regs)
>  {
> +       struct return_consumer *ric = NULL;
>         struct uprobe *uprobe = ri->uprobe;
>         struct uprobe_consumer *uc;
> -       int srcu_idx;
> +       int srcu_idx, iter = 0;

iter -> next_ric_idx  or just ric_idx?

>
>         srcu_idx = srcu_read_lock(&uprobes_srcu);
>         list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
>                                  srcu_read_lock_held(&uprobes_srcu)) {
> +               /*
> +                * If we don't find return consumer, it means uprobe consumer
> +                * was added after we hit uprobe and return consumer did not
> +                * get registered in which case we call the ret_handler only
> +                * if it's not session consumer.
> +                */
> +               ric = return_consumer_find(ri, &iter, uc->id);
> +               if (!ric && uc->session)
> +                       continue;
>                 if (uc->ret_handler)
> -                       uc->ret_handler(uc, ri->func, regs);
> +                       uc->ret_handler(uc, ri->func, regs, ric ? &ric->cookie : NULL);
>         }
>         srcu_read_unlock(&uprobes_srcu, srcu_idx);
>  }

[...]