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

Andrii Nakryiko
2024-09-09T23:44:22Z

On Mon, Sep 9, 2024 at 12:46 AM Jiri Olsa <jolsa@kernel.org> 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@kernel.org>
> ---

Just minor things:

Acked-by: Andrii Nakryiko <andrii@kernel.org>

> 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);
> } …