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

Jiri Olsa
2024-09-10T07:17:43Z

On Mon, Sep 09, 2024 at 04:44:09PM -0700, Andrii Nakryiko wrote:
> 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

ok

>
> >
> > 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

right, makes sense

>
> > /*
> > * 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?

there's null check in kfree, but it's true that we can skip the
whole call and there's the else condition line already, ok

>
> >
> > 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?

sure, ric_idx seems ok to me

thanks,
jirka

>
> >
> > 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);
> > }
>
> …