On Tue, Jul 2, 2024 at 4:55 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> Hi Jiri,
>
> On Mon, 1 Jul 2024 18:41:07 +0200
> 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 keep count of session consumers for uprobe and allocate session_consumer
> > object for each in return_instance object. This allows us to store
> > return values of 'handler' callbacks and data pointers of shared
> > data between both handlers.
> >
> > 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.
> >
> > The control of 'ret_handler' callback execution is done via return
> > value of the 'handler' callback. If it's 0 we install and execute
> > return uprobe, if it's 1 we do not.
> >
> > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> > ---
> > include/linux/uprobes.h | 16 ++++-
> > kernel/events/uprobes.c | 129 +++++++++++++++++++++++++++++++++---
> > kernel/trace/bpf_trace.c | 6 +-
> > kernel/trace/trace_uprobe.c | 12 ++--
> > 4 files changed, 144 insertions(+), 19 deletions(-)
> >
> > diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> > index f46e0ca0169c..903a860a8d01 100644
> > --- a/include/linux/uprobes.h
> > +++ b/include/linux/uprobes.h
> > @@ -34,15 +34,18 @@ enum uprobe_filter_ctx {
> > };
> >
> > struct uprobe_consumer {
> > - int (*handler)(struct uprobe_consumer *self, struct pt_regs *regs);
> > + int (*handler)(struct uprobe_consumer *self, struct pt_regs *regs, __u64 *data);
> > int (*ret_handler)(struct uprobe_consumer *self,
> > unsigned long func,
> > - struct pt_regs *regs);
> > + struct pt_regs *regs, __u64 *data);
> > bool (*filter)(struct uprobe_consumer *self,
> > enum uprobe_filter_ctx ctx,
> > struct mm_struct *mm);
> >
> > struct uprobe_consumer *next;
> > +
> > + bool session; /* marks uprobe session consumer */
> > + unsigned int session_id; /* set when uprobe_consumer is registered */
>
> Hmm, why this has both session and session_id?
session is caller's request to establish session semantics. Jiri, I
think it's better to move it higher next to
handler/ret_handler/filter, that's the part of uprobe_consumer struct
which has read-only caller-provided data (I'm adding offset and
ref_ctr_offset there as well).
> I also think we can use the address of uprobe_consumer itself as a unique id.
+1
>
> Also, if we can set session enabled by default, and skip ret_handler by handler's
> return value, it is more simpler. (If handler returns a specific value, skip ret_handler)
you mean derive if it's a session or not by both handler and
ret_handler being set? I guess this works fine for BPF side, because
there we never had them both set. If this doesn't regress others, I
think it's OK. We just need to make sure we don't unnecessarily
allocate session state for consumers that don't set both handler and
ret_handler. That would be a waste.
>
> > };
> >
> > #ifdef CONFIG_UPROBES
> > @@ -80,6 +83,12 @@ struct uprobe_task {
> > unsigned int depth;
> > };
> >
> > +struct session_consumer {
> > + __u64 cookie;
>
> And this cookie looks not scalable. If we can pass a data to handler, I would like to
> reuse it to pass the target function parameters to ret_handler as kretprobe/fprobe does.
>
> int (*handler)(struct uprobe_consumer *self, struct pt_regs *regs, void *data);
>
> uprobes can collect its uc's required sizes and allocate the memory (shadow stack frame)
> at handler_chain().
The goal here is to keep this simple and fast. I'd prefer to keep it
small and fixed size, if possible. I'm thinking about caching and
reusing return_instance as one of the future optimizations, so if we
can keep this more or less fixed (assuming there is typically not more
than 1 or 2 consumers per uprobe, which seems realistic), this will
provide a way to avoid excessive memory allocations.
>
> > + unsigned int id;
> > + int rc;
> > +};
> > +
> > struct return_instance {
> > struct uprobe *uprobe;
> > unsigned long func;
> > @@ -88,6 +97,9 @@ struct return_instance {
> > bool chained; /* true, if instance is nested */
> >
> > struct return_instance *next; /* keep as stack */
> > +
> > + int sessions_cnt;
> > + struct session_consumer sessions[];
>
> In that case, we don't have this array, but
>
> char data[];
>
> And decode data array, which is a slice of variable length structure;
>
> struct session_consumer {
> struct uprobe_consumer *uc;
> char data[];
> };
>
> The size of session_consumer is uc->session_data_size + sizeof(uc).
>
> What would you think?
>
> Thank you,
>
> > };
> >
> > enum rp_check {
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index 2c83ba776fc7..4da410460f2a 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -63,6 +63,8 @@ struct uprobe {
> > loff_t ref_ctr_offset;
> > unsigned long flags;
> >
> > + unsigned int sessions_cnt;
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