Message-ID: <CAEf4BzYKVbCEGupX47fwM0XSzwwmXs+0sVpcAdp3poFLkjMA6Q@mail.gmail.com>
Date: Wed, 3 Jul 2024 14:43:27 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Oleg Nesterov <oleg@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org, 
	Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>, 
	John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...omium.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCHv2 bpf-next 1/9] uprobe: Add support for session consumer

On Wed, Jul 3, 2024 at 9:09 AM Jiri Olsa <olsajiri@...il.com> wrote:
>
> On Tue, Jul 02, 2024 at 05:13:38PM -0700, Andrii Nakryiko wrote:
> > On Tue, Jul 2, 2024 at 4:55 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > >
> > > Hi Jiri,
> > >
> > > On Mon,  1 Jul 2024 18:41:07 +0200
> > > Jiri Olsa <jolsa@...nel.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 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@...nel.org>
> > > > ---
> > > >  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
>
> and session_id is set when uprobe is registered and used when
> return uprobe is executed to find matching uprobe_consumer,
> plz check handle_uretprobe_chain/session_consumer_find
>
> > 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).
>
> ok, makes sense
>
> >
> > > 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.
>
> hum.. so the current code installs return uprobe if there's ret_handler
> defined in consumer and also the entry 'handler' needs to return 0
>
> if entry 'handler' returns 1 the uprobe is unregistered
>
> we could define new return value from 'handler' to 'not execute the
> 'ret_handler' and have 'handler' return values:
>
>   0 - execute 'ret_handler' if defined
>   1 - remove the uprobe
>   2 - do NOT execute 'ret_handler'  // this current triggers WARN
>
> we could delay the allocation of 'return_instance' until the first
> consumer returns 0, so there's no perf regression
>
> that way we could treat all consumers the same and we wouldn't need
> the session flag..
>
> ok looks like good idea ;-) will try that

Just please double check that we don't pass through 1 or 2 as a return
result for BPF uprobes/multi-uprobes, so that we don't have any
accidental changes of behavior.

>
> thanks,
> jirka
>
> >
> > >
> > > >  };
> > > >
> > > >  #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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