Re: [PATCHv2 RFC bpf-next 0/7] bpf: Add support for ftrace probe

On Thu, Apr 15, 2021 at 8:10 AM Steven Rostedt [off-list ref] wrote:

On Wed, 14 Apr 2021 15:46:49 -0700
Andrii Nakryiko [off-list ref] wrote:

quoted

On Wed, Apr 14, 2021 at 5:19 AM Jiri Olsa [off-list ref] wrote:

quoted

On Tue, Apr 13, 2021 at 06:04:05PM -0700, Andrii Nakryiko wrote:

quoted

On Tue, Apr 13, 2021 at 7:57 AM Jiri Olsa [off-list ref] wrote:

quoted

hi,
sending another attempt on speeding up load of multiple probes
for bpftrace and possibly other tools (first post in [1]).

This patchset adds support to attach bpf program directly to
ftrace probe as suggested by Steven and it speeds up loading
for bpftrace commands like:

   # bpftrace -e 'kfunc:_raw_spin* { @[probe] = count(); }'
   # bpftrace -e 'kfunc:ksys_* { @[probe] = count(); }'

Using ftrace with single bpf program for attachment to multiple
functions is much faster than current approach, where we need to
load and attach program for each probe function.

Ok, so first of all, I think it's super important to allow fast
attachment of a single BPF program to multiple kernel functions (I
call it mass-attachment). I've been recently prototyping a tool
(retsnoop, [0]) that allows attaching fentry/fexit to multiple
functions, and not having this feature turned into lots of extra code
and slow startup/teardown speeds. So we should definitely fix that.

But I think the approach you've taken is not the best one, even though
it's a good starting point for discussion.

First, you are saying function return attachment support is missing,
but is not needed so far. I actually think that without func return
the whole feature is extremely limiting. Not being able to measure
function latency  by tracking enter/exit events is crippling for tons
of useful applications. So I think this should go with both at the
same time.

But guess what, we already have a good BPF infra (BPF trampoline and
fexit programs) that supports func exit tracing. Additionally, it
supports the ability to read input arguments *on function exit*, which
is something that kretprobe doesn't support and which is often a very
limiting restriction, necessitating complicated logic to trace
function entry just to store input arguments. It's a killer feature
and one that makes fexit so much more useful than kretprobe.

The only problem is that currently we have a 1:1:1 relationship
between BPF trampoline, BPF program, and kernel function. I think we
should allow to have a single BPF program, using a single BPF
trampoline, but being able to attach to multiple kernel functions
(1:1:N). This will allow to validate BPF program once, allocate only
one dedicated BPF trampoline, and then (with appropriate attach API)
attach them in a batch mode.

heya,
I had some initial prototypes trying this way, but always ended up
in complicated code, that's why I turned to ftrace_ops.

let's see if it'll make any sense to you ;-)

1) so let's say we have extra trampoline for the program (which
also seems a bit of waste since there will be just single record

BPF trampoline does more than just calls BPF program. At the very
least it saves input arguments for fexit program to be able to access
it. But given it's one BPF trampoline attached to thousands of
functions, I don't see any problem there.

Note, there's a whole infrastructure that does similar things in ftrace.
I wrote the direct call to jump to individual trampolines, because ftrace
was too generic. The only way at the time to get to the arguments was via
the ftrace_regs_caller, which did a full save of regs, because this was
what kprobes needed, and was too expensive for BPF.

I now regret writing the direct callers, and instead should have just done
what I did afterward, which was to make ftrace default to a light weight
trampoline that only saves enough for getting access to the arguments of
the function. And have BPF use that. But I was under the impression that
BPF needed fast access to a single function, and it would not become a
generic trampoline for multiple functions, because that was the argument
used to not enhance ftrace.

Today, ftrace by dafault (on x86) implements a generic way to get the
arguments, and just the arguments which is exactly what BPF would need for
multiple functions. And yes, you even have access to the return code if you
want to "hijack" it. And since it was originally for a individual functions
(and not a batch), I created the direct caller for BPF. But the direct
caller will not be enhanced for multiple functions, as that's not its
purpose. If you want a trampoline to be called back to multiple functions,
then use the infrastructure that was designed for that. Which is what Jiri
had proposed here.

And because the direct caller can mess with the return code, it breaks
function graph tracing. As a temporary work around, we just made function
graph ignore any function that has a direct caller attached to it.

If you want batch processing of BPF programs, you need to first fix the
function graph tracing issue, and allow both BPF attached callers and
function graph to work on the same functions.

I don't know how the BPF code does it, but if you are tracing the exit
of a function, I'm assuming that you hijack the return pointer and replace
it with a call to a trampoline that has access to the arguments. To do

As Jiri replied, BPF trampoline doesn't do it the same way as
kretprobe does it. Which gives the fexit BPF program another critical
advantage over kretprobe -- we know traced function's entry IP in both
entry and exit cases, which allows us to generically correlate them.

I've tried to figure out how to get that entry IP from kretprobe and
couldn't find any way. Do you know if it's possible at all or it's a
fundamental limitation of the way kretprobe is implemented (through
hijacking return address)?

this you need a shadow stack to save the real return as well as the
parameters of the function. This is something that I have patches that do
similar things with function graph.

If you want this feature, lets work together and make this work for both
BPF and ftrace.

Absolutely, ultimately for users it doesn't matter what specific
mechanism is used under the cover. It just seemed like BPF trampoline
has all the useful tracing features (entry IP and input arguments in
fexit) already and is just mostly missing a quick batch attach API. If
we can get the same from ftrace, all the better.

-- Steve