Re: [RFC PATCH bpf-next] ksnoop: kernel argument/return value tracing/display using BTF

On Mon, Jan 4, 2021 at 7:29 AM Alan Maguire [off-list ref] wrote:

BPF Type Format (BTF) provides a description of kernel data structures
and of the types kernel functions utilize as arguments and return values.

A helper was recently added - bpf_snprintf_btf() - that uses that
description to create a string representation of the data provided,
using the BTF id of its type.  For example to create a string
representation of a "struct sk_buff", the pointer to the skb
is provided along with the type id of "struct sk_buff".

Here that functionality is utilized to support tracing kernel
function entry and return using k[ret]probes.  The "struct pt_regs"
context can be used to derive arguments and return values, and
when the user supplies a function name we

- look it up in /proc/kallsyms to find its address/module
- look it up in the BTF kernel data to get types of arguments
  and return value
- store a map representation of the trace information, keyed by
  instruction pointer
- on function entry/return we look up the map to retrieve the BTF
  ids of the arguments/return values and can call bpf_snprintf_btf()
  with these argument/return values along with the type ids to store
  a string representation in the map.
- this is then sent via perf event to userspace where it can be
  displayed.

ksnoop can be used to show function signatures; for example:

This is definitely quite useful!

Is it possible to integrate this with bpftrace? That would save people
from learning yet another tool. ;)

Thanks.