Re: [PATCH bpf-next] bpf: Add drgn script to list progs/maps

[Quentin Monnet <quentin@isovalent.com>]

2020-02-26 18:32 UTC-0800 ~ Andrey Ignatov <rdna@fb.com>
> drgn is a debugger that reads kernel memory and uses DWARF to get types
> and symbols. See [1], [2] and [3] for more details on drgn.
> 
> Since drgn operates on kernel memory it has access to kernel internals
> that user space doesn't. It allows to get extended info about various
> kernel data structures.
> 
> Introduce bpf.py drgn script to list BPF programs and maps and their
> properties unavailable to user space via kernel API.
> 
> The main use-case bpf.py covers is to show BPF programs attached to
> other BPF programs via freplace/fentry/fexit mechanisms introduced
> recently. There is no user-space API to get this info and e.g. bpftool
> can only show all BPF programs but can't show if program A replaces a
> function in program B.
> 

[...]

> 
> Signed-off-by: Andrey Ignatov <rdna@fb.com>
> ---
>   tools/bpf/bpf.py | 149 +++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 149 insertions(+)
>   create mode 100755 tools/bpf/bpf.py
> 
> diff --git a/tools/bpf/bpf.py b/tools/bpf/bpf.py
> new file mode 100755
> index 000000000000..a00d112c0486
> --- /dev/null
> +++ b/tools/bpf/bpf.py
> @@ -0,0 +1,149 @@
> +#!/usr/bin/env drgn
> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> +#
> +# Copyright (c) 2020 Facebook
> +
> +DESCRIPTION = """
> +drgn script to list BPF programs or maps and their properties
> +unavailable via kernel API.
> +
> +See https://github.com/osandov/drgn/ for more details on drgn.
> +"""
> +
> +import argparse
> +import sys
> +
> +from drgn.helpers import enum_type_to_class
> +from drgn.helpers.linux import (
> +    bpf_map_for_each,
> +    bpf_prog_for_each,
> +    hlist_for_each_entry,
> +)
> +
> +
> +BpfMapType = enum_type_to_class(prog.type("enum bpf_map_type"), "BpfMapType")
> +BpfProgType = enum_type_to_class(prog.type("enum bpf_prog_type"), "BpfProgType")
> +BpfProgTrampType = enum_type_to_class(
> +    prog.type("enum bpf_tramp_prog_type"), "BpfProgTrampType"
> +)
> +BpfAttachType = enum_type_to_class(
> +    prog.type("enum bpf_attach_type"), "BpfAttachType"
> +)

Hi Andrey, the script looks neat, thanks for this work!

I tried to run it on my system. Because my kernel is 5.3 and does not 
have "enum bpf_tramp_prog_type", the script crashes on the above 
assignments. But even without that enum, it could be possible to print 
program and map ids and types (even if we don't show the trampolines).

Do you think it would be worth adding error handling on that block, 
something like:

     try:
         BpfMapType = ...
         BpfProgType = ...
         BpfProgTrampType = ...
         BpfAttachType = ...
     except LookupError as e:
         print(e) # Possibly add a hint as kernel being too old?

I understand that printing the BPF extensions is the main interest of 
the script, I'm just thinking it would be nice to use it / tweak it even 
if not on the latest kernel. What do you think?

> +
> +
> +def get_btf_name(btf, btf_id):
> +    type_ = btf.types[btf_id]
> +    if type_.name_off < btf.hdr.str_len:
> +        return btf.strings[type_.name_off].address_of_().string_().decode()
> +    return ""
> +
> +
> +def get_prog_btf_name(bpf_prog):
> +    aux = bpf_prog.aux
> +    if aux.btf:
> +        # func_info[0] points to BPF program function itself.
> +        return get_btf_name(aux.btf, aux.func_info[0].type_id)
> +    return ""
> +
> +
> +def get_prog_name(bpf_prog):
> +    return get_prog_btf_name(bpf_prog) or bpf_prog.aux.name.string_().decode()
> +
> +
> +def attach_type_to_tramp(attach_type):
> +    at = BpfAttachType(attach_type)
> +
> +    if at == BpfAttachType.BPF_TRACE_FENTRY:
> +        return BpfProgTrampType.BPF_TRAMP_FENTRY
> +
> +    if at == BpfAttachType.BPF_TRACE_FEXIT:
> +        return BpfProgTrampType.BPF_TRAMP_FEXIT
> +
> +    return BpfProgTrampType.BPF_TRAMP_REPLACE
> +
> +
> +def get_linked_func(bpf_prog):
> +    kind = attach_type_to_tramp(bpf_prog.expected_attach_type)
> +
> +    linked_prog = bpf_prog.aux.linked_prog
> +    linked_btf_id = bpf_prog.aux.attach_btf_id
> +
> +    linked_prog_id = linked_prog.aux.id.value_()
> +    linked_name = "{}->{}()".format(
> +        get_prog_name(linked_prog),
> +        get_btf_name(linked_prog.aux.btf, linked_btf_id),
> +    )
> +
> +    return "{}->{}: {} {}".format(
> +        linked_prog_id, linked_btf_id.value_(), kind.name, linked_name
> +    )
> +
> +
> +def get_tramp_progs(bpf_prog):
> +    tr = bpf_prog.aux.trampoline
> +    if not tr:
> +        return

Same observation here, I solved it with

     try:
         tr = bpf_prog.aux.trampoline
         if not tr:
             return
     except AttributeError as e:
         print(e)
         return

Best regards,
Quentin