[PATCH bpf-next v1 00/19] bpf: implement bpf iterator for kernel data

Yonghong Song
2020-04-27T20:12:42Z

Motivation:
The current way to dump kernel data structures mostly:
1. /proc system
2. various specific tools like "ss" which requires kernel support.
3. drgn
The dropback for the first two is that whenever you want to dump more, you
need change the kernel. For example, Martin wants to dump socket local
storage with "ss". Kernel change is needed for it to work ([1]).
This is also the direct motivation for this work.

drgn ([2]) solves this proble nicely and no kernel change is not needed.
But since drgn is not able to verify the validity of a particular pointer value,
it might present the wrong results in rare cases.

In this patch set, we introduce bpf iterator. Initial kernel changes are
still needed for interested kernel data, but a later data structure change
will not require kernel changes any more. bpf program itself can adapt
to new data structure changes. This will give certain flexibility with
guaranteed correctness.

In this patch set, kernel seq_ops is used to facilitate iterating through
kernel data, similar to current /proc and many other lossless kernel
dumping facilities. In the future, different iterators can be
implemented to trade off losslessness for other criteria e.g. no
repeated object visits, etc.

User Interface:
1. Similar to prog/map/link, the iterator can be pinned into a
path within a bpffs mount point.
2. The bpftool command can pin an iterator to a file
bpftool iter pin <bpf_prog.o> <path>
3. Use `cat <path>` to dump the contents.
Use `rm -f <path>` to remove the pinned iterator.
4. The anonymous iterator can be created as well.

Please see patch #17 andd #18 for bpf programs and bpf iterator
output examples.

Note that certain iterators are namespace aware. For example,
task and task_file targets only iterate through current pid namespace.
ipv6_route and netlink will iterate through current net namespace.

Please see individual patches for implementation details.

Performance:
The bpf iterator provides in-kernel aggregation abilities
for kernel data. This can greatly improve performance
compared to e.g., iterating all process directories under /proc.
For example, I did an experiment on my VM with an application forking
different number of tasks and each forked process opening various number
of files. The following is the result with the latency with unit of microseconds:

# of forked tasks # of open files # of bpf_prog calls # latency (us)
100 100 11503 7586
1000 1000 1013203 709513
10000 100 1130203 764519

The number of bpf_prog calls may be more than forked tasks multipled by
open files since there are other tasks running on the system.
The bpf program is a do-nothing program. One millions of bpf calls takes
less than one second.

Future Work:
Although the initial motivation is from Martin's sk_local_storage,
this patch didn't implement tcp6 sockets and sk_local_storage.
The /proc/net/tcp6 involves three types of sockets, timewait,
request and tcp6 sockets. Some kind of type casting or other
mechanism is needed to handle all these socket types in one
bpf program. This will be addressed in future work.

Currently, we do not support kernel data generated under module.
This requires some BTF work.

More work for more iterators, e.g., bpf_progs, cgroups, bpf_map elements, etc.

Changelog:
RFC v2 ([3]) -> non-RFC v1:
- rename bpfdump to bpf_iter
- use bpffs instead of a new file system
- use bpf_link to streamline and simplify iterator creation.

References:
[1]: https://lore.kernel.org/bpf/20200225230427.1976129-1-kafai@fb.com
[2]: https://github.com/osandov/drgn
[3]: https://lore.kernel.org/bpf/40e427e2-5b15-e9aa-e2cb-42dc1b53d047@gmail.com/T/

Yonghong Song (19):
net: refactor net assignment for seq_net_private structure
bpf: implement an interface to register bpf_iter targets
bpf: add bpf_map iterator
bpf: allow loading of a bpf_iter program
bpf: support bpf tracing/iter programs for BPF_LINK_CREATE
bpf: support bpf tracing/iter programs for BPF_LINK_UPDATE
bpf: create anonymous bpf iterator
bpf: create file bpf iterator
bpf: add PTR_TO_BTF_ID_OR_NULL support
bpf: add netlink and ipv6_route targets
bpf: add task and task/file targets
bpf: add bpf_seq_printf and bpf_seq_write helpers
bpf: handle spilled PTR_TO_BTF_ID properly when checking
stack_boundary
bpf: support variable length array in tracing programs
tools/libbpf: add bpf_iter support
tools/bpftool: add bpf_iter support for bptool
tools/bpf: selftests: add iterator programs for ipv6_route and netlink
tools/bpf: selftests: add iter progs for bpf_map/task/task_file
tools/bpf: selftests: add bpf_iter selftests

fs/proc/proc_net.c | 5 +-
include/linux/bpf.h | 33 ++
include/linux/seq_file_net.h | 8 +
include/uapi/linux/bpf.h | 38 +-
kernel/bpf/Makefile | 2 +-
kernel/bpf/bpf_iter.c | 358 ++++++++++++++++++
kernel/bpf/btf.c | 38 +-
kernel/bpf/inode.c | 28 ++
kernel/bpf/map_iter.c | 107 ++++++
kernel/bpf/syscall.c | 62 ++-
kernel/bpf/task_iter.c | 319 ++++++++++++++++
kernel/bpf/verifier.c | 47 ++-
kernel/trace/bpf_trace.c | 159 ++++++++
net/ipv6/ip6_fib.c | 71 +++-
net/ipv6/route.c | 30 ++
net/netlink/af_netlink.c | 99 ++++-
scripts/bpf_helpers_doc.py | 2 +
.../bpftool/Documentation/bpftool-iter.rst | 71 ++++
tools/bpf/bpftool/bash-completion/bpftool | 13 +
tools/bpf/bpftool/iter.c | 84 ++++
tools/bpf/bpftool/main.c | 3 +-
tools/bpf/bpftool/main.h | 1 +
tools/include/uapi/linux/bpf.h | 38 +-
tools/lib/bpf/bpf.c | 11 +
tools/lib/bpf/bpf.h | 2 +
tools/lib/bpf/bpf_tracing.h | 23 ++
tools/lib/bpf/libbpf.c | 60 +++
tools/lib/bpf/libbpf.h | 11 +
tools/lib/bpf/libbpf.map | 7 +
.../selftests/bpf/prog_tests/bpf_iter.c | 180 +++++++++
.../selftests/bpf/progs/bpf_iter_bpf_map.c | 32 ++
.../selftests/bpf/progs/bpf_iter_ipv6_route.c | 69 ++++
.../selftests/bpf/progs/bpf_iter_netlink.c | 77 ++++
.../selftests/bpf/progs/bpf_iter_task.c | 29 ++
.../selftests/bpf/progs/bpf_iter_task_file.c | 28 ++
.../selftests/bpf/progs/bpf_iter_test_kern1.c | 4 +
.../selftests/bpf/progs/bpf_iter_test_kern2.c | 4 +
.../selftests/bpf/progs/bpf_iter_test_kern3.c | 18 +
.../bpf/progs/bpf_iter_test_kern_common.h | 22 ++
39 files changed, 2174 insertions(+), 19 deletions(-)
create mode 100644 kernel/bpf/bpf_iter.c
create mode 100644 kernel/bpf/map_iter.c
create mode 100644 kernel/bpf/task_iter.c
create mode 100644 tools/bpf/bpftool/Documentation/bpftool-iter.rst
create mode 100644 tools/bpf/bpftool/iter.c
create mode 100644 tools/testing/selftests/bpf/prog_tests/bpf_iter.c
create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_bpf_map.c
create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_ipv6_route.c
create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_netlink.c
create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_task.c
create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_task_file.c
create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_test_kern1.c
create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_test_kern2.c
create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_test_kern3.c
create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_test_kern_common.h

--
2.24.1