[RFC,bpf-next,v2,00/17] bpf: implement bpf based dumping of kernel data structures

The v1 version is here:
  https://lore.kernel.org/bpf/20200408232520.2675265-1-yhs@fb.com/T/#m058a6817dc3ded9d2db0192ca08486b4a3f4daf0
Compared to v1, I have made changes:
  . use BPF_RAW_TRACEPOINT_OPEN to create an anonymous dumper
  . use BPF_OBJ_PIN with pathname instead dumper name to
    create a file dumper
  . support PTR_TO_BTF_ID_OR_NULL so bpf program will be called when
    the dumping session ends. This gives bpf program an opportunity
    to print footer or accumulate and send summaries for anonymous
    dumper.
  . use BPF_OBJ_GET_INFO_BY_FD to get bpfdump target/dumper info.
Still missing:
  . bpf_seq_printf()/bpf_seq_write() related changes
  . double check seq_ops implementation for bpf_map/task/task_file.
  . libbpf/bpftool implementation
  . tests for new features
  . ...

As there are some discussions regarding to the kernel interface/steps to
create file/anonymous dumpers, I think it will be beneficial for
discussion with this work in progress.

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 based dumping. Initial kernel changes are
  still needed, but a data structure change will not require kernel changes
  any more. bpf program itself is used to adapt to new data structure
  changes. This will give certain flexibility with guaranteed correctness.

  Here, kernel seq_ops is used to facilitate dumping, similar to current
  /proc and many other lossless kernel dumping facilities.

User Interfaces:
  1. A new mount file system, bpfdump at /sys/kernel/bpfdump is introduced.
     Different from /sys/fs/bpf, this is a single user mount. Mount command
     can be:
        mount -t bpfdump bpfdump /sys/kernel/bpfdump
  2. Kernel bpf dumpable data structures are represented as directories
     under /sys/kernel/bpfdump, e.g.,
       /sys/kernel/bpfdump/ipv6_route/
       /sys/kernel/bpfdump/netlink/
       /sys/kernel/bpfdump/bpf_map/
       /sys/kernel/bpfdump/task/
       /sys/kernel/bpfdump/task/file/
     In this patch set, we use "target" to represent a particular bpf
     supported data structure, for example, targets "ipv6_route",
     "netlink", "bpf_map", "task", "task/file", which are actual
     directory hierarchy relative to /sys/kernel/bpfdump/.

     Note that nested structures are supported for sub fields in a major
     data structure. For example, target "task/file" is to examine all open
     files for all tasks (task_struct->files) as reference count and
     locks are needed to access task_struct->files safely.
  3. The bpftool command can be used to create a dumper:
       bpftool dumper pin <bpf_prog.o> <dumper_name>
     where the bpf_prog.o encodes the target information. For example, the
     following dumpers can be created:
       /sys/kernel/bpfdump/ipv6_route/{my1, my2}
       /sys/kernel/bpfdump/task/file/{f1, f2}
  4. Use "cat <dumper>" to dump the contents.
     Use "rm -f <dumper>" to delete the dumper.
  5. An anonymous dumper can be created without pinning to a
     physical file. The fd will return to the application and
     the application can then "read" the contents.

Please see patch #15 and #16 for bpf programs and
bpf dumper output examples.

Two new helpers bpf_seq_printf() and bpf_seq_write() are introduced.
bpf_seq_printf() mostly for file based dumpers and bpf_seq_write()
mostly for anonymous dumpers.

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

For introspection, see patch #14,
  bpftool dumper show {target|dumper}
can show all targets and their context structure type name (for writing bpf
programs), or all dumpers with their associated bpf prog_id.
For any open file descriptors (anonymous or from dumper file),
  cat /proc/<pid>/fdinfo/<fd>
will show target and its associated prog_id as well.

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 is needed
to convert socket_common to these three types of sockets based
on socket state. This will be addressed in future work.

References:
  [1]: https://lore.kernel.org/bpf/20200225230427.1976129-1-kafai@fb.com
  [2]: https://github.com/osandov/drgn

Yonghong Song (17):
  net: refactor net assignment for seq_net_private structure
  bpf: create /sys/kernel/bpfdump mount file system
  bpf: provide a way for targets to register themselves
  bpf: allow loading of a dumper program
  bpf: create file or anonymous dumpers
  bpf: add PTR_TO_BTF_ID_OR_NULL support
  bpf: add netlink and ipv6_route targets
  bpf: add bpf_map target
  bpf: add task and task/file targets
  bpf: add bpf_seq_printf and bpf_seq_write helpers
  bpf: support variable length array in tracing programs
  bpf: implement query for target_proto and file dumper prog_id
  tools/libbpf: libbpf support for bpfdump
  tools/bpftool: add bpf dumper support
  tools/bpf: selftests: add dumper programs for ipv6_route and netlink
  tools/bpf: selftests: add dumper progs for bpf_map/task/task_file
  tools/bpf: selftests: add a selftest for anonymous dumper

 fs/proc/proc_net.c                            |   5 +-
 include/linux/bpf.h                           |  31 +
 include/linux/seq_file_net.h                  |   8 +
 include/uapi/linux/bpf.h                      |  35 +-
 include/uapi/linux/magic.h                    |   1 +
 kernel/bpf/Makefile                           |   1 +
 kernel/bpf/btf.c                              |  30 +-
 kernel/bpf/dump.c                             | 806 ++++++++++++++++++
 kernel/bpf/dump_task.c                        | 320 +++++++
 kernel/bpf/syscall.c                          | 146 +++-
 kernel/bpf/verifier.c                         |  33 +-
 kernel/trace/bpf_trace.c                      | 172 ++++
 net/ipv6/ip6_fib.c                            |  71 +-
 net/ipv6/route.c                              |  29 +
 net/netlink/af_netlink.c                      |  94 +-
 scripts/bpf_helpers_doc.py                    |   2 +
 tools/bpf/bpftool/dumper.c                    | 135 +++
 tools/bpf/bpftool/main.c                      |   3 +-
 tools/bpf/bpftool/main.h                      |   1 +
 tools/include/uapi/linux/bpf.h                |  35 +-
 tools/lib/bpf/bpf.c                           |   9 +-
 tools/lib/bpf/bpf.h                           |   1 +
 tools/lib/bpf/libbpf.c                        |  88 +-
 tools/lib/bpf/libbpf.h                        |   3 +
 tools/lib/bpf/libbpf.map                      |   2 +
 .../selftests/bpf/prog_tests/bpfdump_test.c   |  42 +
 .../selftests/bpf/progs/bpfdump_bpf_map.c     |  33 +
 .../selftests/bpf/progs/bpfdump_ipv6_route.c  |  71 ++
 .../selftests/bpf/progs/bpfdump_netlink.c     |  80 ++
 .../selftests/bpf/progs/bpfdump_task.c        |  29 +
 .../selftests/bpf/progs/bpfdump_task_file.c   |  30 +
 .../selftests/bpf/progs/bpfdump_test_kern.c   |  31 +
 32 files changed, 2343 insertions(+), 34 deletions(-)
 create mode 100644 kernel/bpf/dump.c
 create mode 100644 kernel/bpf/dump_task.c
 create mode 100644 tools/bpf/bpftool/dumper.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/bpfdump_test.c
 create mode 100644 tools/testing/selftests/bpf/progs/bpfdump_bpf_map.c
 create mode 100644 tools/testing/selftests/bpf/progs/bpfdump_ipv6_route.c
 create mode 100644 tools/testing/selftests/bpf/progs/bpfdump_netlink.c
 create mode 100644 tools/testing/selftests/bpf/progs/bpfdump_task.c
 create mode 100644 tools/testing/selftests/bpf/progs/bpfdump_task_file.c
 create mode 100644 tools/testing/selftests/bpf/progs/bpfdump_test_kern.c