[bpf-next,v10,0/7] bpf: Support multi-attach for freplace programs

This series adds support attaching freplace BPF programs to multiple targets.
This is needed to support incremental attachment of multiple XDP programs using
the libxdp dispatcher model.

Patch 1 moves prog_aux->linked_prog and the trampoline to be embedded in
bpf_tracing_link on attach, and freed by the link release logic, and introduces
a mutex to protect the writing of the pointers in prog->aux.

Based on this refactoring (and previously applied patches), it becomes pretty
straight-forward to support multiple-attach for freplace programs (patch 2).
This is simply a matter of creating a second bpf_tracing_link if a target is
supplied. However, for API consistency with other types of link attach, this
option is added to the BPF_LINK_CREATE API instead of extending
bpf_raw_tracepoint_open().

Patch 3 is a port of Jiri Olsa's patch to support fentry/fexit on freplace
programs. His approach of getting the target type from the target program
reference no longer works after we've gotten rid of linked_prog (because the
bpf_tracing_link reference disappears on attach). Instead, we used the saved
reference to the target prog type that is also used to verify compatibility on
secondary freplace attachment.

Patch 4 is the accompanying libbpf update, and patches 5-7 are selftests: patch
5 tests for the multi-freplace functionality itself; patch 6 is Jiri's previous
selftest for the fentry-to-freplace fix; patch 7 is a test for the change
introduced in the previously-applied patches, blocking MODIFY_RETURN functions
from attaching to other BPF programs.

With this series, libxdp and xdp-tools can successfully attach multiple programs
one at a time. To play with this, use the 'freplace-multi-attach' branch of
xdp-tools:

$ git clone --recurse-submodules --branch freplace-multi-attach https://github.com/xdp-project/xdp-tools
$ cd xdp-tools/xdp-loader
$ make
$ sudo ./xdp-loader load veth0 ../lib/testing/xdp_drop.o
$ sudo ./xdp-loader load veth0 ../lib/testing/xdp_pass.o
$ sudo ./xdp-loader status

The series is also available here:
https://git.kernel.org/pub/scm/linux/kernel/git/toke/linux.git/log/?h=bpf-freplace-multi-attach-alt-10

Changelog:

v10:
  - Dial back the s/tgt_/dst_/ replacement a bit
  - Fix smatch warning (from ktest robot)
  - Rebase to bpf-next, drop already-applied patches

v9:
  - Clarify commit message of patch 3
  - Add new struct bpf_attach_target_info for returning from
    bpf_check_attach_target() and passing to bpf_trampoline_get()
  - Move trampoline key computation into a helper
  - Make sure we don't break bpffs debug umh
  - Add some comment blocks explaining the logic flow in
    bpf_tracing_prog_attach()
  - s/tgt_/dst_/ in prog->aux, and for local variables using those members
  - Always drop dst_trampoline and dst_prog from prog->aux on first attach
  - Don't remove syscall fmod_ret test from selftest benchmarks
  - Add saved_ prefix to dst_{prog,attach}_type members in prog_aux
  - Drop prog argument from check_attach_modify_return()
  - Add comment about possible NULL of tr_link->tgt_prog on link_release()

v8:
  - Add a separate error message when trying to attach FMOD_REPLACE to tgt_prog
  - Better error messages in bpf_program__attach_freplace()
  - Don't lock mutex when setting tgt_* pointers in prog create and verifier
  - Remove fmod_ret programs from benchmarks in selftests (new patch 11)
  - Fix a few other nits in selftests

v7:
  - Add back missing ptype == prog->type check in link_create()
  - Use tracing_bpf_link_attach() instead of separate freplace_bpf_link_attach()
  - Don't break attachment of bpf_iters in libbpf (by clobbering link_create.iter_info)

v6:
  - Rebase to latest bpf-next
  - Simplify logic in bpf_tracing_prog_attach()
  - Don't create a new attach_type for link_create(), disambiguate on prog->type
    instead
  - Use raw_tracepoint_open() in libbpf bpf_program__attach_ftrace() if called
    with NULL target
  - Switch bpf_program__attach_ftrace() to take function name as parameter
    instead of btf_id
  - Add a patch disallowing MODIFY_RETURN programs from attaching to other BPF
    programs, and an accompanying selftest (patches 1 and 10)

v5:
  - Fix typo in inline function definition of bpf_trampoline_get()
  - Don't put bpf_tracing_link in prog->aux, use a mutex to protect tgt_prog and
    trampoline instead, and move them to the link on attach.
  - Restore Jiri as author of the last selftest patch

v4:
  - Cleanup the refactored check_attach_btf_id() to make the logic easier to follow
  - Fix cleanup paths for bpf_tracing_link
  - Use xchg() for removing the bpf_tracing_link from prog->aux and restore on (some) failures
  - Use BPF_LINK_CREATE operation to create link with target instead of extending raw_tracepoint_open
  - Fold update of tools/ UAPI header into main patch
  - Update arg dereference patch to use skeletons and set_attach_target()

v3:
  - Get rid of prog_aux->linked_prog entirely in favour of a bpf_tracing_link
  - Incorporate Jiri's fix for attaching fentry to freplace programs

v2:
  - Drop the log arguments from bpf_raw_tracepoint_open
  - Fix kbot errors
  - Rebase to latest bpf-next

---

Jiri Olsa (1):
      selftests/bpf: Adding test for arg dereference in extension trace

Toke Høiland-Jørgensen (6):
      bpf: move prog->aux->linked_prog and trampoline into bpf_link on attach
      bpf: support attaching freplace programs to multiple attach points
      bpf: Fix context type resolving for extension programs
      libbpf: add support for freplace attachment in bpf_link_create
      selftests: add test for multiple attachments of freplace program
      selftests: Add selftest for disallowing modify_return attachment to freplace


 include/linux/bpf.h                           |   2 +
 include/uapi/linux/bpf.h                      |   9 +-
 kernel/bpf/btf.c                              |   9 +-
 kernel/bpf/syscall.c                          | 132 +++++++++--
 kernel/bpf/verifier.c                         |  10 +
 tools/include/uapi/linux/bpf.h                |   9 +-
 tools/lib/bpf/bpf.c                           |  18 +-
 tools/lib/bpf/bpf.h                           |   3 +-
 tools/lib/bpf/libbpf.c                        |  44 +++-
 tools/lib/bpf/libbpf.h                        |   3 +
 tools/lib/bpf/libbpf.map                      |   1 +
 .../selftests/bpf/prog_tests/fexit_bpf2bpf.c  | 212 +++++++++++++++---
 .../selftests/bpf/prog_tests/trace_ext.c      | 111 +++++++++
 .../selftests/bpf/progs/fmod_ret_freplace.c   |  14 ++
 .../bpf/progs/freplace_get_constant.c         |  15 ++
 .../selftests/bpf/progs/test_trace_ext.c      |  18 ++
 .../bpf/progs/test_trace_ext_tracing.c        |  25 +++
 17 files changed, 573 insertions(+), 62 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/trace_ext.c
 create mode 100644 tools/testing/selftests/bpf/progs/fmod_ret_freplace.c
 create mode 100644 tools/testing/selftests/bpf/progs/freplace_get_constant.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_trace_ext.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_trace_ext_tracing.c