[bpf-next,v6,0/9] bpf: Support private stack for bpf progs

Message

Yonghong Song Oct. 20, 2024, 7:13 p.m. UTC

The main motivation for private stack comes from nested scheduler in
sched-ext from Tejun. The basic idea is that
 - each cgroup will its own associated bpf program,
 - bpf program with parent cgroup will call bpf programs
   in immediate child cgroups.

Let us say we have the following cgroup hierarchy:
  root_cg (prog0):
    cg1 (prog1):
      cg11 (prog11):
        cg111 (prog111)
        cg112 (prog112)
      cg12 (prog12):
        cg121 (prog121)
        cg122 (prog122)
    cg2 (prog2):
      cg21 (prog21)
      cg22 (prog22)
      cg23 (prog23)

In the above example, prog0 will call a kfunc which will call prog1 and
prog2 to get sched info for cg1 and cg2 and then the information is
summarized and sent back to prog0. Similarly, prog11 and prog12 will be
invoked in the kfunc and the result will be summarized and sent back to
prog1, etc. The following illustrates a possible call sequence:
   ... -> bpf prog A -> kfunc -> ops.<callback_fn> (bpf prog B) ...

Currently, for each thread, the x86 kernel allocate 16KB stack. Each
bpf program (including its subprograms) has maximum 512B stack size to
avoid potential stack overflow. Nested bpf programs further increase the
risk of stack overflow. To avoid potential stack overflow caused by bpf
programs, this patch set supported private stack and bpf program stack
space is allocated during verification time. Such private stack is applied
to tracing programs like kprobe/uprobe, perf_event, tracepoint, raw
tracepoint and struct_ops progs. For struct_ops progs, if the callback
stub function name has format like
  <st_ops_name>__<member_name>__priv_stack
that callback func prog will use the private stack. For other tracing
programs, if the prog (including subprogs, but not including callback
functions) stack depth is greater than or equals to 128 bytes, private
stack will be used.

But more than one instance of the same bpf program may run in the system.
To make things simple, percpu private stack is allocated for each program,
so if the same program is running on different cpus concurrently, we won't
have any issue. Note that the kernel already have logic to prevent the
recursion for the same bpf program on the same cpu (kprobe, fentry, etc.).

This patch set implemented a percpu private stack based approach for x86
arch. Please see each individual patch for details.

Change logs:
  v5 -> v6:
    - v5 link: https://lore.kernel.org/bpf/[e-mail]/
    - Instead of using (or not using) private stack at struct_ops level,
      each prog in struct_ops can decide whether to use private stack or not.
  v4 -> v5:
    - v4 link: https://lore.kernel.org/bpf/[e-mail]/
    - Remove bpf_prog_call() related implementation.
    - Allow (opt-in) private stack for sched-ext progs.
  v3 -> v4:
    - v3 link: https://lore.kernel.org/bpf/[e-mail]/
      There is a long discussion in the above v3 link trying to allow private
      stack to be used by kernel functions in order to simplify implementation.
      But unfortunately we didn't find a workable solution yet, so we return
      to the approach where private stack is only used by bpf programs.
    - Add bpf_prog_call() kfunc.
  v2 -> v3:
    - Instead of per-subprog private stack allocation, allocate private
      stacks at main prog or callback entry prog. Subprogs not main or callback
      progs will increment the inherited stack pointer to be their
      frame pointer.
    - Private stack allows each prog max stack size to be 512 bytes, intead
      of the whole prog hierarchy to be 512 bytes.
    - Add some tests.

Yonghong Song (9):
  bpf: Allow each subprog having stack size of 512 bytes
  bpf: Rename bpf_struct_ops_arg_info to bpf_struct_ops_func_info
  bpf: Support private stack for struct ops programs
  bpf: Mark each subprog with proper private stack modes
  bpf, x86: Refactor func emit_prologue
  bpf, x86: Create a helper for certain "reg <op>= imm" operations
  bpf, x86: Add jit support for private stack
  selftests/bpf: Add tracing prog private stack tests
  selftests/bpf: Add struct_ops prog private stack tests

 arch/x86/net/bpf_jit_comp.c                   | 187 +++++++++++----
 include/linux/bpf.h                           |  16 +-
 include/linux/bpf_verifier.h                  |   4 +
 include/linux/filter.h                        |   1 +
 kernel/bpf/bpf_struct_ops.c                   |  71 +++---
 kernel/bpf/core.c                             |  24 ++
 kernel/bpf/verifier.c                         | 139 +++++++++--
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  77 +++++++
 .../selftests/bpf/bpf_testmod/bpf_testmod.h   |   6 +
 .../bpf/prog_tests/struct_ops_private_stack.c | 106 +++++++++
 .../selftests/bpf/prog_tests/verifier.c       |   2 +
 .../bpf/progs/struct_ops_private_stack.c      |  62 +++++
 .../bpf/progs/struct_ops_private_stack_fail.c |  62 +++++
 .../progs/struct_ops_private_stack_recur.c    |  50 ++++
 .../bpf/progs/verifier_private_stack.c        | 216 ++++++++++++++++++
 15 files changed, 933 insertions(+), 90 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/struct_ops_private_stack.c
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_private_stack.c
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_private_stack_fail.c
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_private_stack_recur.c
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_private_stack.c