[bpf-next,v9,0/3] bpf, x86: allow function arguments up to 12 for TRACING

Message

Menglong Dong July 12, 2023, 8:47 a.m. UTC

From: Menglong Dong <[e-mail]>

For now, the BPF program of type BPF_PROG_TYPE_TRACING can only be used
on the kernel functions whose arguments count less than or equal to 6, if
not considering '> 8 bytes' struct argument. This is not friendly at all,
as too many functions have arguments count more than 6. According to the
current kernel version, below is a statistics of the function arguments
count:

argument count | function count
7              | 704
8              | 270
9              | 84
10             | 47
11             | 47
12             | 27
13             | 22
14             | 5
15             | 0
16             | 1

Therefore, let's enhance it by increasing the function arguments count
allowed in arch_prepare_bpf_trampoline(), for now, only x86_64.

In the 1st patch, we save/restore regs with BPF_DW size to make the code
in save_regs()/restore_regs() simpler.

In the 2nd patch, we make arch_prepare_bpf_trampoline() support to copy
function arguments in stack for x86 arch. Therefore, the maximum
arguments can be up to MAX_BPF_FUNC_ARGS for FENTRY, FEXIT and
MODIFY_RETURN. Meanwhile, we clean the potential garbage value when we
copy the arguments on-stack.

And the 3rd patch is for the testcases of the this series.

Changes since v8:
- change the way to test fmod_ret in the 3rd patch

Changes since v7:
- split the testcases, and add fentry_many_args/fexit_many_args to
  DENYLIST.aarch64 in 3rd patch

Changes since v6:
- somit nits from commit message and comment in the 1st patch
- remove the inline in get_nr_regs() in the 1st patch
- rename some function and various in the 1st patch

Changes since v5:
- adjust the commit log of the 1st patch, avoiding confusing people that
  bugs exist in current code
- introduce get_nr_regs() to get the space that used to pass args on
  stack correct in the 2nd patch
- add testcases to tracing_struct.c instead of fentry_test.c and
  fexit_test.c

Changes since v4:
- consider the case of the struct in arguments can't be hold by regs
- add comment for some code
- add testcases for MODIFY_RETURN
- rebase to the latest

Changes since v3:
- try make the stack pointer 16-byte aligned. Not sure if I'm right :)
- introduce clean_garbage() to clean the grabage when argument count is 7
- use different data type in bpf_testmod_fentry_test{7,12}
- add testcase for grabage values in ctx

Changes since v2:
- keep MAX_BPF_FUNC_ARGS still
- clean garbage value in upper bytes in the 2nd patch
- move bpf_fentry_test{7,12} to bpf_testmod.c and rename them to
  bpf_testmod_fentry_test{7,12} meanwhile in the 3rd patch

Changes since v1:
- change the maximun function arguments to 14 from 12
- add testcases (Jiri Olsa)
- instead EMIT4 with EMIT3_off32 for "lea" to prevent overflow

Menglong Dong (3):
  bpf, x86: save/restore regs with BPF_DW size
  bpf, x86: allow function arguments up to 12 for TRACING
  selftests/bpf: add testcase for TRACING with 6+ arguments

 arch/x86/net/bpf_jit_comp.c                   | 246 +++++++++++++++---
 net/bpf/test_run.c                            |  14 +-
 tools/testing/selftests/bpf/DENYLIST.aarch64  |   2 +
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  49 +++-
 .../selftests/bpf/prog_tests/fentry_test.c    |  43 ++-
 .../selftests/bpf/prog_tests/fexit_test.c     |  43 ++-
 .../selftests/bpf/prog_tests/modify_return.c  |  13 +-
 .../selftests/bpf/prog_tests/tracing_struct.c |  19 ++
 .../selftests/bpf/progs/fentry_many_args.c    |  39 +++
 .../selftests/bpf/progs/fexit_many_args.c     |  40 +++
 .../selftests/bpf/progs/modify_return.c       |  40 +++
 .../selftests/bpf/progs/tracing_struct.c      |  54 ++++
 12 files changed, 545 insertions(+), 57 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/fentry_many_args.c
 create mode 100644 tools/testing/selftests/bpf/progs/fexit_many_args.c