[PATCH bpf-next v2 0/2] bpf, x64: Fix tailcall hierarchy

[Leon Hwang <hffilwlqm@gmail.com>]

The patchset fixes a tailcall hierarchy issue.

The issue is confirmed in the discussions of "bpf, x64: Fix tailcall infinite
loop"[0].

But, the issue is only resolved on x86.

Hopefully, the issue on aarch64 and s390x will be resolved soon.

This CI history[1] confirms the issue on aarch64 and s390x.

I provide a long commit message in the first patch to describe how the issue
happens and how this patchset resolves the issue in details.

In short, it uses PERCPU tail_call_cnt to store the temporary tail_call_cnt.

First, at the prologue of bpf prog, it initialise the PERCPU
tail_call_cnt by setting current CPU's tail_call_cnt to 0.

Then, when a tailcall happens, it fetches and increments current CPU's
tail_call_cnt, and compares to MAX_TAIL_CALL_CNT.

v1 -> v2:
  * Solution changes from extra run-time call insn to percpu tail_call_cnt.
  * Address comments from Alexei:
    * Use percpu tail_call_cnt.
    * Use asm to make sure no callee saved registers are touched.

RFC v2 -> v1:
  * Solution changes from propagating tail_call_cnt with its pointer to extra
    run-time call insn.
  * Address comments from Maciej:
    * Replace all memcpy(prog, x86_nops[5], X86_PATCH_SIZE) with
        emit_nops(&prog, X86_PATCH_SIZE)

RFC v1 -> RFC v2:
  * Address comments from Stanislav:
    * Separate moving emit_nops() as first patch.

Links:
[0] https://lore.kernel.org/bpf/6203dd01-789d-f02c-5293-def4c1b18aef@gmail.com/
[1] https://github.com/kernel-patches/bpf/pull/6476/checks

Leon Hwang (2):
  bpf, x64: Fix tailcall hierarchy
  selftests/bpf: Add testcases for tailcall hierarchy fixing

 arch/x86/net/bpf_jit_comp.c                   | 128 +++---
 .../selftests/bpf/prog_tests/tailcalls.c      | 418 ++++++++++++++++++
 .../bpf/progs/tailcall_bpf2bpf_hierarchy1.c   |  34 ++
 .../bpf/progs/tailcall_bpf2bpf_hierarchy2.c   |  55 +++
 .../bpf/progs/tailcall_bpf2bpf_hierarchy3.c   |  46 ++
 5 files changed, 624 insertions(+), 57 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy1.c
 create mode 100644 tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c
 create mode 100644 tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c


base-commit: 499e99ea0e8020bfc84b2327d4c37e45dc40bbd1
-- 
2.42.1