[bpf-next,v3,0/3] bpf, x64: Fix tailcall hierarchy

Message

Leon Hwang April 2, 2024, 3:26 p.m. UTC

The patchset fixes a tailcall hierarchy issue.

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

The issue is only resolved on x86.

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

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

In short, it stores tail_call_cnt at task_struct.

First, at the prologue of bpf prog, it initialise the tail_call_cnt at
task_struct like "current->bpf_tail_call_cnt = 0;".

Then, when a tailcall happens, it compares the tail_call_cnt with
MAX_TAIL_CALL_CNT, and then increment it.

v2 -> v3:
  * Solution changes from percpu tail_call_cnt to tail_call_cnt at task_struct.

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/[e-mail]/
[1] https://github.com/kernel-patches/bpf/pull/6721/checks

Leon Hwang (3):
  bpf: Add bpf_tail_call_cnt to task_struct
  bpf, x64: Fix tailcall hierarchy
  selftests/bpf: Add testcases for tailcall hierarchy fixing

 arch/x86/net/bpf_jit_comp.c                   | 137 +++---
 include/linux/sched.h                         |   2 +
 .../selftests/bpf/prog_tests/tailcalls.c      | 418 ++++++++++++++++++
 .../bpf/progs/tailcall_bpf2bpf_hierarchy1.c   |  38 ++
 .../bpf/progs/tailcall_bpf2bpf_hierarchy2.c   |  63 +++
 .../bpf/progs/tailcall_bpf2bpf_hierarchy3.c   |  50 +++
 6 files changed, 652 insertions(+), 56 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: 8f1ff3cf139bc1269eebae5d43ffbe482675f360