[PATCH v6 bpf-next 0/3] bpf: Fix tailcall hierarchy

Leon Hwang
2024-07-14T12:39:17Z

This patchset fixes a tailcall hierarchy issue.

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

The issue has been resolved on both x86_64 and arm64[1].

I provide a long commit message in the "bpf, x64: Fix tailcall hierarchy"
patch to describe how the issue happens and how this patchset resolves the
issue in details.

How does this patchset resolve the issue?

In short, it stores tail_call_cnt on the stack of main prog, and propagates
tail_call_cnt_ptr to its subprogs.

First, at the prologue of main prog, it initializes tail_call_cnt and
prepares tail_call_cnt_ptr. And at the prologue of subprog, it reuses
the tail_call_cnt_ptr from caller.

Then, when a tailcall happens, it increments tail_call_cnt by its pointer.

v5 -> v6:
* Address comments from Eduard:
* Add JITed dumping along annotating comments in "bpf, x64: Fix
tailcall hierarchy".
* Rewrite two selftests with RUN_TESTS macro.

v4 -> v5:
* Solution changes from tailcall run ctx to tail_call_cnt and its pointer.
It's because v4 solution is unable to handle the case that there is no
tailcall in subprog but there is tailcall in EXT prog which attaches to
the subprog.

v3 -> v4:
* Solution changes from per-task tail_call_cnt to tailcall run ctx.
As for per-cpu/per-task solution, there is a case it is unable to handle[2].

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/6203dd01-789d-f02c-5293-def4c1b18aef@gmail.com/
[1] https://github.com/kernel-patches/bpf/pull/7350/checks
[2] https://lore.kernel.org/bpf/CAADnVQK1qF+uBjwom2s2W-yEmgd_3rGi5Nr+KiV3cW0T+UPPfA@mail.gmail.com/

Leon Hwang (3):
bpf, x64: Fix tailcall hierarchy
bpf, arm64: Fix tailcall hierarchy
selftests/bpf: Add testcases for tailcall hierarchy fixing

arch/arm64/net/bpf_jit_comp.c | 57 +++-
arch/x86/net/bpf_jit_comp.c | 107 ++++--
.../selftests/bpf/prog_tests/tailcalls.c | 320 ++++++++++++++++++
.../bpf/progs/tailcall_bpf2bpf_hierarchy1.c | 34 ++
.../bpf/progs/tailcall_bpf2bpf_hierarchy2.c | 70 ++++
.../bpf/progs/tailcall_bpf2bpf_hierarchy3.c | 62 ++++
.../progs/tailcall_bpf2bpf_hierarchy_fentry.c | 35 ++
tools/testing/selftests/bpf/progs/tc_dummy.c | 12 +
8 files changed, 653 insertions(+), 44 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
create mode 100644 tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy_fentry.c
create mode 100644 tools/testing/selftests/bpf/progs/tc_dummy.c

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2.44.0