From 0ceb2f21d3e02b310ed246a210b6cecea24e19b2 Mon Sep 17 00:00:00 2001 From: KaFai Wan Date: Sun, 18 Oct 2026 08:53:45 +0800 Subject: [PATCH] bpf: Determine freplace staticness from BTF linkage bpf_check_attach_target() rejects a freplace whose target is a static function, using the subprog "unreliable" flag as a proxy for "static". That flag is raised whenever a subprogram's BTF prototype disagrees with the registers at its call site. `147400e9c27c ("bpf: Compute parameters liveness in bpf_compute_live_registers()")` made that work for static callees: a static function with an unused argument no longer matches its BTF prototype at the call site and is marked unreliable. Since LLVM commit `29b36a16f111 ("[BPF] Emit ABI-accurate BTF prototypes for DW_CC_nocall")` in 23.1 [1] that premise is gone: BTFDebug may now drop arguments removed by the optimizer, so for __noinline static int subprog(struct __sk_buff *ctx) { return bpf_ktime_get_ns(); } compiled with clang 23 the BTF describes subprog() as [75] FUNC_PROTO '(anon)' ret_type_id=2 vlen=0 [76] FUNC 'subprog' type_id=75 linkage=static There is then no argument left to compare, the subprog stays reliable, and the check in bpf_check_attach_target() is skipped. The freplace is then rejected by the later linkage check instead, reporting "subprog() is not a global function", and the selftest "exceptions" fails. Determine staticness from the BTF linkage instead, and keep the unreliable check for global functions whose BTF does not match their call site. There is no functional change: freplacing a static function was rejected before and is still rejected with the same -EINVAL. Selftest "exceptions" is updated to match. [1] https://github.com/llvm/llvm-project/pull/198426 Fixes: 147400e9c27c ("bpf: Compute parameters liveness in bpf_compute_live_registers()") Signed-off-by: KaFai Wan Signed-off-by: Alexei Starovoitov Link: https://patch.msgid.link/20261018005348.2195130-2-kafai.wan@linux.dev --- kernel/bpf/verifier.c | 3 ++- tools/testing/selftests/bpf/prog_tests/exceptions.c | 2 +- 2 files changed, 3 insertions(+), 2 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 63fff641d985..74e5ac248a10 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -22301,7 +22301,8 @@ int bpf_check_attach_target(struct bpf_verifier_log *log, } conservative = aux->func_info_aux[subprog].unreliable; if (prog_extension) { - if (conservative) { + if (aux->func_info_aux[subprog].linkage != BTF_FUNC_GLOBAL || + conservative) { bpf_log(log, "Cannot replace static functions\n"); return -EINVAL; diff --git a/tools/testing/selftests/bpf/prog_tests/exceptions.c b/tools/testing/selftests/bpf/prog_tests/exceptions.c index cea01d638f6f..1bd15c4c125b 100644 --- a/tools/testing/selftests/bpf/prog_tests/exceptions.c +++ b/tools/testing/selftests/bpf/prog_tests/exceptions.c @@ -333,7 +333,7 @@ static void test_exceptions_success(void) bpf_program__fd(skel->progs.exception_throw_subprog), "throwing_subprog"), "set_attach_target")) goto done; - }), "throwing_subprog() is not a global function", 0); + }), "Cannot replace static functions", 0); if (test__start_subtest("non-throwing extension -> non-throwing subprog")) /* non-throwing extension -> non-throwing global subprog : OK */