From 3917b1012ee2fef6da16d7450d4267dcb3e93363 Mon Sep 17 00:00:00 2001 From: Ihor Solodrai Date: Mon, 13 Jul 2026 16:52:22 -0700 Subject: [PATCH 1/2] bpf: Fix tracing of kfuncs with implicit args A kfunc marked with KF_IMPLICIT_ARGS flag takes implicit arguments (such as bpf_prog_aux) that the verifier injects at load time. resolve_btfids strips those from the kfunc's BTF-visible prototype and keeps the real kernel ABI in a counterpart _impl prototype [1]. fentry/fexit/fmod_ret/fsession programs may attach to the BPF kernel functions, including those with implicit args. However bpf_check_attach_target() and bpf_check_attach_btf_id_multi() extract the struct btf_func_model from the wrong BTF prototype of the kfunc. The btf_func_model is later read to construct the trampoline, which then causes the injected implicit argument to be clobbered and the kfunc dereferencing garbage. Add btf_attach_func_proto() to resolve the real ABI prototype of the kfunc the way the call site does: by looking up the _impl prototype for a KF_IMPLICIT_ARGS kfunc. Use it at both attach-target model construction sites. To enable this, make two supporting changes: * pass bpf_verifier_log instead of bpf_verifier_env to find_kfunc_impl_proto(), so it can be reused from the attach path * add btf_kfunc_check_flag() to test a flag across all of a kfunc's hook sets, because a program attaching to a kfunc is not in the kfunc's call-set KF_IMPLICIT_ARGS must be consistent across the sets, so btf_kfunc_check_flag() returns -EINVAL on inconsistency. btf_kfunc_check_flag() reads the kfunc's flags from the target's kfunc_set_tab. For a module BTF that table is stable only after the module is live, so take a module reference around the read, mirroring how the kfunc call path gates the same lookup with btf_try_get_module(). The remaining call sites of btf_distill_func_proto() are safe as is. The BPF_TRACE_ITER case distills a registered iterator's prototype, and bpf_struct_ops_desc_init() distills the function-pointer members of a struct_ops type. Neither is a kfunc, and so can't have implicit arguments. [1] https://lore.kernel.org/all/20260120222638.3976562-1-ihor.solodrai@linux.dev/ Fixes: 64e1360524b9 ("bpf: Verifier support for KF_IMPLICIT_ARGS") Reported-by: Tejun Heo Signed-off-by: Ihor Solodrai Link: https://github.com/sched-ext/scx/issues/3687#issuecomment-4906694106 Link: https://patch.msgid.link/20260713235223.1639022-2-ihor.solodrai@linux.dev Signed-off-by: Eduard Zingerman --- include/linux/btf.h | 1 + kernel/bpf/btf.c | 29 +++++++++++++++++++ kernel/bpf/verifier.c | 66 ++++++++++++++++++++++++++++++++++--------- 3 files changed, 83 insertions(+), 13 deletions(-) diff --git a/include/linux/btf.h b/include/linux/btf.h index 240401d9b25b..c09b7994de4e 100644 --- a/include/linux/btf.h +++ b/include/linux/btf.h @@ -578,6 +578,7 @@ const char *btf_str_by_offset(const struct btf *btf, u32 offset); struct btf *btf_parse_vmlinux(void); struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog); u32 *btf_kfunc_flags(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog); +int btf_kfunc_check_flag(const struct btf *btf, u32 kfunc_btf_id, u32 flag); bool btf_kfunc_is_allowed(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog); u32 *btf_kfunc_is_modify_return(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog); diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index 64572f85edc8..c4673a54c4ba 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -9114,6 +9114,35 @@ u32 *btf_kfunc_flags(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_p return btf_kfunc_id_set_contains(btf, hook, kfunc_btf_id); } +/* + * Check a single KF_* @flag on a kfunc across all of its hook sets. + * Returns: + * * 1 if @flag is set + * * 0 if @flag is not set + * * -EINVAL if @flag is set inconsistently across the sets + * * -ENOENT if kfunc_btf_id is not a registered kfunc + */ +int btf_kfunc_check_flag(const struct btf *btf, u32 kfunc_btf_id, u32 flag) +{ + enum btf_kfunc_hook hook; + int res = -ENOENT; + bool is_set; + u32 *flags; + + for (hook = 0; hook < BTF_KFUNC_HOOK_MAX; hook++) { + flags = btf_kfunc_id_set_contains(btf, hook, kfunc_btf_id); + if (!flags) + continue; + is_set = *flags & flag; + if (res < 0) + res = is_set; + else if (res != is_set) + return -EINVAL; + } + + return res; +} + u32 *btf_kfunc_is_modify_return(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog) { diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 9f1333676365..99444eae917e 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2584,24 +2584,25 @@ static struct btf *find_kfunc_desc_btf(struct bpf_verifier_env *env, s16 offset) #define KF_IMPL_SUFFIX "_impl" -static const struct btf_type *find_kfunc_impl_proto(struct bpf_verifier_env *env, +static const struct btf_type *find_kfunc_impl_proto(struct bpf_verifier_log *log, struct btf *btf, const char *func_name) { - char *buf = env->tmp_str_buf; const struct btf_type *func; + char buf[KSYM_NAME_LEN]; s32 impl_id; int len; - len = snprintf(buf, TMP_STR_BUF_LEN, "%s%s", func_name, KF_IMPL_SUFFIX); - if (len < 0 || len >= TMP_STR_BUF_LEN) { - verbose(env, "function name %s%s is too long\n", func_name, KF_IMPL_SUFFIX); + len = snprintf(buf, sizeof(buf), "%s%s", func_name, KF_IMPL_SUFFIX); + if (len < 0 || len >= sizeof(buf)) { + bpf_log(log, "function name %s%s is too long\n", + func_name, KF_IMPL_SUFFIX); return NULL; } impl_id = btf_find_by_name_kind(btf, buf, BTF_KIND_FUNC); if (impl_id <= 0) { - verbose(env, "cannot find function %s in BTF\n", buf); + bpf_log(log, "cannot find function %s in BTF\n", buf); return NULL; } @@ -2653,7 +2654,7 @@ static int fetch_kfunc_meta(struct bpf_verifier_env *env, * can be found through the counterpart _impl kfunc. */ if (kfunc_flags && (*kfunc_flags & KF_IMPLICIT_ARGS)) - func_proto = find_kfunc_impl_proto(env, btf, func_name); + func_proto = find_kfunc_impl_proto(&env->log, btf, func_name); else func_proto = btf_type_by_id(btf, func->type); @@ -18880,6 +18881,47 @@ static int btf_id_allow_sleepable(u32 btf_id, unsigned long addr, const struct b return -EINVAL; } +/* + * Resolve the prototype describing a trace target's real ABI. A + * KF_IMPLICIT_ARGS kfunc has its injected args stripped from the public + * prototype, so use the _impl prototype; other targets use their own. + */ +static const struct btf_type * +btf_attach_func_proto(struct bpf_verifier_log *log, struct btf *btf, u32 func_id) +{ + const struct btf_type *func; + struct module *mod = NULL; + const char *name; + int implicit; + + func = btf_type_by_id(btf, func_id); + if (!func || !btf_type_is_func(func)) + return NULL; + name = btf_name_by_offset(btf, func->name_off); + + /* + * btf_kfunc_check_flag() reads kfunc_set_tab, which for a module is + * stable only once it is live; hold a module ref across the read to + * exclude a concurrent module load. + */ + if (btf_is_module(btf)) { + mod = btf_try_get_module(btf); + if (!mod) + return NULL; + } + implicit = btf_kfunc_check_flag(btf, func_id, KF_IMPLICIT_ARGS); + module_put(mod); + + if (implicit == -EINVAL) { + bpf_log(log, "kfunc %s has inconsistent KF_IMPLICIT_ARGS\n", name); + return NULL; + } + if (implicit > 0) + return find_kfunc_impl_proto(log, btf, name); + + return btf_type_by_id(btf, func->type); +} + int bpf_check_attach_target(struct bpf_verifier_log *log, const struct bpf_prog *prog, const struct bpf_prog *tgt_prog, @@ -19128,8 +19170,8 @@ int bpf_check_attach_target(struct bpf_verifier_log *log, if (prog_extension && btf_check_type_match(log, prog, btf, t)) return -EINVAL; - t = btf_type_by_id(btf, t->type); - if (!btf_type_is_func_proto(t)) + t = btf_attach_func_proto(log, btf, btf_id); + if (!t || !btf_type_is_func_proto(t)) return -EINVAL; if ((prog->aux->saved_dst_prog_type || prog->aux->saved_dst_attach_type) && @@ -19412,10 +19454,8 @@ int bpf_check_attach_btf_id_multi(struct btf *btf, struct bpf_prog *prog, u32 bt tname = btf_name_by_offset(btf, t->name_off); if (!tname) return -EINVAL; - if (!btf_type_is_func(t)) - return -EINVAL; - t = btf_type_by_id(btf, t->type); - if (!btf_type_is_func_proto(t)) + t = btf_attach_func_proto(NULL, btf, btf_id); + if (!t || !btf_type_is_func_proto(t)) return -EINVAL; err = btf_distill_func_proto(NULL, btf, t, tname, &tgt_info->fmodel); if (err < 0) From 0af15f3057a60a038864d47fa4b54d1c3c3c654c Mon Sep 17 00:00:00 2001 From: Kumar Kartikeya Dwivedi Date: Mon, 13 Jul 2026 16:52:23 -0700 Subject: [PATCH 2/2] selftests/bpf: Cover tracing implicit kfunc args KF_IMPLICIT_ARGS kfuncs have a BPF-call prototype and a real kernel target prototype. Add a tracing selftest that attaches fentry and fexit programs to bpf_kfunc_implicit_arg(), runs a syscall BPF program that calls it, and checks that the tracing context exposes both the explicit argument and the implicit prog aux pointer. Co-developed-by: Ihor Solodrai Signed-off-by: Kumar Kartikeya Dwivedi Signed-off-by: Ihor Solodrai Link: https://patch.msgid.link/20260713235223.1639022-3-ihor.solodrai@linux.dev Signed-off-by: Eduard Zingerman --- .../prog_tests/kfunc_implicit_args_tracing.c | 36 +++++++++ .../bpf/progs/kfunc_implicit_args_tracing.c | 77 +++++++++++++++++++ 2 files changed, 113 insertions(+) create mode 100644 tools/testing/selftests/bpf/prog_tests/kfunc_implicit_args_tracing.c create mode 100644 tools/testing/selftests/bpf/progs/kfunc_implicit_args_tracing.c diff --git a/tools/testing/selftests/bpf/prog_tests/kfunc_implicit_args_tracing.c b/tools/testing/selftests/bpf/prog_tests/kfunc_implicit_args_tracing.c new file mode 100644 index 000000000000..61cc5aaba025 --- /dev/null +++ b/tools/testing/selftests/bpf/prog_tests/kfunc_implicit_args_tracing.c @@ -0,0 +1,36 @@ +// SPDX-License-Identifier: GPL-2.0 +/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */ + +#include +#include "kfunc_implicit_args_tracing.skel.h" + +void test_kfunc_implicit_args_tracing(void) +{ + struct kfunc_implicit_args_tracing *skel; + LIBBPF_OPTS(bpf_test_run_opts, topts); + int err, fd; + + skel = kfunc_implicit_args_tracing__open_and_load(); + if (!ASSERT_OK_PTR(skel, "open_and_load")) + return; + + err = kfunc_implicit_args_tracing__attach(skel); + if (!ASSERT_OK(err, "attach")) + goto cleanup; + + fd = bpf_program__fd(skel->progs.trigger_implicit_arg); + err = bpf_prog_test_run_opts(fd, &topts); + if (!ASSERT_OK(err, "test_run")) + goto cleanup; + + ASSERT_EQ(topts.retval, 5, "kfunc_retval"); + ASSERT_EQ(skel->bss->fentry_arg_cnt, 2, "fentry_arg_cnt"); + ASSERT_NEQ(skel->bss->fentry_aux_arg, 0, "fentry_aux_arg"); + ASSERT_EQ(skel->bss->fentry_result, 1, "fentry_result"); + ASSERT_EQ(skel->bss->fexit_arg_cnt, 2, "fexit_arg_cnt"); + ASSERT_NEQ(skel->bss->fexit_aux_arg, 0, "fexit_aux_arg"); + ASSERT_EQ(skel->bss->fexit_result, 1, "fexit_result"); + +cleanup: + kfunc_implicit_args_tracing__destroy(skel); +} diff --git a/tools/testing/selftests/bpf/progs/kfunc_implicit_args_tracing.c b/tools/testing/selftests/bpf/progs/kfunc_implicit_args_tracing.c new file mode 100644 index 000000000000..995f8b8b5b9e --- /dev/null +++ b/tools/testing/selftests/bpf/progs/kfunc_implicit_args_tracing.c @@ -0,0 +1,77 @@ +// SPDX-License-Identifier: GPL-2.0 +/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */ + +#include +#include +#include +#include + +extern int bpf_kfunc_implicit_arg(int a) __weak __ksym; + +char _license[] SEC("license") = "GPL"; + +/* Shared arg checks; reports arg count and aux, returns 1 on success. */ +static __always_inline __u64 +check_implicit_args(void *ctx, __u64 *arg_cnt, __u64 *aux_arg) +{ + __u64 a = 0, aux = 0, z = 0; + __u64 result; + __s64 err; + + *arg_cnt = bpf_get_func_arg_cnt(ctx); + result = *arg_cnt == 2; + + err = bpf_get_func_arg(ctx, 0, &a); + result &= err == 0 && (int)a == 5; + + err = bpf_get_func_arg(ctx, 1, &aux); + *aux_arg = aux; + result &= err == 0 && aux != 0; + + err = bpf_get_func_arg(ctx, 2, &z); + result &= err == -EINVAL; + + return result; +} + +__u64 fentry_result; +__u64 fentry_arg_cnt; +__u64 fentry_aux_arg; + +SEC("fentry/bpf_kfunc_implicit_arg") +int BPF_PROG(trace_implicit_arg_fentry) +{ + __u64 ret = 0; + __s64 err; + + fentry_result = check_implicit_args(ctx, &fentry_arg_cnt, &fentry_aux_arg); + + err = bpf_get_func_ret(ctx, &ret); + fentry_result &= err == -EOPNOTSUPP; + + return 0; +} + +__u64 fexit_result; +__u64 fexit_arg_cnt; +__u64 fexit_aux_arg; + +SEC("fexit/bpf_kfunc_implicit_arg") +int BPF_PROG(trace_implicit_arg_fexit) +{ + __u64 ret = 0; + __s64 err; + + fexit_result = check_implicit_args(ctx, &fexit_arg_cnt, &fexit_aux_arg); + + err = bpf_get_func_ret(ctx, &ret); + fexit_result &= err == 0 && ret == 5; + + return 0; +} + +SEC("syscall") +int trigger_implicit_arg(void *ctx) +{ + return bpf_kfunc_implicit_arg(5); +}