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Sat, 22 Feb 2025 22:28:05 -0800 (PST) From: Yafang Shao To: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, song@kernel.org, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, jolsa@kernel.org, jpoimboe@kernel.org, peterz@infradead.org Cc: bpf@vger.kernel.org, linux-kernel@vger.kernel.org, Yafang Shao Subject: [PATCH v2 bpf-next 2/3] bpf: Reject attaching fexit to functions annotated with __noreturn Date: Sun, 23 Feb 2025 14:27:34 +0800 Message-Id: <20250223062735.3341-3-laoar.shao@gmail.com> X-Mailer: git-send-email 2.37.1 (Apple Git-137.1) In-Reply-To: <20250223062735.3341-1-laoar.shao@gmail.com> References: <20250223062735.3341-1-laoar.shao@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Patchwork-Delegate: bpf@iogearbox.net If we attach fexit to a function annotated with __noreturn, it will cause an issue that the bpf trampoline image will be left over even if the bpf link has been destroyed. Take attaching do_exit() for example. The fexit works as follows, bpf_trampoline + __bpf_tramp_enter + percpu_ref_get(&tr->pcref); + call do_exit() + __bpf_tramp_exit + percpu_ref_put(&tr->pcref); Since do_exit() never returns, the refcnt of the trampoline image is never decremented, preventing it from being freed. That can be verified with as follows, $ bpftool link show <<<< nothing output $ grep "bpf_trampoline_[0-9]" /proc/kallsyms ffffffffc04cb000 t bpf_trampoline_6442526459 [bpf] <<<< leftover With this change, attaching fexit probes to functions like do_exit() will be rejected. $ ./fexit libbpf: prog 'fexit': BPF program load failed: -EINVAL libbpf: prog 'fexit': -- BEGIN PROG LOAD LOG -- Attaching fexit to __noreturn functions is rejected. Signed-off-by: Yafang Shao --- kernel/bpf/verifier.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 9971c03adfd5..329af451c936 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -22841,6 +22841,13 @@ BTF_ID(func, __rcu_read_unlock) #endif BTF_SET_END(btf_id_deny) +/* The functions annotated with __noreturn are denied. */ +BTF_SET_START(fexit_deny) +#define NORETURN(fn) BTF_ID(func, fn) +#include +#undef NORETURN +BTF_SET_END(fexit_deny) + static bool can_be_sleepable(struct bpf_prog *prog) { if (prog->type == BPF_PROG_TYPE_TRACING) { @@ -22929,6 +22936,10 @@ static int check_attach_btf_id(struct bpf_verifier_env *env) } else if (prog->type == BPF_PROG_TYPE_TRACING && btf_id_set_contains(&btf_id_deny, btf_id)) { return -EINVAL; + } else if (prog->expected_attach_type == BPF_TRACE_FEXIT && + btf_id_set_contains(&fexit_deny, btf_id)) { + verbose(env, "Attaching fexit to __noreturn functions is rejected.\n"); + return -EINVAL; } key = bpf_trampoline_compute_key(tgt_prog, prog->aux->attach_btf, btf_id);