Merge branch 'bpf-reject-attaching-fexit-fmod_ret-to-noreturn-functions'

Yafang Shao says:

====================
Attaching fexit probes to functions marked with __noreturn may lead to
unpredictable behavior. To avoid this, we will reject attaching probes to
such functions. Currently, there is no ideal solution, so we will hardcode
a check for all __noreturn functions.

Once a more robust solution is implemented, this workaround can be removed.

v4->v5:
- Remove unnecessary functions (Alexei)
- Use BTF_ID directly (Alexei)

v3->v4: https://lore.kernel.org/bpf/20250317121735.86515-1-laoar.shao@gmail.com/
- Reject also fmod_ret (Alexei)
- Fix build warnings and remove unnecessary functions (Alexei)

v1->v2: https://lore.kernel.org/bpf/20250223062735.3341-1-laoar.shao@gmail.com/
- keep tools/objtool/noreturns.h as is (Josh)
- Add noreturns.h to objtool/sync-check.sh (Josh)
- Add verbose for the reject and simplify the test case (Song)

v1: https://lore.kernel.org/bpf/20250211023359.1570-1-laoar.shao@gmail.com/
====================

Link: https://patch.msgid.link/20250318114447.75484-1-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -23215,6 +23215,33 @@ BTF_ID(func, __rcu_read_unlock)
 #endif
 BTF_SET_END(btf_id_deny)
 
+/* fexit and fmod_ret can't be used to attach to __noreturn functions.
+ * Currently, we must manually list all __noreturn functions here. Once a more
+ * robust solution is implemented, this workaround can be removed.
+ */
+BTF_SET_START(noreturn_deny)
+#ifdef CONFIG_IA32_EMULATION
+BTF_ID(func, __ia32_sys_exit)
+BTF_ID(func, __ia32_sys_exit_group)
+#endif
+#ifdef CONFIG_KUNIT
+BTF_ID(func, __kunit_abort)
+BTF_ID(func, kunit_try_catch_throw)
+#endif
+#ifdef CONFIG_MODULES
+BTF_ID(func, __module_put_and_kthread_exit)
+#endif
+#ifdef CONFIG_X86_64
+BTF_ID(func, __x64_sys_exit)
+BTF_ID(func, __x64_sys_exit_group)
+#endif
+BTF_ID(func, do_exit)
+BTF_ID(func, do_group_exit)
+BTF_ID(func, kthread_complete_and_exit)
+BTF_ID(func, kthread_exit)
+BTF_ID(func, make_task_dead)
+BTF_SET_END(noreturn_deny)
+
 static bool can_be_sleepable(struct bpf_prog *prog)
 {
 	if (prog->type == BPF_PROG_TYPE_TRACING) {
@@ -23301,6 +23328,11 @@ 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 ||
+		   prog->expected_attach_type == BPF_MODIFY_RETURN) &&
+		   btf_id_set_contains(&noreturn_deny, btf_id)) {
+		verbose(env, "Attaching fexit/fmod_ret to __noreturn functions is rejected.\n");
+		return -EINVAL;
 	}
 
 	key = bpf_trampoline_compute_key(tgt_prog, prog->aux->attach_btf, btf_id);

tools/testing/selftests/bpf/prog_tests/fexit_noreturns.c

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include "fexit_noreturns.skel.h"
+
+void test_fexit_noreturns(void)
+{
+	RUN_TESTS(fexit_noreturns);
+}

tools/testing/selftests/bpf/progs/fexit_noreturns.c

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+SEC("fexit/do_exit")
+__failure __msg("Attaching fexit/fmod_ret to __noreturn functions is rejected.")
+int BPF_PROG(noreturns)
+{
+	return 0;
+}