selftests/bpf: Add unit tests with __bpf_trap() kfunc

Add some inline-asm tests and C tests where __bpf_trap() or
__builtin_trap() is used in the code. The __builtin_trap()
test is guarded with llvm21 ([1]) since otherwise the compilation
failure will happen.

  [1] llvm/llvm-project#131731

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>

Author: Yonghong Song

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

@@ -14,6 +14,7 @@
 #include "verifier_bounds_deduction_non_const.skel.h"
 #include "verifier_bounds_mix_sign_unsign.skel.h"
 #include "verifier_bpf_get_stack.skel.h"
+#include "verifier_bpf_trap.skel.h"
 #include "verifier_bswap.skel.h"
 #include "verifier_btf_ctx_access.skel.h"
 #include "verifier_btf_unreliable_prog.skel.h"
@@ -148,6 +149,7 @@ void test_verifier_bounds_deduction(void)     { RUN(verifier_bounds_deduction);
 void test_verifier_bounds_deduction_non_const(void)     { RUN(verifier_bounds_deduction_non_const); }
 void test_verifier_bounds_mix_sign_unsign(void) { RUN(verifier_bounds_mix_sign_unsign); }
 void test_verifier_bpf_get_stack(void)        { RUN(verifier_bpf_get_stack); }
+void test_verifier_bpf_trap(void)             { RUN(verifier_bpf_trap); }
 void test_verifier_bswap(void)                { RUN(verifier_bswap); }
 void test_verifier_btf_ctx_access(void)       { RUN(verifier_btf_ctx_access); }
 void test_verifier_btf_unreliable_prog(void)  { RUN(verifier_btf_unreliable_prog); }

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

@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+#if __clang_major__ >= 21
+SEC("socket")
+__description("__builtin_trap with simple c code")
+__failure __msg("unexpected __bpf_trap() due to uninitialized variable?")
+void bpf_builtin_trap_with_simple_c(void)
+{
+	__builtin_trap();
+}
+#endif
+
+SEC("socket")
+__description("__bpf_trap with simple c code")
+__failure __msg("unexpected __bpf_trap() due to uninitialized variable?")
+void bpf_trap_with_simple_c(void)
+{
+	__bpf_trap();
+}
+
+SEC("socket")
+__description("__bpf_trap as the second-from-last insn")
+__failure __msg("unexpected __bpf_trap() due to uninitialized variable?")
+__naked void bpf_trap_at_func_end(void)
+{
+	asm volatile (
+	"r0 = 0;"
+	"call %[__bpf_trap];"
+	"exit;"
+	:
+	: __imm(__bpf_trap)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("dead code __bpf_trap in the middle of code")
+__success
+__naked void dead_bpf_trap_in_middle(void)
+{
+	asm volatile (
+	"r0 = 0;"
+	"if r0 == 0 goto +1;"
+	"call %[__bpf_trap];"
+	"r0 = 2;"
+	"exit;"
+	:
+	: __imm(__bpf_trap)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("reachable __bpf_trap in the middle of code")
+__failure __msg("unexpected __bpf_trap() due to uninitialized variable?")
+__naked void live_bpf_trap_in_middle(void)
+{
+	asm volatile (
+	"r0 = 0;"
+	"if r0 == 1 goto +1;"
+	"call %[__bpf_trap];"
+	"r0 = 2;"
+	"exit;"
+	:
+	: __imm(__bpf_trap)
+	: __clobber_all);
+}
+
+char _license[] SEC("license") = "GPL";