selftests/bpf: Add test for showing a void BTF type

Extend the snprintf_btf test with type_ids from the vmlinux BTF that
used to NULL-deref in the BTF show path: a "const void", checked to
render the "<unsupported kind:0>" placeholder, and a BTF_KIND_VAR,
checked to resolve and render without error.

The program renders from its own buffer and the test picks a VAR whose
resolved type fits it, so the render stays in bounds.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://lore.kernel.org/r/20260901104924.346187-6-jiayuan.chen@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -1,7 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <test_progs.h>
 #include <linux/btf.h>
+#include <bpf/btf.h>
 #include "netif_receive_skb.skel.h"
+#include "snprintf_btf_void.skel.h"
 
 /* Demonstrate that bpf_snprintf_btf succeeds and that various data types
  * are formatted correctly.
@@ -58,3 +60,80 @@ void serial_test_snprintf_btf(void)
 cleanup:
 	netif_receive_skb__destroy(skel);
 }
+
+/*
+ * bpf_snprintf_btf() renders a type_id taken straight from the vmlinux BTF.
+ * Two such type_ids used to NULL-deref in the BTF show path:
+ *   - a "const void" (a modifier resolving to void) in btf_modifier_show()
+ *   - a BTF_KIND_VAR in btf_var_show() (base BTF has no resolved_ids)
+ * A fixed kernel renders both without crashing.
+ */
+static long run(struct snprintf_btf_void *skel, __u32 type_id)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, topts);
+	char ctx[8] = {};
+
+	skel->bss->type_id = type_id;
+	topts.ctx_in = ctx;
+	topts.ctx_size_in = sizeof(ctx);
+	if (!ASSERT_OK(bpf_prog_test_run_opts(bpf_program__fd(skel->progs.dump_type),
+					      &topts), "test_run"))
+		return -1;
+	return skel->bss->ret;
+}
+
+void test_snprintf_btf_void(void)
+{
+	const struct btf_type *t;
+	struct snprintf_btf_void *skel;
+	int i, n, cv = 0, var = 0;
+	struct btf *btf;
+
+	btf = btf__parse("/sys/kernel/btf/vmlinux", NULL);
+	if (!btf) {
+		test__skip();
+		return;
+	}
+
+	skel = snprintf_btf_void__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		goto out_btf;
+
+	n = btf__type_cnt(btf);
+	for (i = 1; i < n && !(cv && var); i++) {
+		t = btf__type_by_id(btf, i);
+		if (!cv && btf_kind(t) == BTF_KIND_CONST && t->type == 0)
+			cv = i;
+		/* Pick a VAR small enough to render from the program's buffer. */
+		if (!var && btf_kind(t) == BTF_KIND_VAR) {
+			long sz = btf__resolve_size(btf, t->type);
+
+			if (sz > 0 && sz <= (long)sizeof(skel->bss->obj))
+				var = i;
+		}
+	}
+
+	/* "const void" renders the "<unsupported kind:0>" placeholder. */
+	if (test__start_subtest("const_void")) {
+		if (cv) {
+			ASSERT_EQ(run(skel, cv),
+				  sizeof("<unsupported kind:0>") - 1, "ret");
+			ASSERT_STREQ(skel->bss->out, "<unsupported kind:0>",
+				     "placeholder");
+		} else {
+			test__skip();
+		}
+	}
+
+	/* A BTF_KIND_VAR must resolve and render without error. */
+	if (test__start_subtest("var")) {
+		if (var)
+			ASSERT_GT(run(skel, var), 0, "ret");
+		else
+			test__skip();
+	}
+
+	snprintf_btf_void__destroy(skel);
+out_btf:
+	btf__free(btf);
+}

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

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "btf_ptr.h"
+#include <bpf/bpf_helpers.h>
+
+__u32 type_id;
+/* A buffer we own to render the selected type from, kept in bounds. */
+char obj[256];
+char out[64];
+long ret;
+
+SEC("raw_tp/sys_enter")
+int dump_type(void *ctx)
+{
+	struct btf_ptr ptr = {
+		.ptr = obj,
+		.type_id = type_id,
+		.flags = 0,
+	};
+
+	ret = bpf_snprintf_btf(out, sizeof(out), &ptr, sizeof(ptr), 0);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";