selftests/bpf: Test changing packet data from global functions with a kfunc

The verifier should invalidate all packet pointers after a packet data
changing kfunc is called. So, similar to commit 3f23ee5
("selftests/bpf: test for changing packet data from global functions"),
test changing packet data from global functions to make sure packet
pointers are indeed invalidated.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250925170013.1752561-2-ameryhung@gmail.com

Author: ameryhung

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

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 /* Converted from tools/testing/selftests/bpf/verifier/sock.c */
 
-#include <linux/bpf.h>
+#include "vmlinux.h"
 #include <bpf/bpf_helpers.h>
 #include "bpf_misc.h"
 
@@ -1068,6 +1068,34 @@ int invalidate_pkt_pointers_from_global_func(struct __sk_buff *sk)
 	return TCX_PASS;
 }
 
+__noinline
+long xdp_pull_data2(struct xdp_md *x, __u32 len)
+{
+	return bpf_xdp_pull_data(x, len);
+}
+
+__noinline
+long xdp_pull_data1(struct xdp_md *x, __u32 len)
+{
+	return xdp_pull_data2(x, len);
+}
+
+/* global function calls bpf_xdp_pull_data(), which invalidates packet
+ * pointers established before global function call.
+ */
+SEC("xdp")
+__failure __msg("invalid mem access")
+int invalidate_xdp_pkt_pointers_from_global_func(struct xdp_md *x)
+{
+	int *p = (void *)(long)x->data;
+
+	if ((void *)(p + 1) > (void *)(long)x->data_end)
+		return XDP_DROP;
+	xdp_pull_data1(x, 0);
+	*p = 42; /* this is unsafe */
+	return XDP_PASS;
+}
+
 __noinline
 int tail_call(struct __sk_buff *sk)
 {