selftests/bpf: Add a test for ptr_to_map_value on stack for helper access

Change bpf_iter_task.c such that pointer to map_value may appear
on the stack for bpf_seq_printf() to access. Without previous
verifier patch, the bpf_iter test will fail.
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201210013350.943985-1-yhs@fb.com

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

@@ -11,9 +11,10 @@ int dump_task(struct bpf_iter__task *ctx)
 {
 	struct seq_file *seq = ctx->meta->seq;
 	struct task_struct *task = ctx->task;
+	static char info[] = "    === END ===";
 
 	if (task == (void *)0) {
-		BPF_SEQ_PRINTF(seq, "    === END ===\n");
+		BPF_SEQ_PRINTF(seq, "%s\n", info);
 		return 0;
 	}
 

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

@@ -108,8 +108,9 @@
 	BPF_EXIT_INSN(),
 	},
 	.fixup_map_hash_8b = { 3 },
-	.errstr = "invalid indirect read from stack off -8+0 size 8",
-	.result = REJECT,
+	.errstr_unpriv = "invalid indirect read from stack off -8+0 size 8",
+	.result_unpriv = REJECT,
+	.result = ACCEPT,
 },
 {
 	"unpriv: mangle pointer on stack 1",