selftests/bpf: Skip callback tests if jit is disabled in test_verifier

If CONFIG_BPF_JIT_ALWAYS_ON is not set and bpf_jit_enable is 0, there
exist 6 failed tests.

  [root@linux bpf]# echo 0 > /proc/sys/net/core/bpf_jit_enable
  [root@linux bpf]# echo 0 > /proc/sys/kernel/unprivileged_bpf_disabled
  [root@linux bpf]# ./test_verifier | grep FAIL
  torvalds#106/p inline simple bpf_loop call FAIL
  torvalds#107/p don't inline bpf_loop call, flags non-zero FAIL
  torvalds#108/p don't inline bpf_loop call, callback non-constant FAIL
  torvalds#109/p bpf_loop_inline and a dead func FAIL
  torvalds#110/p bpf_loop_inline stack locations for loop vars FAIL
  torvalds#111/p inline bpf_loop call in a big program FAIL
  Summary: 768 PASSED, 15 SKIPPED, 6 FAILED

The test log shows that callbacks are not allowed in non-JITed programs,
interpreter doesn't support them yet, thus these tests should be skipped
if jit is disabled, copy some check functions from the other places under
tools directory, and then handle this case in do_test_single().

With this patch:

  [root@linux bpf]# echo 0 > /proc/sys/net/core/bpf_jit_enable
  [root@linux bpf]# echo 0 > /proc/sys/kernel/unprivileged_bpf_disabled
  [root@linux bpf]# ./test_verifier | grep FAIL
  Summary: 768 PASSED, 21 SKIPPED, 0 FAILED

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>

tools/testing/selftests/bpf/test_verifier.c

@@ -21,6 +21,7 @@
 #include <sched.h>
 #include <limits.h>
 #include <assert.h>
+#include <fcntl.h>
 
 #include <linux/unistd.h>
 #include <linux/filter.h>
@@ -1397,6 +1398,34 @@ static bool is_skip_insn(struct bpf_insn *insn)
 	return memcmp(insn, &skip_insn, sizeof(skip_insn)) == 0;
 }
 
+static bool is_ldimm64_insn(struct bpf_insn *insn)
+{
+	return insn->code == (BPF_LD | BPF_IMM | BPF_DW);
+}
+
+static bool insn_is_pseudo_func(struct bpf_insn *insn)
+{
+	return is_ldimm64_insn(insn) && insn->src_reg == BPF_PSEUDO_FUNC;
+}
+
+static bool is_jit_enabled(void)
+{
+	const char *jit_sysctl = "/proc/sys/net/core/bpf_jit_enable";
+	bool enabled = false;
+	int sysctl_fd;
+
+	sysctl_fd = open(jit_sysctl, 0, O_RDONLY);
+	if (sysctl_fd != -1) {
+		char tmpc;
+
+		if (read(sysctl_fd, &tmpc, sizeof(tmpc)) == 1)
+			enabled = (tmpc != '0');
+		close(sysctl_fd);
+	}
+
+	return enabled;
+}
+
 static int null_terminated_insn_len(struct bpf_insn *seq, int max_len)
 {
 	int i;
@@ -1662,6 +1691,16 @@ static void do_test_single(struct bpf_test *test, bool unpriv,
 		goto close_fds;
 	}
 
+	if (!is_jit_enabled()) {
+		for (i = 0; i < prog_len; i++, prog++) {
+			if (insn_is_pseudo_func(prog)) {
+				printf("SKIP (callbacks are not allowed in non-JITed programs)\n");
+				skips++;
+				goto close_fds;
+			}
+		}
+	}
+
 	alignment_prevented_execution = 0;
 
 	if (expected_ret == ACCEPT || expected_ret == VERBOSE_ACCEPT) {