selftest/bpf: Add a test that reads various addresses.

Add a function to bpf_testmod that returns invalid kernel and user addresses.
Then attach an fexit program to that function that tries to read
memory through these addresses.

This logic checks that bpf_probe_read_kernel and BPF_PROBE_MEM logic is sane.

Signed-off-by: Alexei Starovoitov <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

···
 	return sum;
 }
 
+__weak noinline struct file *bpf_testmod_return_ptr(int arg)
+{
+	static struct file f = {};
+
+	switch (arg) {
+	case 1: return (void *)EINVAL;		/* user addr */
+	case 2: return (void *)0xcafe4a11;	/* user addr */
+	case 3: return (void *)-EINVAL;		/* canonical, but invalid */
+	case 4: return (void *)(1ull << 60);	/* non-canonical and invalid */
+	case 5: return (void *)~(1ull << 30);	/* trigger extable */
+	case 6: return &f;			/* valid addr */
+	case 7: return (void *)((long)&f | 1);	/* kernel tricks */
+	default: return NULL;
+	}
+}
+
 noinline ssize_t
 bpf_testmod_test_read(struct file *file, struct kobject *kobj,
 		      struct bin_attribute *bin_attr,
···
 		.off = off,
 		.len = len,
 	};
+	int i = 1;
+
+	while (bpf_testmod_return_ptr(i))
+		i++;
 
 	/* This is always true. Use the check to make sure the compiler
 	 * doesn't remove bpf_testmod_loop_test.

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

···
 	return 0;
 }
 
+SEC("fexit/bpf_testmod_return_ptr")
+int BPF_PROG(handle_fexit_ret, int arg, struct file *ret)
+{
+	long buf = 0;
+
+	bpf_probe_read_kernel(&buf, 8, ret);
+	bpf_probe_read_kernel(&buf, 8, (char *)ret + 256);
+	*(volatile long long *)ret;
+	*(volatile int *)&ret->f_mode;
+	return 0;
+}
+
 __u32 fmod_ret_read_sz = 0;
 
 SEC("fmod_ret/bpf_testmod_test_read")