bpf: Add selftests for BPF_MODIFY_RETURN

Test for two scenarios:

  * When the fmod_ret program returns 0, the original function should
    be called along with fentry and fexit programs.
  * When the fmod_ret program returns a non-zero value, the original
    function should not be called, no side effect should be observed and
    fentry and fexit programs should be called.

The result from the kernel function call and whether a side-effect is
observed is returned via the retval attr of the BPF_PROG_TEST_RUN (bpf)
syscall.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200304191853.1529-8-kpsingh@chromium.org

1 files changed, 21 insertions, 1 deletions

diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 3600f098e7c6..4c921f5154e0 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -10,6 +10,7 @@
 #include <net/bpf_sk_storage.h>
 #include <net/sock.h>
 #include <net/tcp.h>
+#include <linux/error-injection.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/bpf_test_run.h>
@@ -143,6 +144,14 @@ int noinline bpf_fentry_test6(u64 a, void *b, short c, int d, void *e, u64 f)
 	return a + (long)b + c + d + (long)e + f;
 }
 
+int noinline bpf_modify_return_test(int a, int *b)
+{
+	*b += 1;
+	return a + *b;
+}
+
+ALLOW_ERROR_INJECTION(bpf_modify_return_test, ERRNO);
+
 static void *bpf_test_init(const union bpf_attr *kattr, u32 size,
 			   u32 headroom, u32 tailroom)
 {
@@ -168,7 +177,9 @@ int bpf_prog_test_run_tracing(struct bpf_prog *prog,
 			      const union bpf_attr *kattr,
 			      union bpf_attr __user *uattr)
 {
-	int err = -EFAULT;
+	u16 side_effect = 0, ret = 0;
+	int b = 2, err = -EFAULT;
+	u32 retval = 0;
 
 	switch (prog->expected_attach_type) {
 	case BPF_TRACE_FENTRY:
@@ -181,10 +192,19 @@ int bpf_prog_test_run_tracing(struct bpf_prog *prog,
 		    bpf_fentry_test6(16, (void *)17, 18, 19, (void *)20, 21) != 111)
 			goto out;
 		break;
+	case BPF_MODIFY_RETURN:
+		ret = bpf_modify_return_test(1, &b);
+		if (b != 2)
+			side_effect = 1;
+		break;
 	default:
 		goto out;
 	}
 
+	retval = ((u32)side_effect << 16) | ret;
+	if (copy_to_user(&uattr->test.retval, &retval, sizeof(retval)))
+		goto out;
+
 	err = 0;
 out:
 	trace_bpf_test_finish(&err);