Re: [PATCH bpf-next v4 2/2] selftests/bpf: check whether s32 is sufficient for kfunc offset

Hi,

On 2/8/2022 2:33 AM, Yonghong Song wrote:

On 2/5/22 8:31 PM, Hou Tao wrote:

quoted

In add_kfunc_call(), bpf_kfunc_desc->imm with type s32 is used to
represent the offset of called kfunc from __bpf_call_base, so
add a test to ensure that the offset will not be overflowed.

Signed-off-by: Hou Tao <redacted>
---
  .../selftests/bpf/prog_tests/ksyms_module.c   | 42 +++++++++++++++++++
  1 file changed, 42 insertions(+)

[...]

quoted

+    /* Ensure kfunc call is supported */
+    skel = test_ksyms_module__open_and_load();
+    if (!ASSERT_OK_PTR(skel, "test_ksyms_module__open"))
+        return;
+
+    err = kallsyms_find("bpf_testmod_test_mod_kfunc", &kfunc_addr);
+    if (!ASSERT_OK(err, "find kfunc addr"))
+        goto cleanup;
+
+    err = kallsyms_find("__bpf_call_base", &base_addr);
+    if (!ASSERT_OK(err, "find base addr"))
+        goto cleanup;
+
+    used_offset = kfunc_addr - base_addr;
+    actual_offset = kfunc_addr - base_addr;
+    ASSERT_EQ((long long)used_offset, actual_offset, "kfunc offset
overflowed");

I am a little bit confused about motivation here. Maybe I missed something. If
we indeed have kfunc offset overflow,
should kernel verifier just reject the program? Specially,
we should make the above test_ksyms_module__open_and_load()
fail?

In add_kfunc_call(), the calculation of imm doesn't consider the overflow
of s32. So test_ksyms_module__open_and_load() will succeed. I think the
better solution is to put the overflow check in add_kfunc_call(), so will
drop this patch and add the overflow check in add_kfunc_call() instead.

Regards,
Tao

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