On 6/23/25 14:12, Ilya Leoshkevich wrote:
> On Mon, 2025-06-23 at 08:05 +0200, Viktor Malik wrote:
>> On 6/20/25 20:06, Alexei Starovoitov wrote:
>>> On Fri, Jun 20, 2025 at 5:33 AM Viktor Malik <vmalik@xxxxxxxxxx>
>>> wrote:
>>>>
>>>>> +SEC("syscall") __retval(USER_PTR_ERR) int
>>>>> test_strnstr_user_ptr2(void *ctx) { return bpf_strnstr("hello",
>>>>> user_ptr, 1); }
>>>>
>>>> For some reason, these tests are failing on s390x. I'll
>>>> investigate.
>>>
>>> I suspect this is the reason for failures:
>>>
>>> +char *user_ptr = (char *)1;
>>> +char *invalid_kern_ptr = (char *)-1;
>>
>> Actually, the kernel address works fine, it's the userspace addresses
>> causing the problem (user_ptr and NULL). On s390,
>> __get_kernel_nofault
>> always returns 0 for these addresses instead of going to the
>> exception
>> table.
>>
>>> Ilya,
>>>
>>> Please suggest user/kern addresses to use for these tests.
>>
>> FWIW, I've also tried a couple other random userspace addresses, for
>> all
>> of them __get_kernel_nofault returned 0.
>>
>> In string kfuncs, 0 is treated as the end of the string (not an
>> error),
>> so, unless some s390 expert has a better solution, the best I can
>> think
>> of here is to disable the userspace addresses tests on s390.
>>
>> Viktor
>
> Unfortunately NULL is a valid kernel pointer on s390; this is very
> annoying, but unlikely to change any time soon.
>
> Also, s390 has overlapping kernel and user address spaces. This means
> that you cannot deduce by the value of an address whether it's a
> kernel or a user address; something like 0x400000 can be both valid
> kernel and user address. Normal dereferences access the kernel address
> space; in order to access the user address space, one has to use magic
> machine instructions.
>
> So I would disable both NULL and invalid user_ptr tests on s390, since
> they do not apply. I would still test for an invalid kernel_ptr though;
> accessing (char *)-1 should cause an exception on s390.
That is indeed the case.
Thanks for the clarification! I'll send v7 with the problematic tests
disabled for s390.
