Re: [PATCH bpf] selftests/bpf: Add a copyright notice to lpm_trie_map_get_next_key

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Mon, 2024-11-04 at 09:34 +0800, Hou Tao wrote:
> Hi,
> 
> On 11/3/2024 2:04 PM, Byeonguk Jeong wrote:
> > Hi,
> > 
> > The selftest "verifier_bits_iter/bad words" has been failed with
> > retval 115, while I did not touched anything but a comment.
> > 
> > Do you have any idea why it failed? I am not sure whether it
> > indicates
> > any bugs in the kernel.
> > 
> > Best,
> > Byeonguk
> 
> Sorry for the inconvenience. It seems the test case
> "verifier_bits_iter/bad words" is flaky. It may fail randomly, such
> as
> in [1]. I think calling bpf_probe_read_kernel_common() on 3GB addr
> under
> s390 host may succeed and the content of the memory address will
> decide
> whether the test case will succeed or not. Do not know the reason why
> reading 3GB address succeeds under s390. Hope to get some insight
> from
> Ilya.  I think we could fix the failure first by using NULL as the
> address of bad words just like null_pointer test case does. Will
> merge
> the test in bad_words into the null_pointer case.

Hi,

s390 kernel runs in a completely separate address space, there is no
user/kernel split at TASK_SIZE. The same address may be valid in both
the kernel and the user address spaces, there is no way to tell by
looking at it. The config option related to this property is
ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE.

Also, unfortunately, 0 is a valid address in the s390 kernel address
space.

I wonder if we could use -4095 as an address that cannot be
dereferenced on all platforms?

Best regards,
Ilya