Re: BTF compatibility issue across builds

[Yonghong Song <yhs@fb.com>]

On 2/10/22 2:34 PM, Alexei Starovoitov wrote:
> On Thu, Feb 10, 2022 at 10:17 AM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>>
>> On 2/10/22 2:01 AM, Michal Suchánek wrote:
>>> Hello,
>>>
>>> On Mon, Jan 31, 2022 at 09:36:44AM -0800, Yonghong Song wrote:
>>>>
>>>>
>>>> On 1/27/22 7:10 AM, Shung-Hsi Yu wrote:
>>>>> Hi,
>>>>>
>>>>> We recently run into module load failure related to split BTF on openSUSE
>>>>> Tumbleweed[1], which I believe is something that may also happen on other
>>>>> rolling distros.
>>>>>
>>>>> The error looks like the follow (though failure is not limited to ipheth)
>>>>>
>>>>>        BPF:[103111] STRUCT BPF:size=152 vlen=2 BPF: BPF:Invalid name BPF:
>>>>>
>>>>>        failed to validate module [ipheth] BTF: -22
>>>>>
>>>>> The error comes down to trying to load BTF of *kernel modules from a
>>>>> different build* than the runtime kernel (but the source is the same), where
>>>>> the base BTF of the two build is different.
>>>>>
>>>>> While it may be too far stretched to call this a bug, solving this might
>>>>> make BTF adoption easier. I'd natively think that we could further split
>>>>> base BTF into two part to avoid this issue, where .BTF only contain exported
>>>>> types, and the other (still residing in vmlinux) holds the unexported types.
>>>>
>>>> What is the exported types? The types used by export symbols?
>>>> This for sure will increase btf handling complexity.
>>>
>>> And it will not actually help.
>>>
>>> We have modversion ABI which checks the checksum of the symbols that the
>>> module imports and fails the load if the checksum for these symbols does
>>> not match. It's not concerned with symbols not exported, it's not
>>> concerned with symbols not used by the module. This is something that is
>>> sustainable across kernel rebuilds with minor fixes/features and what
>>> distributions watch for.
>>>
>>> Now with BTF the situation is vastly different. There are at least three
>>> bugs:
>>>
>>>    - The BTF check is global for all symbols, not for the symbols the
>>>      module uses. This is not sustainable. Given the BTF is supposed to
>>>      allow linking BPF programs that were built in completely different
>>>      environment with the kernel it is completely within the scope of BTF
>>>      to solve this problem, it's just neglected.
>>>    - It is possible to load modules with no BTF but not modules with
>>>      non-matching BTF. Surely the non-matching BTF could be discarded.
>>>    - BTF is part of vermagic. This is completely pointless since modules
>>>      without BTF can be loaded on BTF kernel. Surely it would not be too
>>>      difficult to do the reverse as well. Given BTF must pass extra check
>>>      to be used having it in vermagic is just useless moise.
>>>
>>>>> Does that sound like something reasonable to work on?
>>>>>
>>>>>
>>>>> ## Root case (in case anyone is interested in a verbose version)
>>>>>
>>>>> On openSUSE Tumbleweed there can be several builds of the same source. Since
>>>>> the source is the same, the binaries are simply replaced when a package with
>>>>> a larger build number is installed during upgrade.
>>>>>
>>>>> In our case, a rebuild is triggered[2], and resulted in changes in base BTF.
>>>>> More precisely, the BTF_KIND_FUNC{,_PROTO} of i2c_smbus_check_pec(u8 cpec,
>>>>> struct i2c_msg *msg) and inet_lhash2_bucket_sk(struct inet_hashinfo *h,
>>>>> struct sock *sk) was added to the base BTF of 5.15.12-1.3. Those functions
>>>>> are previously missing in base BTF of 5.15.12-1.1.
>>>>
>>>> As stated in [2] below, I think we should understand why rebuild is
>>>> triggered. If the rebuild for vmlinux is triggered, why the modules cannot
>>>> be rebuild at the same time?
>>>
>>> They do get rebuilt. However, if you are running the kernel and install
>>> the update you get the new modules with the old kernel. If the install
>>> script fails to copy the kernel to your EFI partition based on the fact
>>> a kernel with the same filename is alreasy there you get the same.
>>>
>>> If you have 'stable' distribution adding new symbols is normal and it
>>> does not break module loading without BTF but it breaks BTF.
>>
>> Okay, I see. One possible solution is that if kernel module btf
>> does not match vmlinux btf, the kernel module btf will be ignored
>> with a dmesg warning but kernel module load will proceed as normal.
>> I think this might be also useful for bpf lskel kernel modules as
>> well which tries to be portable (with CO-RE) for different kernels.
> 
> That sounds like #2 that Michal is proposing:
> "It is possible to load modules with no BTF but not modules with
>   non-matching BTF. Surely the non-matching BTF could be discarded."
> 
> That's probably the simplest way forward.
> 
> The patch
> https://patchwork.kernel.org/project/netdevbpf/patch/20220209052141.140063-1-connoro@google.com/
> shouldn't be necessary too.

Right the patch tried to address this issue and if we allow
non-matching BTF is ignored and then treaking DEBUG_INFO_BTF_MODULES
is not necessary.