Date:   Wed, 22 Feb 2023 12:36:34 -0800
From:   John Moon <quic_johmoo@...cinc.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Randy Dunlap <rdunlap@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "Nicolas Schier" <nicolas@...sle.eu>,
        <linux-kbuild@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...r.kernel.org>,
        <linux-arm-msm@...r.kernel.org>,
        Trilok Soni <quic_tsoni@...cinc.com>,
        "Bjorn Andersson" <andersson@...nel.org>,
        Todd Kjos <tkjos@...gle.com>,
        "Matthias Maennich" <maennich@...gle.com>,
        Giuliano Procida <gprocida@...gle.com>,
        <kernel-team@...roid.com>, Jordan Crouse <jorcrous@...zon.com>
Subject: Re: [PATCH RESEND 1/1] check-uapi: Introduce check-uapi.sh



On 2/19/2023 12:52 AM, Masahiro Yamada wrote:
> On Sat, Feb 18, 2023 at 5:31 PM Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
>>
>> On Sat, Feb 18, 2023 at 09:17:12AM +0100, Greg Kroah-Hartman wrote:
>>> On Fri, Feb 17, 2023 at 12:22:34PM -0800, John Moon wrote:
>>>> While the kernel community has been good at maintaining backwards
>>>> compatibility with kernel UAPIs, it would be helpful to have a tool
>>>> to check if a patch introduces changes that break backwards
>>>> compatibility.
>>>>
>>>> To that end, introduce check-uapi.sh: a simple shell script that
>>>> checks for changes to UAPI headers using libabigail.
>>>>
>>>> libabigail is "a framework which aims at helping developers and
>>>> software distributors to spot some ABI-related issues like interface
>>>> incompatibility in ELF shared libraries by performing a static
>>>> analysis of the ELF binaries at hand."
>>>>
>>>> The script uses one of libabigail's tools, "abidiff", to compile the
>>>> changed header before and after the patch to detect any changes.
>>>>
>>>> abidiff "compares the ABI of two shared libraries in ELF format. It
>>>> emits a meaningful report describing the differences between the two
>>>> ABIs."
>>>>
>>>> Signed-off-by: John Moon <quic_johmoo@...cinc.com>
>>>> ---
>>>>   scripts/check-uapi.sh | 245 ++++++++++++++++++++++++++++++++++++++++++
>>>>   1 file changed, 245 insertions(+)
>>>>   create mode 100755 scripts/check-uapi.sh
>>>
>>> Ok, this is very cool, thank you so much for doing this.
>>>
>>> I know Randy Dunlap was also looking into this previously, so I've cc:ed
>>> him and bounced him the original.
>>>
>>> I tried this out, and at first glance, this felt like it was just "too
>>> fast" in that nothing actually was being tested.  So I manually added a
>>> field to a structure I know would break the abi, and:
>>>
>>>        $ ./scripts/check-uapi.sh
>>>        !!! ABI differences detected in include/uapi/linux/usb/ch9.h (compared to file at HEAD^1) !!!
>>>
>>>            [C] 'struct usb_ctrlrequest' changed:
>>>              type size changed from 64 to 72 (in bits)
>>>              1 data member insertion:
>>>                '__u8 abi_break', at offset 16 (in bits) at ch9.h:216:1
>>>              3 data member changes:
>>>                '__le16 wValue' offset changed from 16 to 24 (in bits) (by +8 bits)
>>>                '__le16 wIndex' offset changed from 32 to 40 (in bits) (by +8 bits)
>>>                '__le16 wLength' offset changed from 48 to 56 (in bits) (by +8 bits)
>>>
>>>        0/1 UAPI header file changes are backwards compatible
>>>        UAPI header ABI check failed
>>>
>>> So it worked!
>>
>> Ok, I take it back, it doesn't seem to work :(
>>
>> It only "catches" a change from the last commit, but if you have an
>> intermediate commit (i.e change something in HEAD^ but not HEAD), it
>> does not detect it at all.
>>
>> And if you give it an old version, it doesn't check that either (hint,
>> try passing in a very old kernel version, that returns instantly and
>> doesn't actually build anything).

I'll make an update to improve this behavior. Should be able to specify 
the commit in which a change is made and which past commits to check 
against.

>>
>> So it's a good first cut as an example, but as it doesn't really work
>> correctly yet, we can't take it.  Care to fix it up to work so that it
>> can be usable?
>>
>> thanks,
>>
>> greg k-h
> 
> 
> 
> This tool does not even work with changes in HEAD.
> 
> I attached two test-case patches.
> 
> This patch does not mention any requirement for
> the build host, but neither of them works for me,
> on Ubuntu 22.04.
> 
> I guess people will find more bad cases
> if they use older distros as the build host.
> (I know why it does not work, though)
> 
> 
> 
> 
> For patch 0001:
> 
> $ ./scripts/check-uapi.sh
> Couldn't compile current version of UAPI header
> include/uapi/linux/cec-funcs.h...
> In file included from <command-line>:
> /tmp/tmp.gYBwfiWTqX/usr/include/linux/cec-funcs.h.pre: In function
> ‘cec_msg_set_audio_volume_level’:
> /tmp/tmp.gYBwfiWTqX/usr/include/linux/cec-funcs.h.pre:1575:23: error:
> ‘CEC_MSG_SET_AUDIO_VOLUME_LEVEL’ undeclared (first use in this
> function)
>   1575 |         msg->msg[1] = CEC_MSG_SET_AUDIO_VOLUME_LEVEL;
>        |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /tmp/tmp.gYBwfiWTqX/usr/include/linux/cec-funcs.h.pre:1575:23: note:
> each undeclared identifier is reported only once for each function it
> appears in
> 0/1 UAPI header file changes are backwards compatible
> UAPI header ABI check failed
> 
> 
> 
> For patch 0002:
> 
> $ ./scripts/check-uapi.sh
> Couldn't compile current version of UAPI header include/uapi/linux/signal.h...
> In file included from /tmp/tmp.wm5RykUr3y/usr/include/linux/signal.h.pre:5,
>                   from <command-line>:
> /usr/include/x86_64-linux-gnu/asm/signal.h:103:9: error: unknown type
> name ‘size_t’
>    103 |         size_t ss_size;
>        |         ^~~~~~
> 0/1 UAPI header file changes are backwards compatible
> UAPI header ABI check failed
> 

Thanks for the example patches! We understand the issue and I think 
there's a simple solution. We can install all of the current UAPI 
headers into the tmp directory and include those. That way, the user's 
system headers shouldn't enter into the equation.

> 
> 
> 
> 
> 
> BTW, I recommend you to not pick up a patch before having
> any reviewer read the code.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> --
> Best Regards
> Masahiro Yamada

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