linux-kernel - Re: [PATCH v5 1/2] check-uapi: Introduce check-uapi.sh

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Date:   Thu, 13 Apr 2023 10:15:24 -0700
From:   John Moon <quic_johmoo@...cinc.com>
To:     Mark Wielaard <mark@...mp.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Masahiro Yamada <masahiroy@...nel.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@...ts.infradead.org>,
        <linux-arm-msm@...r.kernel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Arnd Bergmann" <arnd@...db.de>,
        Bjorn Andersson <andersson@...nel.org>,
        Todd Kjos <tkjos@...gle.com>,
        Matthias Maennich <maennich@...gle.com>,
        Giuliano Procida <gprocida@...gle.com>,
        <kernel-team@...roid.com>, <libabigail@...rceware.org>,
        Jordan Crouse <jorcrous@...zon.com>,
        Trilok Soni <quic_tsoni@...cinc.com>,
        Satya Durga Srinivasu Prabhala <quic_satyap@...cinc.com>,
        Elliot Berman <quic_eberman@...cinc.com>,
        "Guru Das Srinagesh" <quic_gurus@...cinc.com>
Subject: Re: [PATCH v5 1/2] check-uapi: Introduce check-uapi.sh

On 4/13/2023 7:37 AM, Mark Wielaard wrote:
> Hi,
> 
> On Wed, 2023-04-12 at 09:37 -0700, John Moon via Libabigail wrote:
>> On 4/11/2023 11:14 PM, Greg Kroah-Hartman wrote:
>>>> Would you find the tool more useful if it simply filtered out all instances
>>>> where the size of the type did not change? This would filter out the
>>>> following which the tool currently flags:
>>>>
>>>> - enum expansions
>>>> - reserved field expansions
>>>> - expansions of a struct with a flex array at the end
>>>> - type changes
>>>> - re-ordering of existing members
>>>> - ...others?
>>>
>>> Obviously not, as some of those are real breakages, and some are not at
>>> all.
>>>
>>> Please understand what is an abi breakage.  Adding new enums is not.
>>> Using a reserved field is not.  Reording existing members IS.
>>>
>>
>> Yes, understood that method would miss certain classes of breakages. I
>> was suggesting it as a way to improve the signal-to-noise ratio of the
>> tool since we don't currently have an algorithm for determining
>> breakages with 100% accuracy.
> 
> Note that you can check the exit code of libabigail's abidiff to see
> whether something is an incompatible abi change or not, see:
> https://sourceware.org/libabigail/manual/abidiff.html#return-values
> 
> You can also of course use suppressions to instruct abidiff to avoid
> reporting changes involving certain ABI artifacts:
> https://sourceware.org/libabigail/manual/libabigail-concepts.html#suppr-spec-label
> 
> Cheers,
> 
> Mark

Checking the ABIDIFF_ABI_INCOMPATIBLE_CHANGE flag in the return code is 
a good idea, but checking it doesn't change what the tool is currently 
outputting (i.e. the flag is set for all the changes currently 
reported). I think this is because of some filtering we're doing based 
on grepping stdout, but checking the return code would be more stable.

The suppressions may work for some cases, but I fear they would be too 
eager in other cases. Looking at the docs, I'm not sure how we could 
express something like:

"suppress changed enumerators if they end in 'MAX' or 'LAST' and appear 
at the end of the enumeration"

or

"suppress data member insertions into a struct if the last member in the 
struct has its size reduced by sizeof(new_member) and is named 'pad' or 
'reserved'"

They're complicated cases to detect in a general way.

Thanks,
John