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From:   Giuliano Procida <gprocida@google.com>
Date:   Thu, 13 Apr 2023 18:12:21 +0100
Message-ID: <CAGvU0HnepJwhUdMPDPY7q8nAnMNU=2_XaZHkPhFO0hmaseSPEQ@mail.gmail.com>
Subject: Re: [PATCH v5 1/2] check-uapi: Introduce check-uapi.sh
To:     Mark Wielaard <mark@klomp.org>
Cc:     John Moon <quic_johmoo@quicinc.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
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        Nicolas Schier <nicolas@fjasle.eu>,
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        linux-arm-msm@vger.kernel.org,
        Randy Dunlap <rdunlap@infradead.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Bjorn Andersson <andersson@kernel.org>,
        Todd Kjos <tkjos@google.com>,
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        Jordan Crouse <jorcrous@amazon.com>,
        Trilok Soni <quic_tsoni@quicinc.com>,
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Hi all.

On Thu, 13 Apr 2023 at 15:37, Mark Wielaard <mark@klomp.org> 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

libabigail's abidiff already hides certain differences by default.
You can turn this behaviour off with --harmless.

Note that abidiff without --harmless treats certain ABI differences
asymmetrically,
hiding them one way around but not the other.

The ABI diff tool I designed for Android always treats differences symmetrically
and will only suppress certain kinds of diff if specially requested
(which we don't
do any more in production). [Technically, we also ignore qualifier changes on
function parameter and return types, but we achieve that by stripping them out
unconditionally.]

Once we get around to UAPI monitoring, we'll do the same there. We can always
review the SNR later.

Regards,
Giuliano.

> Cheers,
>
> Mark
>
> --
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