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

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Date:   Wed, 12 Apr 2023 08:14:47 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     John Moon <quic_johmoo@...cinc.com>
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 Tue, Apr 11, 2023 at 11:36:48AM -0700, John Moon wrote:
> > > 
> > > Others do not seem to be intentional:
> > > 
> > >   Addition/use of flex arrays:
> > >     - include/uapi/linux/rseq.h (f7b01bb0b57f)
> > >     - include/uapi/scsi/scsi_bsg_mpi3mr.h (c6f2e6b6eaaf)
> > 
> > That is not a breakage, that's a tool problem.
> > 
> > >   Type change:
> > >     - include/uapi/scsi/scsi_bsg_ufs.h (3f5145a615238)
> > 
> > Again, not a real breakage, size is still the same.
> > 
> 
> 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.

> These changes aren't _always_ safe, but if you assume the kernel developer
> is doing something reasonable, then maybe it's okay. Maybe we could hide
> these checks behind something like a "--pedantic" flag?

Again, no, that list above has totally different things in it, some are
completly safe, others totally break the abi.  Do NOT lump them all
together as that is wrong.

thanks,

greg k-h