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Date:   Fri, 6 May 2022 11:15:23 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Johannes Berg' <johannes@...solutions.net>,
        Keith Packard <keithp@...thp.com>,
        Kees Cook <keescook@...omium.org>
CC:     "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Francis Laniel <laniel_francis@...vacyrequired.com>,
        Daniel Axtens <dja@...ens.net>,
        "Dan Williams" <dan.j.williams@...el.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Guenter Roeck <linux@...ck-us.net>,
        "Daniel Vetter" <daniel.vetter@...ll.ch>,
        Tadeusz Struk <tadeusz.struk@...aro.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Andrew Gabbasov" <andrew_gabbasov@...tor.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Gross <agross@...nel.org>,
        Andy Lavr <andy.lavr@...il.com>,
        Arend van Spriel <aspriel@...il.com>,
        Baowen Zheng <baowen.zheng@...igine.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Bradley Grove <linuxdrivers@...otech.com>,
        "brcm80211-dev-list.pdl@...adcom.com" 
        <brcm80211-dev-list.pdl@...adcom.com>,
        Christian Brauner <brauner@...nel.org>,
        Christian Göttsche <cgzones@...glemail.com>,
        Christian Lamparter <chunkeey@...glemail.com>,
        Chris Zankel <chris@...kel.net>,
        Cong Wang <cong.wang@...edance.com>,
        "David Gow" <davidgow@...gle.com>,
        David Howells <dhowells@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Dennis Dalessandro <dennis.dalessandro@...nelisnetworks.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        "Dmitry Kasatkin" <dmitry.kasatkin@...il.com>,
        Eli Cohen <elic@...dia.com>,
        "Eric Dumazet" <edumazet@...gle.com>,
        Eric Paris <eparis@...isplace.org>,
        "Eugeniu Rosca" <erosca@...adit-jv.com>,
        Felipe Balbi <balbi@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Franky Lin <franky.lin@...adcom.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        Gregory Greenman <gregory.greenman@...el.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "Hante Meuleman" <hante.meuleman@...adcom.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Hulk Robot <hulkci@...wei.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        James Morris <jmorris@...ei.org>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
        Johan Hedberg <johan.hedberg@...il.com>,
        John Keeping <john@...anate.com>,
        Juergen Gross <jgross@...e.com>, Kalle Valo <kvalo@...nel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "kunit-dev@...glegroups.com" <kunit-dev@...glegroups.com>,
        Kuniyuki Iwashima <kuniyu@...zon.co.jp>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        "Lars-Peter Clausen" <lars@...afoo.de>,
        Lee Jones <lee.jones@...aro.org>,
        Leon Romanovsky <leon@...nel.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        "linux1394-devel@...ts.sourceforge.net" 
        <linux1394-devel@...ts.sourceforge.net>,
        "linux-afs@...ts.infradead.org" <linux-afs@...ts.infradead.org>,
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        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "linux-xtensa@...ux-xtensa.org" <linux-xtensa@...ux-xtensa.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        Loic Poulain <loic.poulain@...aro.org>,
        Louis Peens <louis.peens@...igine.com>,
        Luca Coelho <luciano.coelho@...el.com>,
        "Luiz Augusto von Dentz" <luiz.dentz@...il.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        Marcel Holtmann <marcel@...tmann.org>,
        Mark Brown <broonie@...nel.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "Max Filippov" <jcmvbkbc@...il.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Muchun Song <songmuchun@...edance.com>,
        Nathan Chancellor <nathan@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nuno Sá <nuno.sa@...log.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Paul Moore <paul@...l-moore.com>,
        "Rich Felker" <dalias@...ifal.cx>,
        Rob Herring <robh+dt@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        "selinux@...r.kernel.org" <selinux@...r.kernel.org>,
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        Simon Horman <simon.horman@...igine.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Stefan Richter <stefanr@...6.in-berlin.de>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Takashi Iwai <tiwai@...e.com>, Tom Rix <trix@...hat.com>,
        Udipto Goswami <quic_ugoswami@...cinc.com>,
        "wcn36xx@...ts.infradead.org" <wcn36xx@...ts.infradead.org>,
        Wei Liu <wei.liu@...nel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        Xiu Jianfeng <xiujianfeng@...wei.com>,
        "Yang Yingliang" <yangyingliang@...wei.com>
Subject: RE: [PATCH 02/32] Introduce flexible array struct memcpy() helpers

From: Johannes Berg
> Sent: 05 May 2022 21:13
> On Thu, 2022-05-05 at 13:08 -0700, Keith Packard wrote:
> 
> 
> > I bet you've already considered the simpler form:
> >
> >         struct something *instance = mem_to_flex_dup(byte_array, count, GFP_KERNEL);
> >         if (IS_ERR(instance))
> >             return PTR_ERR(instance);
> >
> 
> Sadly, this doesn't work in any way because mem_to_flex_dup() needs to
> know at least the type, hence passing 'instance', which is simpler than
> passing 'struct something'.

You can use:
         struct something *instance;
         mem_to_flex_dup(instance, byte_array, count, GFP_KERNEL);
         if (IS_ERR(instance))
             return PTR_ERR(instance);
and have mem_to_flex_dup() (which must be a #define) update 'instance'.
(You can require &instance - and just precede all the uses with
an extra '*' to make it more obvious the variable is updated.
But there is little point requiring it be NULL.)

If you really want to define the variable mid-block you can use:
         mem_to_flex_dup(struct something *, instance, byte_array, count, GFP_KERNEL);

but I really hate having declarations anywhere other than the top of
a function because it makes them hard for the 'mk1 eyeball' to spot.

	David

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