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Date:   Mon, 26 Sep 2022 17:15:54 +0200
From:   =?utf-8?B?0L3QsNCx?= <nabijaczleweli@nabijaczleweli.xyz>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Bagas Sanjaya <bagasdotme@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Federico Vaga <federico.vaga@vaga.pv.it>,
        Alex Shi <alexs@kernel.org>,
        Yanteng Si <siyanteng@loongson.cn>,
        Hu Haowen <src.res@email.cn>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        linux-doc-tw-discuss@lists.sourceforge.net,
        Eric Biederman <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Linux MM <linux-mm@kvack.org>
Subject: Re: [PATCH v4 02/18] a.out: remove define-only CMAGIC, previously
 magic number
Message-ID: <20220926151554.7gxd6unp5727vw3c@tarta.nabijaczleweli.xyz>
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Hi!

On Mon, Sep 26, 2022 at 10:16:02AM +0200, Geert Uytterhoeven wrote:
> Thanks for your patch, which is now commit 53c2bd679017277f
> ("a.out: remove define-only CMAGIC, previously magic number") in
> driver-core/driver-core-next.
>=20
> On Fri, Sep 16, 2022 at 12:40 AM =D0=BD=D0=B0=D0=B1 <nabijaczleweli@nabij=
aczleweli.xyz> wrote:
> > The last user was removed in 5.1 in
> > commit 08300f4402ab ("a.out: remove core dumping support")
> > but this is part of the UAPI headers, so this may want to either wait
> > until a.out is removed entirely, or be removed from the magic number doc
> > and silently remain in the header
>=20
> Indeed. This is part of uapi, and might break some unknown
> userspace, while the gain is limited.  Do we really want to reduce
> include/uapi/linux/a.out.h piecewise (e.g. N_BADMAG() seems to be
> unused, too), instead of keeping it until a.out support is removed
> completely?

Not really, but it looked like a magic number in the magic-number.rst
sense due to the field being "magic" and the unintuitive type naming:
I hadn't realised it's part of the on-disk format
re-examination shows that it very well may be (have been).

> Anyway, even at that point, it might be wise to keep the header file
> around, as people have expressed the desire to run a.out binaries
> through a userspace-compatibility wrapper.

Agreed. Scissor-patch that reverts the removal below.

> > A cursory glance on DCS didn't show any user code actually using this
> > value
>=20
> What is DCS?

Debian Code Search; in this case my query was:
  https://codesearch.debian.net/search?q=3D%5CbCMAGIC%5Cb&literal=3D0

There's a few false positives here but all results that are using CMAGIC
to mean this CMAGIC (and aren't hurd code copied from linux)
just re-define it.

> >  Documentation/process/magic-number.rst                    | 1 -
> >  Documentation/translations/it_IT/process/magic-number.rst | 1 -
> >  Documentation/translations/zh_CN/process/magic-number.rst | 1 -
> >  Documentation/translations/zh_TW/process/magic-number.rst | 1 -
> >  include/uapi/linux/a.out.h                                | 3 ---
> >  5 files changed, 7 deletions(-)
> >=20
> > diff --git a/include/uapi/linux/a.out.h b/include/uapi/linux/a.out.h
> > index 5fafde3798e5..bb15da96df2a 100644
> > --- a/include/uapi/linux/a.out.h
> > +++ b/include/uapi/linux/a.out.h
> > @@ -70,9 +70,6 @@ enum machine_type {
> >     The first page is unmapped to help trap NULL pointer references */
> >  #define QMAGIC 0314
> >
> > -/* Code indicating core file.  */
> > -#define CMAGIC 0421
> > -
> >  #if !defined (N_BADMAG)
> >  #define N_BADMAG(x)      (N_MAGIC(x) !=3D OMAGIC         \
> >                         && N_MAGIC(x) !=3D NMAGIC         \
>=20
> Gr{oetje,eeting}s,
>                         Geert

Best,
=D0=BD=D0=B0=D0=B1

-- >8 --
Subject: [PATCH] a.out: restore CMAGIC

Part of UAPI and the on-disk format:
this means that it's not a magic number per magic-number.rst,
and it's best to leave it untouched to avoid breaking userspace
and suffer the same fate as a.out in general

Fixes: commit 53c2bd679017 ("a.out: remove define-only CMAGIC,
 previously magic number")
Signed-off-by: Ahelenia Ziemia=C5=84ska <nabijaczleweli@nabijaczleweli.xyz>
---
 include/uapi/linux/a.out.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/uapi/linux/a.out.h b/include/uapi/linux/a.out.h
index bb15da96df2a..5fafde3798e5 100644
--- a/include/uapi/linux/a.out.h
+++ b/include/uapi/linux/a.out.h
@@ -70,6 +70,9 @@ enum machine_type {
    The first page is unmapped to help trap NULL pointer references */
 #define QMAGIC 0314
=20
+/* Code indicating core file.  */
+#define CMAGIC 0421
+
 #if !defined (N_BADMAG)
 #define N_BADMAG(x)	  (N_MAGIC(x) !=3D OMAGIC		\
 			&& N_MAGIC(x) !=3D NMAGIC		\
--=20
2.30.2


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