Message-Id: <1a1f118e-9a7c-4c66-b956-d21eb36fce48@app.fastmail.com>
Date: Thu, 26 Sep 2024 13:54:09 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: "David Hildenbrand" <david@...hat.com>, "Arnd Bergmann" <arnd@...nel.org>,
 linux-mm@...ck.org
Cc: "Jason A . Donenfeld" <Jason@...c4.com>,
 "Alexander Viro" <viro@...iv.linux.org.uk>,
 "Alexandre Torgue" <alexandre.torgue@...s.st.com>,
 "Andreas Larsson" <andreas@...sler.com>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Ard Biesheuvel" <ardb@...nel.org>,
 "Christian Brauner" <brauner@...nel.org>,
 "Christoph Hellwig" <hch@....de>,
 "Christophe Leroy" <christophe.leroy@...roup.eu>,
 "Damien Le Moal" <dlemoal@...nel.org>,
 "Greg Ungerer" <gerg@...ux-m68k.org>, "Helge Deller" <deller@....de>,
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Subject: Re: [PATCH 5/5] [RFC] mm: Remove MAP_UNINITIALIZED support

On Thu, Sep 26, 2024, at 08:46, David Hildenbrand wrote:
> On 25.09.24 23:06, Arnd Bergmann wrote:
>
> The first, uncontroversial step could indeed be to make 
> MAP_UNINITIALIZED a nop, but still leave the definitions in mman.h etc 
> around.
>
> This is the same we did with MAP_DENYWRITE. There might be some weird 
> user out there, and carelessly reusing the bit could result in trouble. 
> (people might argue that they are not using it with MAP_HUGETLB, so it 
> would work)
>
> Going forward and removing MAP_UNINITIALIZED is a bit more 
> controversial, but maybe there really isn't any other user around. 
> Software that is not getting recompiled cannot be really identified by 
> letting it rest in -next only.
>
> My take would be to leave MAP_UNINITIALIZED in the headers in some form 
> for documentation purposes.

I don't think there is much point in doing this in multiple
steps, either we want to break it at compile time or leave
it silently doing nothing. There is also very little
difference in practice because applications almost always
use sys/mman.h instead of linux/mman.h.

FWIW, the main user appears to be the uClibc and uclibc-ng
malloc() implementation for NOMMU targets:

https://git.uclibc.org/uClibc/commit/libc/stdlib/malloc/malloc.c?id=00673f93826bf1f

Both of these also define this constant itself as 0x4000000
for all architectures.

There are a few others that I could find with Debian codesearch:

https://sources.debian.org/src/monado/21.0.0+git2905.e26a272c1~dfsg1-2/src/external/tracy/client/tracy_rpmalloc.cpp/?hl=890#L889
https://sources.debian.org/src/systemtap/5.1-4/testsuite/systemtap.syscall/mmap.c/?hl=224#L224
https://sources.debian.org/src/fuzzel/1.11.1+ds-1/shm.c/?hl=488#L488
https://sources.debian.org/src/notcurses/3.0.7+dfsg.1-1/src/lib/fbuf.h/?hl=35#L35
https://sources.debian.org/src/lmms/1.2.2+dfsg1-6/src/3rdparty/rpmalloc/rpmalloc/rpmalloc/rpmalloc.c/?hl=1753#L1753

All of these will fall back to not passing MAP_UNINITIALIZED
if it's not defined, which is what happens on glibc and musl.

       Arnd

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