Message-ID: <079aea54-a3ab-413e-aa4f-0dce06c8e142@gmx.de>
Date: Thu, 26 Sep 2024 11:18:11 +0200
From: Helge Deller <deller@....de>
To: Arnd Bergmann <arnd@...nel.org>, linux-mm@...ck.org
Cc: Arnd Bergmann <arnd@...db.de>, "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>, David Hildenbrand <david@...hat.com>,
 Greg Ungerer <gerg@...ux-m68k.org>, Kees Cook <kees@...nel.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Matt Turner <mattst88@...il.com>, Max Filippov <jcmvbkbc@...il.com>,
 Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>,
 Nicholas Piggin <npiggin@...il.com>,
 Richard Henderson <richard.henderson@...aro.org>,
 Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
 Vladimir Murzin <vladimir.murzin@....com>, Vlastimil Babka <vbabka@...e.cz>,
 linux-stm32@...md-mailman.stormreply.com, linux-kernel@...r.kernel.org,
 linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 5/5] [RFC] mm: Remove MAP_UNINITIALIZED support

On 9/25/24 23:06, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
>
> MAP_UNINITIALIZED was added back in 2009 for NOMMU kernels, specifically
> for blackfin, which is long gone. MAP_HUGE_SHIFT/MAP_HUGE_MASK were
> added in 2012 for architectures supporting hugepages, which at the time
> did not overlap with the ones supporting NOMMU.
>
> Adding the macro under an #ifdef was obviously a mistake, which
> Christoph Hellwig tried to address by making it unconditionally defined
> to 0x4000000 as part of the series to support RISC-V NOMMU kernels. At
> this point linux/mman.h contained two conflicting definitions for bit 26,
> though the two are still mutually exclusive at runtime in all supported
> configurations.
>
> According to the commit 854e9ed09ded ("mm: support madvise(MADV_FREE)")
> description, it was previously used internally by facebook, which
> would have resulted in MAP_HUGE_1MB turning into MAP_HUGE_2MB
> with MAP_UNINITIALIZED enabled, and every other page size implying
> MAP_UNINITIALIZED. I assume there are no remaining out of tree users
> on MMU-enabled kernels today.
>
> I do not see any sensible way to redefine the macros for the ABI in
> a way avoids breaking something. The only ideas so far are:
>
>   - do nothing, try to document the bug, hope for the best
>
>   - remove the kernel implementation and redefine MAP_UNINITIALIZED to
>     zero in the header to silently turn it off for everyone. There are
>     few NOMMU users left, and the ones that do use NOMMU usually turn
>     off MMAP_ALLOW_UNINITIALIZED, as it still has the potential to cause
>     bugs and even security issues on systems with a memory protection
>     unit.
>
>   - remove both the implementation and the macro to force a build
>     failure for anyone trying to use the feature. This way we can
>     see who complains and whether we need to put it back in some
>     form or change the userspace sources to no longer pass the flag.
>
> Implement the third option here for the sake of discussion.

Usually I'd vote for option #2, which means remove the kernel implementation and
redefine MAP_UNINITIALIZED to zero in the header.
A few years back this turned out to be the "most compatible" solution.

But today, and specifically regarding MAP_UNINITIALIZED, I think we should get rid of it now.
This flag is useless and build issues will force people to drop it.

Helge

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