linux-kernel - Re: [PATCH RESEND] mm/pagewalk: split walk_page_range

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <764d452d-446a-4b5e-aa03-e0d901126931@redhat.com>
Date: Wed, 4 Jun 2025 10:12:40 +0200
From: David Hildenbrand <david@...hat.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Barry Song <baohua@...nel.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Muchun Song <muchun.song@...ux.dev>,
 Oscar Salvador <osalvador@...e.de>, Huacai Chen <chenhuacai@...nel.org>,
 WANG Xuerui <kernel@...0n.name>, Jonas Bonn <jonas@...thpole.se>,
 Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
 Stafford Horne <shorne@...il.com>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexandre Ghiti <alex@...ti.fr>, Jann Horn <jannh@...gle.com>,
 loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
 linux-openrisc@...r.kernel.org, linux-riscv@...ts.infradead.org,
 linux-mm@...ck.org
Subject: Re: [PATCH RESEND] mm/pagewalk: split walk_page_range_novma() into
 kernel/user parts

On 04.06.25 10:07, Mike Rapoport wrote:
> On Wed, Jun 04, 2025 at 09:39:30AM +0200, David Hildenbrand wrote:
>> On 03.06.25 21:22, Lorenzo Stoakes wrote:
>>> The walk_page_range_novma() function is rather confusing - it supports two
>>> modes, one used often, the other used only for debugging.
>>>
>>> The first mode is the common case of traversal of kernel page tables, which
>>> is what nearly all callers use this for.
>>
>> ... and what people should be using it for 🙂
>>
>>>
>>> Secondly it provides an unusual debugging interface that allows for the
>>> traversal of page tables in a userland range of memory even for that memory
>>> which is not described by a VMA.
>>>
>>> This is highly unusual and it is far from certain that such page tables
>>> should even exist, but perhaps this is precisely why it is useful as a
>>> debugging mechanism.
>>>
>>> As a result, this is utilised by ptdump only. Historically, things were
>>> reversed - ptdump was the only user, and other parts of the kernel evolved
>>> to use the kernel page table walking here.
>>>
>>> Since we have some complicated and confusing locking rules for the novma
>>> case, it makes sense to separate the two usages into their own functions.
>>>
>>> Doing this also provide self-documentation as to the intent of the caller -
>>> are they doing something rather unusual or are they simply doing a standard
>>> kernel page table walk?
>>>
>>> We therefore maintain walk_page_range_novma() for this single usage, and
>>> document the function as such.
>>
>> If we have to keep this dangerous interface, it should probably be
>>
>> walk_page_range_debug() or walk_page_range_dump()
> 
> We can also move it from include/linux/pagewalk.h to mm/internal.h

Agreed.

-- 
Cheers,

David / dhildenb