linux-kernel - Re: [PATCH v3 07/22] checkpatch: Warn on page table access without accessors

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Message-ID: <273b638e-8251-4faf-929a-87432a48abdc@kernel.org>
Date: Thu, 13 Nov 2025 20:17:22 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Samuel Holland <samuel.holland@...ive.com>, Joe Perches
 <joe@...ches.com>, Palmer Dabbelt <palmer@...belt.com>,
 Paul Walmsley <pjw@...nel.org>, linux-riscv@...ts.infradead.org,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc: devicetree@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>,
 linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...nel.org>,
 Michal Hocko <mhocko@...e.com>, Conor Dooley <conor@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Alexandre Ghiti <alex@...ti.fr>,
 Emil Renner Berthing <kernel@...il.dk>, Rob Herring <robh+dt@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>, "Liam R . Howlett"
 <Liam.Howlett@...cle.com>, Andy Whitcroft <apw@...onical.com>,
 Dwaipayan Ray <dwaipayanray1@...il.com>,
 Lukas Bulwahn <lukas.bulwahn@...il.com>
Subject: Re: [PATCH v3 07/22] checkpatch: Warn on page table access without
 accessors

On 13.11.25 03:36, Samuel Holland wrote:
> On 2025-11-12 8:21 PM, Joe Perches wrote:
>> On Wed, 2025-11-12 at 17:45 -0800, Samuel Holland wrote:
>>> Architectures may have special rules for accessing the hardware page
>>> tables (for example, atomicity/ordering requirements), so the generic MM
>>> code provides the pXXp_get() and set_pXX() hooks for architectures to
>>> implement. These accessor functions are often omitted where a raw
>>> pointer dereference is believed to be safe (i.e. race-free). However,
>>> RISC-V needs to use these hooks to rewrite the page table values at
>>> read/write time on some platforms. A raw pointer dereference will no
>>> longer produce the correct value on those platforms, so the generic code
>>> must always use the accessor functions.
>> []
>>> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
>> []
>>> @@ -7721,6 +7721,13 @@ sub process {
>>>   				ERROR("MISSING_SENTINEL", "missing sentinel in ID array\n" . "$here\n$stat\n");
>>>   			}
>>>   		}
>>> +
>>> +# check for raw dereferences of hardware page table pointers
>>> +		if ($realfile !~ m@...ch/@ &&
>>> +		    $line =~ /(?<!pte_t |p[mu4g]d_t |izeof\()\*\(?(vmf(\.|->))?(pte|p[mu4g]d)p?\b/) {
>>> +			WARN("PAGE_TABLE_ACCESSORS",
>>> +			     "Use $3p_get()/set_$3() instead of dereferencing page table pointers\n" . $herecurr);
>>> +		}
>>>   	}
>>
>> Seems like a lot of matches
>>
>> $ git grep -P '(?<!pte_t |p[mu4g]d_t |izeof\()\*\(?(vmf(\.|->))?(pte|p[mu4g]d)p?\b' | \
>>    grep -v '^arch/' | wc -l
>> 766

That is indeed concerning.

I recall that we discussed an alternative approach with Ryan in the 
past: I don't remember all the details, but essentially it was about 
using separate types, such that dereferencing would not get you the type 
the other functions would be expecting. Such that the compiler will bark 
when you try to dereference.

-- 
Cheers

David