linux-kernel - Re: [PATCH v2 0/5] implement lightweight guard pages

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Message-ID: <f000d21f-dd04-462a-9d34-d0e7f0f7dc2e@redhat.com>
Date: Wed, 23 Oct 2024 11:13:47 +0200
From: David Hildenbrand <david@...hat.com>
To: Vlastimil Babka <vbabka@...e.cz>, Dmitry Vyukov <dvyukov@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: fw@...eb.enyo.de, James.Bottomley@...senpartnership.com,
 Liam.Howlett@...cle.com, akpm@...ux-foundation.org, arnd@...db.de,
 brauner@...nel.org, chris@...kel.net, deller@....de, hch@...radead.org,
 ink@...assic.park.msu.ru, jannh@...gle.com, jcmvbkbc@...il.com,
 jeffxu@...omium.org, jhubbard@...dia.com, linux-alpha@...r.kernel.org,
 linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-mips@...r.kernel.org, linux-mm@...ck.org,
 linux-parisc@...r.kernel.org, mattst88@...il.com, muchun.song@...ux.dev,
 paulmck@...nel.org, richard.henderson@...aro.org, shuah@...nel.org,
 sidhartha.kumar@...cle.com, surenb@...gle.com, tsbogend@...ha.franken.de,
 willy@...radead.org, elver@...gle.com,
 Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 0/5] implement lightweight guard pages

On 23.10.24 11:06, Vlastimil Babka wrote:
> On 10/23/24 10:56, Dmitry Vyukov wrote:
>>>
>>> Overall while I sympathise with this, it feels dangerous and a pretty major
>>> change, because there'll be something somewhere that will break because it
>>> expects faults to be swallowed that we no longer do swallow.
>>>
>>> So I'd say it'd be something we should defer, but of course it's a highly
>>> user-facing change so how easy that would be I don't know.
>>>
>>> But I definitely don't think a 'introduce the ability to do cheap PROT_NONE
>>> guards' series is the place to also fundmentally change how user access
>>> page faults are handled within the kernel :)
>>
>> Will delivering signals on kernel access be a backwards compatible
>> change? Or will we need a different API? MADV_GUARD_POISON_KERNEL?
>> It's just somewhat painful to detect/update all userspace if we add
>> this feature in future. Can we say signal delivery on kernel accesses
>> is unspecified?
> 
> Would adding signal delivery to guard PTEs only help enough the ASAN etc
> usecase? Wouldn't it be instead possible to add some prctl to opt-in the
> whole ASANized process to deliver all existing segfaults as signals instead
> of -EFAULT ?

Not sure if it is an "instead", you might have to deliver the signal in 
addition to letting the syscall fail (not that I would be an expert on 
signal delivery :D ).

prctl sounds better, or some way to configure the behavior on VMA 
ranges; otherwise we would need yet another marker, which is not the end 
of the world but would make it slightly more confusing.

-- 
Cheers,

David / dhildenb