linux-kernel - Re: [PATCH v2 2/5] mm: add PTE_MARKER

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Message-ID: <5ee11594-c40a-4ef1-a6c2-f1e38da0e9a3@lucifer.local>
Date: Mon, 21 Oct 2024 22:13:11 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
        David Hildenbrand <david@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Muchun Song <muchun.song@...ux.dev>,
        Richard Henderson <richard.henderson@...aro.org>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E . J . Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>, Arnd Bergmann <arnd@...db.de>,
        linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linux-arch@...r.kernel.org,
        Shuah Khan <shuah@...nel.org>, Christian Brauner <brauner@...nel.org>,
        linux-kselftest@...r.kernel.org,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>,
        Jeff Xu <jeffxu@...omium.org>, Christoph Hellwig <hch@...radead.org>,
        linux-api@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v2 2/5] mm: add PTE_MARKER_GUARD PTE marker

+cc Dave Hansen

On Mon, Oct 21, 2024 at 09:42:53PM +0100, Lorenzo Stoakes wrote:
> On Mon, Oct 21, 2024 at 03:45:31PM +0200, Vlastimil Babka wrote:
> > On 10/20/24 18:20, Lorenzo Stoakes wrote:
> > > Add a new PTE marker that results in any access causing the accessing
> > > process to segfault.
> >
> > Should we distinguish it from other segfaults? Is there a way? I can see
> > memory protection keys use SEGV_PKUERR, but no idea if we have any free values.
>
> Wasn't even aware that existed!!
>
> I'm not sure a process can do anything particularly useful with this
> information though?  Hitting a guard page would indicate a programming
> error rather than something that might allow meaningful feedback to a user
> like memory protection keys.
>
> Do you think there's enough value int his to warrant digging in? And indeed
> I imagine bits are in short supply for this and would need a strong
> argument to get... so yeah I don't think too worthwhile most likely!
>
> Thanks for the suggestion though!

To put it on list - Dave Hansen commented on IRC that it would be safer to
avoid this for now due to this being an ABI change, and reasonable to
perhaps add it later if required, so that seems a sensible way forward.

Thanks!

[snip]