linux-kernel - Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c

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Message-ID: <60b0b01b-d397-4e47-8e85-295241ea3230@redhat.com>
Date: Mon, 14 Jul 2025 17:52:56 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Pedro Falcato <pfalcato@...e.de>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Jeff Xu <jeffxu@...omium.org>
Subject: Re: [PATCH 2/5] mm/mseal: move madvise() logic to mm/madvise.c

On 14.07.25 17:45, Lorenzo Stoakes wrote:
> On Mon, Jul 14, 2025 at 05:41:45PM +0200, David Hildenbrand wrote:
>> On 14.07.25 17:31, Pedro Falcato wrote:
>>> On Mon, Jul 14, 2025 at 05:03:03PM +0200, David Hildenbrand wrote:
>>>> [...]
>>>>
>>>> But now I wonder, why is it okay to discard anon pages in a MAP_PRIVATE file
>>>> mapping?
>>>
>>> IIRC this was originally suggested by Linus, on one of the versions introducing
>>> mseal. But the gist is that discarding pages is okay if you could already zero
>>> them manually, using e.g memset. Hence the writeability checks.
>>
>> What you can do is
>>
>> a) mmap(PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, ...)
>>
>> b) modify content (write, whatever)
>>
>> c) mprotect(PROT_READ)
>>
>> d) mseal()
>>
>> But then still do
>>
>> madvise(MADV_DONTNEED)
>>
>> to discard.
>>
>>
>> There is no writability anymore.
> 
> Well, you can mprotect() writable it again :)

Isn't that what sealing ... prohibits?

-- 
Cheers,

David / dhildenb