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

[David Hildenbrand <david@redhat.com>]

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