linux-kernel - Re: MREMAP_FIXED unmaps dest on error

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Date:   Mon, 3 Apr 2023 18:31:52 +0500
From:   stsp <stsp2@...dex.ru>
To:     David Hildenbrand <david@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     linux-mm@...ck.org,
        Jakub Matěna <matenajakub@...il.com>,
        Hugh Dickins <hughd@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: MREMAP_FIXED unmaps dest on error

Hi,

03.04.2023 16:58, David Hildenbrand пишет:
> On 30.03.23 17:48, stsp wrote:
>> Hello.
>>
>> The attached test-case demonstrates a
>> bug in mremap(). If MREMAP_FIXED is used
>> over an existing mapping and mremap() fails,
>> destination area gets unmapped.
>> AFAIK the failed syscall should have no
>> observable effects.
>
> I remember that holds for various mapping-related syscalls: if 
> something goes wrong, the end result is not guaranteed to be what we 
> had before the syscall.
>
> For example, if you use mmap(MAP_FIXED) to replace part of an exiting 
> mapping, we first munmap what's there and then try to mmap the new 
> mapping. If something goes wrong while doing that, we cannot simple 
> undo what we already did.
>
> Long story short: the semantics of these syscalls has never been to 
> leave the system in the state as it was before in case anything goes 
> wrong.
>
>
> As another example, if you do an mprotect() that covers multiple VMAS, 
> and there is an issue with the last VMA, all but the last VMA will 
> have their permissions changed.
>
Thanks for info.
Is this documented in a man page?
I wonder how do you deal with mmap() and
mprotect() on such occasions. mremap()
is an extension, but mmap() and mprotect()
are from posix, so is it a compliant impl?

Also my example shows another bug
that the VMAs are not merged after I
restore the protection of one of them,
allowing them to merge.