* Re: MREMAP_FIXED unmaps dest on error
[not found] <18c36a78-4082-fab6-c7c9-69a249516803@yandex.ru>
@ 2023-04-03 11:58 ` David Hildenbrand
0 siblings, 0 replies; only message in thread
From: David Hildenbrand @ 2023-04-03 11:58 UTC (permalink / raw)
To: stsp, Linux Kernel Mailing List
Cc: linux-mm, Jakub Matěna, Hugh Dickins, Vlastimil Babka,
Peter Zijlstra (Intel)
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,
David / dhildenb
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