linux-kernel - Re: [PATCH v1] mm: fix unmap_mapping

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Date: Wed, 20 Dec 2023 09:53:43 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: "jiajun.xie" <jiajun.xie.sh@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mm: fix unmap_mapping_range high bits shift bug

On Wed, 20 Dec 2023 13:28:39 +0800 "jiajun.xie" <jiajun.xie.sh@...il.com> wrote:

> From: Jiajun Xie <jiajun.xie.sh@...il.com>
> 
> The bug happens when highest bit of holebegin is 1, suppose
> holebign is 0x8000000111111000, after shift, hba would be
> 0xfff8000000111111, then vma_interval_tree_foreach would look
> it up fail or leads to the wrong result.
> 
> error call seq e.g.:
> - mmap(..., offset=0x8000000111111000)
>   |- syscall(mmap, ... unsigned long, off):
>      |- ksys_mmap_pgoff( ... , off >> PAGE_SHIFT);
> 
>   here pgoff is correctly shifted to 0x8000000111111,
>   but pass 0x8000000111111000 as holebegin to unmap
>   would then cause terrible result, as shown below:
> 
> - unmap_mapping_range(..., loff_t const holebegin)
>   |- pgoff_t hba = holebegin >> PAGE_SHIFT;
>           /* hba = 0xfff8000000111111 unexpectedly */
> 
> turn holebegin to be unsigned first would fix the bug.
> 

Thanks.  Are you able to describe the runtime effects of this
(obviously bad, but it's good to spell it out) and under what
circumstances it occurs?