linux-kernel - Re: [PATCH 4/5] mm/mseal: separate out and simplify VMA gap check

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Message-ID: <mmqwb7m76ty6gmazw7db35rfow5vq66a72qa7rwjhnkh7o3j6t@2bk55s6jqb5f>
Date: Mon, 14 Jul 2025 11:35:44 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>, Vlastimil Babka <vbabka@...e.cz>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jeff Xu <jeffxu@...omium.org>
Subject: Re: [PATCH 4/5] mm/mseal: separate out and simplify VMA gap check

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [250714 09:08]:
> The check_mm_seal() function is doing something general - checking whether
> a range contains only VMAs (or rather that it does NOT contain any unmapped
> regions).
> 
> Generalise this and put the logic in mm/vma.c - introducing
> range_contains_unmapped(). Additionally we can simplify the logic, we are
> simply checking whether the last vma->vm_end has either a VMA starting
> after it or ends before the end parameter.
> 
> No functional change intended.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

I do fear that people will find this function and try and use it
internally, it may make our jobs of avoiding this being expanded more
annoying.

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

> ---
>  mm/mseal.c | 38 +++-----------------------------------
>  mm/vma.c   | 18 ++++++++++++++++++
>  mm/vma.h   |  3 +++
>  3 files changed, 24 insertions(+), 35 deletions(-)
> 
> diff --git a/mm/mseal.c b/mm/mseal.c
> index adbcc65e9660..8e4c605af700 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -37,34 +37,6 @@ static int mseal_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	return ret;
>  }
> 
> -/*
> - * Check for do_mseal:
> - * 1> start is part of a valid vma.
> - * 2> end is part of a valid vma.
> - * 3> No gap (unallocated address) between start and end.
> - * 4> map is sealable.
> - */
> -static int check_mm_seal(unsigned long start, unsigned long end)
> -{
> -	struct vm_area_struct *vma;
> -	unsigned long nstart = start;
> -	VMA_ITERATOR(vmi, current->mm, start);
> -
> -	/* going through each vma to check. */
> -	for_each_vma_range(vmi, vma, end) {
> -		if (vma->vm_start > nstart)
> -			/* unallocated memory found. */
> -			return -ENOMEM;
> -
> -		if (vma->vm_end >= end)
> -			return 0;
> -
> -		nstart = vma->vm_end;
> -	}
> -
> -	return -ENOMEM;
> -}
> -
>  /*
>   * Apply sealing.
>   */
> @@ -184,14 +156,10 @@ int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
>  	if (mmap_write_lock_killable(mm))
>  		return -EINTR;
> 
> -	/*
> -	 * First pass, this helps to avoid
> -	 * partial sealing in case of error in input address range,
> -	 * e.g. ENOMEM error.
> -	 */
> -	ret = check_mm_seal(start, end);
> -	if (ret)
> +	if (range_contains_unmapped(mm, start, end)) {
> +		ret = -ENOMEM;
>  		goto out;
> +	}
> 
>  	/*
>  	 * Second pass, this should success, unless there are errors
> diff --git a/mm/vma.c b/mm/vma.c
> index b3d880652359..b57545568ae6 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -3203,3 +3203,21 @@ int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
> 
>  	return 0;
>  }
> +
> +/* Does the [start, end) range contain any unmapped memory? */
> +bool range_contains_unmapped(struct mm_struct *mm,
> +		unsigned long start, unsigned long end)
> +{
> +	struct vm_area_struct *vma;
> +	unsigned long prev_end = start;
> +	VMA_ITERATOR(vmi, current->mm, start);
> +
> +	for_each_vma_range(vmi, vma, end) {
> +		if (vma->vm_start > prev_end)
> +			return true;
> +
> +		prev_end = vma->vm_end;
> +	}
> +
> +	return prev_end < end;
> +}
> diff --git a/mm/vma.h b/mm/vma.h
> index d17f560cf53d..bfe9a04e6018 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -598,4 +598,7 @@ int create_init_stack_vma(struct mm_struct *mm, struct vm_area_struct **vmap,
>  int relocate_vma_down(struct vm_area_struct *vma, unsigned long shift);
>  #endif
> 
> +bool range_contains_unmapped(struct mm_struct *mm,
> +		unsigned long start, unsigned long end);
> +
>  #endif	/* __MM_VMA_H */
> --
> 2.50.1