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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Jul 14, 2025 at 11:35:44AM -0400, Liam R. Howlett wrote:
> * Lorenzo Stoakes <lorenzo.stoakes@oracle.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@oracle.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.

Hmm, surely we should have some ability to dissuade within mm :)

Thing is I don't love having this function in mm/mseal.c when it has
nothing to do with mseal()'ing.

If people want to be weird about gaps they can pretty trivially implement
something like this anyway. Probably. Possibly. Maybe?

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

Thanks!

>
> > ---
> >  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