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[PATCH v3 4/5] mm/mseal: Simplify and rename VMA gap check

Lorenzo Stoakes posted 5 patches 2 months, 3 weeks ago
[PATCH v3 4/5] mm/mseal: Simplify and rename VMA gap check
Posted by Lorenzo Stoakes 2 months, 3 weeks ago 
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).

So rename this function to range_contains_unmapped().

Additionally 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.

This check is rather dubious, so it is sensible to keep it local to
mm/mseal.c as at a later stage it may be removed, and we don't want any
other mm code to perform such a check.

No functional change intended.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Acked-by: David Hildenbrand <david@redhat.com>
---
 mm/mseal.c | 36 +++++++++++-------------------------
 1 file changed, 11 insertions(+), 25 deletions(-)

diff --git a/mm/mseal.c b/mm/mseal.c
index adbcc65e9660..61c07b1369cb 100644
--- a/mm/mseal.c
+++ b/mm/mseal.c
@@ -37,32 +37,22 @@ 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)
+/* Does the [start, end) range contain any unmapped memory? */
+static bool range_contains_unmapped(struct mm_struct *mm,
+		unsigned long start, unsigned long end)
 {
 	struct vm_area_struct *vma;
-	unsigned long nstart = start;
+	unsigned long prev_end = 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;
+		if (vma->vm_start > prev_end)
+			return true;
 
-		nstart = vma->vm_end;
+		prev_end = vma->vm_end;
 	}
 
-	return -ENOMEM;
+	return prev_end < end;
 }
 
 /*
@@ -184,14 +174,10 @@ 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
-- 
2.50.1
Re: [PATCH v3 4/5] mm/mseal: Simplify and rename VMA gap check
Posted by Jeff Xu 2 months, 2 weeks ago 
Hi Lorenzo,

On Wed, Jul 16, 2025 at 10:38 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> 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).
>
> So rename this function to range_contains_unmapped().
>
> Additionally 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.
>
> This check is rather dubious, so it is sensible to keep it local to
> mm/mseal.c as at a later stage it may be removed, and we don't want any
> other mm code to perform such a check.
>
> No functional change intended.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/mseal.c | 36 +++++++++++-------------------------
>  1 file changed, 11 insertions(+), 25 deletions(-)
>
> diff --git a/mm/mseal.c b/mm/mseal.c
> index adbcc65e9660..61c07b1369cb 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -37,32 +37,22 @@ 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)
Is it possible to leave the check_mm_seal() function together with its
header comments? My original reason was to have a contract that
documents the exact entry check for mseal(). That way, no matter how
the code is refactored in the future, as long as the contract remains
true, I won't need to worry about behavior changes for mseal(). This
could be helpful if you move range_contains_unmapped into vma.c in the
future.

Note: "4> map is sealable." can be removed,  which is obsolete, we no
longer use sealable flags.

Thanks and regards,
-Jeff
> +/* Does the [start, end) range contain any unmapped memory? */
> +static bool range_contains_unmapped(struct mm_struct *mm,
> +               unsigned long start, unsigned long end)
>  {
>         struct vm_area_struct *vma;
> -       unsigned long nstart = start;
> +       unsigned long prev_end = 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;
> +               if (vma->vm_start > prev_end)
> +                       return true;
>
> -               nstart = vma->vm_end;
> +               prev_end = vma->vm_end;
>         }
>
> -       return -ENOMEM;
> +       return prev_end < end;
>  }
>
>  /*
> @@ -184,14 +174,10 @@ 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
> --
> 2.50.1
>
Re: [PATCH v3 4/5] mm/mseal: Simplify and rename VMA gap check
Posted by Lorenzo Stoakes 2 months, 1 week ago 
On Thu, Jul 24, 2025 at 11:40:59AM -0700, Jeff Xu wrote:
> Hi Lorenzo,
>
> On Wed, Jul 16, 2025 at 10:38 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > 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).
> >
> > So rename this function to range_contains_unmapped().
> >
> > Additionally 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.
> >
> > This check is rather dubious, so it is sensible to keep it local to
> > mm/mseal.c as at a later stage it may be removed, and we don't want any
> > other mm code to perform such a check.
> >
> > No functional change intended.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> > Acked-by: David Hildenbrand <david@redhat.com>
> > ---
> >  mm/mseal.c | 36 +++++++++++-------------------------
> >  1 file changed, 11 insertions(+), 25 deletions(-)
> >
> > diff --git a/mm/mseal.c b/mm/mseal.c
> > index adbcc65e9660..61c07b1369cb 100644
> > --- a/mm/mseal.c
> > +++ b/mm/mseal.c
> > @@ -37,32 +37,22 @@ 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)
> Is it possible to leave the check_mm_seal() function together with its
> header comments? My original reason was to have a contract that
> documents the exact entry check for mseal(). That way, no matter how
> the code is refactored in the future, as long as the contract remains
> true, I won't need to worry about behavior changes for mseal(). This
> could be helpful if you move range_contains_unmapped into vma.c in the
> future.
>
> Note: "4> map is sealable." can be removed,  which is obsolete, we no
> longer use sealable flags.

Sure, I will add in a comment to make this abundantly clear.

>
> Thanks and regards,
> -Jeff
> > +/* Does the [start, end) range contain any unmapped memory? */
> > +static bool range_contains_unmapped(struct mm_struct *mm,
> > +               unsigned long start, unsigned long end)
> >  {
> >         struct vm_area_struct *vma;
> > -       unsigned long nstart = start;
> > +       unsigned long prev_end = 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;
> > +               if (vma->vm_start > prev_end)
> > +                       return true;
> >
> > -               nstart = vma->vm_end;
> > +               prev_end = vma->vm_end;
> >         }
> >
> > -       return -ENOMEM;
> > +       return prev_end < end;
> >  }
> >
> >  /*
> > @@ -184,14 +174,10 @@ 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
> > --
> > 2.50.1
> >

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