Re: [PATCH v3 05/40] mm/vma: ensure mmap_prepare doesn't set actions on a mergeable vma
From: Gregory Price <gourry@gourry.net>
Date: 2026-09-24 18:00:23
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On Thu, Sep 17, 2026 at 05:22:14PM +0100, Lorenzo Stoakes (ARM) wrote:
quoted hunk ↗ jump to hunk
When a user requests an mmap_action be performed in mmap_prepare, this involves populating the VMA range with data. However, if the VMA is mergeable, it might then mistakenly be merged with another VMA without having populated the range. Every mmap action currently available sets VMA flags such that the VMA cannot be merged. However, to ensure that no future mmap action falls foul of this, assert that this is the case upon mmap_prepare validation. Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org> --- mm/vma.c | 9 +++++++++ 1 file changed, 9 insertions(+)diff --git a/mm/vma.c b/mm/vma.c index d6ed10cefc8f..62f2ce1ad5a1 100644 --- a/mm/vma.c +++ b/mm/vma.c@@ -2809,6 +2809,15 @@ static int mmap_validate(unsigned long prev_start, unsigned long prev_end, int mmap_prepare_validate(const struct vm_area_desc *prev_desc, const struct vm_area_desc *desc) { + /* + * It is not valid to execute mmap actions for VMAs which can be merged, + * as any such merge would leave portions of the mapping incorrectly + * unmapped. + */ + if (vma_flags_can_merge(&desc->vma_flags) && + WARN_ON_ONCE(desc->action.type != MMAP_NOTHING)) + return -EINVAL; +
If you wanted to make this unit-testable, you could pull it out into a
separate function:
static bool mmap_action_is_valid(const struct vm_area_desc *desc)
{
return desc->action.type == MMAP_NOTHING ||
!vma_flags_can_merge(&desc->vma_flags);
}
then write:
if (WARN_ON_ONCE(!mmap_action_is_valid(desc)))
return -EINVAL;
And you can write a unit test directly against mmap_action_is_valid
otherwise
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>