On Mon Jun 29, 2026 at 3:25 PM EDT, Lorenzo Stoakes wrote:
> The core do_mmap() function accepts a vm_flags_t parameter which it then
> manipulates before passing to mmap_region() to do the heavy lifting of the
> memory mapping.
>
> Update do_mmap() to instead accept a vma_flags_t parameter, and adjust all
> the logic within do_mmap() to manipulate this instead.
>
> This is as part of the ongoing effort to convert VMA flags from a system
> word size to a bitmap type which allows us to unrestrict the number of VMA
> flags, as well as gain control over how VMA flag manipulation occurs.
>
> We do not cascade these changes to all functions which accept vm_flags_t,
> but rather use vma_flags_to_legacy() where necessary, specifically
> deferring converting calc_vm_prot_bits(), calc_vm_flag_bits() and
> __get_unmapped_area() to vma_flags_t.
>
> Also utilise the new vma_flags_can_grow() predicate which correctly handles
> the case of architectures without upward growing stacks.
>
> As part of this change, introduce VMA_SHADOW_STACK so we can correctly
> handle the case of the shadow stack not being defined.
>
> No functional change intended.
>
> Signed-off-by: Lorenzo Stoakes <[email protected]>
> ---
> arch/mips/kernel/vdso.c | 4 +--
> fs/aio.c | 2 +-
> include/linux/memfd.h | 6 ++--
> include/linux/mm.h | 6 ++--
> ipc/shm.c | 3 +-
> mm/memfd.c | 15 ++++-----
> mm/mmap.c | 67 ++++++++++++++++++++++++-----------------
> mm/nommu.c | 3 +-
> mm/util.c | 10 +++---
> mm/vma.c | 7 ++---
> mm/vma.h | 2 +-
> 11 files changed, 69 insertions(+), 56 deletions(-)
>
<snip>
>
> -static int check_write_seal(vm_flags_t *vm_flags_ptr)
> +static int check_write_seal(vma_flags_t *vma_flags_ptr)
> {
> - vm_flags_t vm_flags = *vm_flags_ptr;
> - vm_flags_t mask = vm_flags & (VM_SHARED | VM_WRITE);
> -
> /* If a private mapping then writability is irrelevant. */
> - if (!(mask & VM_SHARED))
> + if (!vma_flags_test(vma_flags_ptr, VMA_SHARED_BIT))
> return 0;
>
> /*
> * New PROT_WRITE and MAP_SHARED mmaps are not allowed when
> * write seals are active.
> */
> - if (mask & VM_WRITE)
> + if (vma_flags_test(vma_flags_ptr, VMA_WRITE_BIT))
> return -EPERM;
>
> /*
> * This is a read-only mapping, disallow mprotect() from making a
> * write-sealed mapping writable in future.
> */
> - *vm_flags_ptr &= ~VM_MAYWRITE;
> + vma_flags_clear(vma_flags_ptr, VMA_MAYWRITE_BIT);
>
> return 0;
> }
This function alone changed its original behavior, since vm_flags is a
snapshot of *vm_flags_ptr, but after the change this snapshot is gone.
But its only caller memfd_check_seals_mmap() gets vm_flags_ptr from the
input parameter of do_mmap(), so the overall behavior does not change.
<snip>
> + case MAP_DROPPABLE: {
> + vma_flags_t droppable = VMA_DROPPABLE;
> +
> + if (vma_flags_empty(&droppable))
> return -EOPNOTSUPP;
> + vma_flags_set_mask(&vma_flags, droppable);
> +
> /*
> * A locked or stack area makes no sense to be
> droppable.
> *
> @@ -515,23 +527,24 @@ unsigned long do_mmap(struct file *file, unsigned long
> addr,
> */
> if (flags & (MAP_LOCKED | MAP_HUGETLB))
> return -EINVAL;
> - if (vm_flags & (VM_GROWSDOWN | VM_GROWSUP))
> + if (vma_flags_can_grow(&vma_flags))
> return -EINVAL;
>
> - vm_flags |= VM_DROPPABLE;
> -
Lance pointed out the reordering of setting VMA_DROPPABLE and checking
of can_grow, but these flags are not overlapped and there is no parallel
writer to vma_flags. So it is still no functional change, just not
mechanical changes. :)
Otherwise, LGTM.
Reviewed-by: Zi Yan <[email protected]>
--
Best Regards,
Yan, Zi
