On Mon, Jul 06, 2026 at 10:10:32PM -0400, Zi Yan wrote:
> 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.
Right yeah, the snapshot was always just a convenience thing :)
>
> <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. :)
Right yes exactly :)
>
> Otherwise, LGTM.
>
> Reviewed-by: Zi Yan <[email protected]>
Thanks!
>
> --
> Best Regards,
> Yan, Zi
>
Cheers, Lorenzo
