Re: [PATCH 04/15] mm: add vm_ops->mapped hook

Lorenzo Stoakes (Oracle)
2026-03-13T11:58:51Z

On Fri, Mar 13, 2026 at 04:02:36AM -0700, Usama Arif wrote:
> On Thu, 12 Mar 2026 20:27:19 +0000 "Lorenzo Stoakes (Oracle)" <ljs@kernel.org> wrote:
>
> > Previously, when a driver needed to do something like establish a reference
> > count, it could do so in the mmap hook in the knowledge that the mapping
> > would succeed.
> >
> > With the introduction of f_op->mmap_prepare this is no longer the case, as
> > it is invoked prior to actually establishing the mapping.
> >
> > To take this into account, introduce a new vm_ops->mapped callback which is
> > invoked when the VMA is first mapped (though notably - not when it is
> > merged - which is correct and mirrors existing mmap/open/close behaviour).
> >
> > We do better that vm_ops->open() here, as this callback can return an
> > error, at which point the VMA will be unmapped.
> >
> > Note that vm_ops->mapped() is invoked after any mmap action is
> > complete (such as I/O remapping).
> >
> > We intentionally do not expose the VMA at this point, exposing only the
> > fields that could be used, and an output parameter in case the operation
> > needs to update the vma->vm_private_data field.
> >
> > In order to deal with stacked filesystems which invoke inner filesystem's
> > mmap() invocations, add __compat_vma_mapped() and invoke it on
> > vfs_mmap() (via compat_vma_mmap()) to ensure that the mapped callback is
> > handled when an mmap() caller invokes a nested filesystem's mmap_prepare()
> > callback.
> >
> > We can now also remove call_action_complete() and invoke
> > mmap_action_complete() directly, as we separate out the rmap lock logic to
> > be called in __mmap_region() instead via maybe_drop_file_rmap_lock().
> >
> > We also abstract unmapping of a VMA on mmap action completion into its own
> > helper function, unmap_vma_locked().
> >
> > Additionally, update VMA userland test headers to reflect the change.
> >
> > Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> > ---
> > include/linux/fs.h | 9 +++-
> > include/linux/mm.h | 17 +++++++
> > mm/internal.h | 10 ++++
> > mm/util.c | 86 ++++++++++++++++++++++++---------
> > mm/vma.c | 41 +++++++++++-----
> > tools/testing/vma/include/dup.h | 34 ++++++++++++-
> > 6 files changed, 158 insertions(+), 39 deletions(-)
> >
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index a2628a12bd2b..c390f5c667e3 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -2059,13 +2059,20 @@ static inline bool can_mmap_file(struct file *file)
> > }
> >
> > int compat_vma_mmap(struct file *file, struct vm_area_struct *vma);
> > +int __vma_check_mmap_hook(struct vm_area_struct *vma);
> >
> > static inline int vfs_mmap(struct file *file, struct vm_area_struct *vma)
> > {
> > + int err;
> > +
> > if (file->f_op->mmap_prepare)
> > return compat_vma_mmap(file, vma);
> >
> > - return file->f_op->mmap(file, vma);
> > + err = file->f_op->mmap(file, vma);
> > + if (err)
> > + return err;
> > +
> > + return __vma_check_mmap_hook(vma);
> > }
> >
> > static inline int vfs_mmap_prepare(struct file *file, struct vm_area_desc *desc)
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 12a0b4c63736..7333d5db1221 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -759,6 +759,23 @@ struct vm_operations_struct {
> > * Context: User context. May sleep. Caller holds mmap_lock.
> > */
> > void (*close)(struct vm_area_struct *vma);
> > + /**
> > + * @mapped: Called when the VMA is first mapped in the MM. Not called if
> > + * the new VMA is merged with an adjacent VMA.
> > + *
> > + * The @vm_private_data field is an output field allowing the user to
> > + * modify vma->vm_private_data as necessary.
> > + *
> > + * ONLY valid if set from f_op->mmap_prepare. Will result in an error if
> > + * set from f_op->mmap.
> > + *
> > + * Returns %0 on success, or an error otherwise. On error, the VMA will
> > + * be unmapped.
> > + *
> > + * Context: User context. May sleep. Caller holds mmap_lock.
> > + */
> > + int (*mapped)(unsigned long start, unsigned long end, pgoff_t pgoff,
> > + const struct file *file, void **vm_private_data);
> > /* Called any time before splitting to check if it's allowed */
> > int (*may_split)(struct vm_area_struct *vma, unsigned long addr);
> > int (*mremap)(struct vm_area_struct *vma);
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 7bfa85b5e78b..f0f2cf1caa36 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -158,6 +158,8 @@ static inline void *folio_raw_mapping(const struct folio *folio)
> > * mmap hook and safely handle error conditions. On error, VMA hooks will be
> > * mutated.
> > *
> > + * IMPORTANT: f_op->mmap() is deprecated, prefer f_op->mmap_prepare().
> > + *
> > * @file: File which backs the mapping.
> > * @vma: VMA which we are mapping.
> > *
> > @@ -201,6 +203,14 @@ static inline void vma_close(struct vm_area_struct *vma)
> > /* unmap_vmas is in mm/memory.c */
> > void unmap_vmas(struct mmu_gather *tlb, struct unmap_desc *unmap);
> >
> > +static inline void unmap_vma_locked(struct vm_area_struct *vma)
> > +{
> > + const size_t len = vma_pages(vma) << PAGE_SHIFT;
> > +
> > + mmap_assert_locked(vma->vm_mm);
> > + do_munmap(vma->vm_mm, vma->vm_start, len, NULL);
> > +}
> > +
> > #ifdef CONFIG_MMU
> >
> > static inline void get_anon_vma(struct anon_vma *anon_vma)
> > diff --git a/mm/util.c b/mm/util.c
> > index dba1191725b6..2b0ed54008d6 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -1163,6 +1163,55 @@ void flush_dcache_folio(struct folio *folio)
> > EXPORT_SYMBOL(flush_dcache_folio);
> > #endif
> >
> > +static int __compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
> > +{
> > + struct vm_area_desc desc = {
> > + .mm = vma->vm_mm,
> > + .file = file,
> > + .start = vma->vm_start,
> > + .end = vma->vm_end,
> > +
> > + .pgoff = vma->vm_pgoff,
> > + .vm_file = vma->vm_file,
> > + .vma_flags = vma->flags,
> > + .page_prot = vma->vm_page_prot,
> > +
> > + .action.type = MMAP_NOTHING, /* Default */
> > + };
> > + int err;
> > +
> > + err = vfs_mmap_prepare(file, &desc);
> > + if (err)
> > + return err;
> > +
> > + err = mmap_action_prepare(&desc, &desc.action);
> > + if (err)
> > + return err;
> > +
> > + set_vma_from_desc(vma, &desc);
> > + return mmap_action_complete(vma, &desc.action);
> > +}
> > +
> > +static int __compat_vma_mapped(struct file *file, struct vm_area_struct *vma)
> > +{
> > + const struct vm_operations_struct *vm_ops = vma->vm_ops;
> > + void *vm_private_data = vma->vm_private_data;
> > + int err;
> > +
> > + if (!vm_ops->mapped)
> > + return 0;
> > +
>
> Hello!
>
> Can vm_ops be NULL here? __compat_vma_mapped() is called from
> compat_vma_mmap(), which is reached when a filesystem provides
> mmap_prepare. If the mmap_prepare hook does not set desc->vm_ops,
> vma->vm_ops will be NULL and this dereferences a NULL pointer.

I _think_ for this to ever be invoked, you would need to be dealing with a
file-backed VMA so vm_ops->fault would HAVE to be defined.

But you're right anyway as a matter of principle we should check it! Will fix.

>
> For e.g. drivers/char/mem.c, mmap_zero_prepare() would trigger
> a NULL pointer dereference here.
>
> Would need to do
> if (!vm_ops || !vm_ops->mapped)
> return 0;
>
> here

Yes.

>
>
> > + err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff, file,
> > + &vm_private_data);
> > + if (err)
> > + unmap_vma_locked(vma);
>
> when mapped() returns an error, unmap_vma_locked(vma) is called
> but execution continues into the vm_private_data update below. After
> unmap_vma_locked() the VMA may be freed (do_munmap can remove the VMA
> entirely), so accessing vma->vm_private_data after that is a
> use-after-free.

Very good point :) will fix thanks!

Probably:

if (err)
unmap_vma_locked(vma);
else if (vm_private_data != vma->vm_private_data)
vma->vm_private_data = vm_private_data;

return err;

Would be fine.

>
> Probably need to do:
> if (err) {
> unmap_vma_locked(vma);
> return err;
> }
>
> > + /* Update private data if changed. */
> > + if (vm_private_data != vma->vm_private_data)
> > + vma->vm_private_data = vm_private_data;
> > +
> > + return err;
> > +}
> > +
> > /**
> > * compat_vma_mmap() - Apply the file's .mmap_prepare() hook to an
> > * existing VMA and execute any requested actions.
> > @@ -1191,34 +1240,26 @@ EXPORT_SYMBOL(flush_dcache_folio);
> > */
> > int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
> > {
> > - struct vm_area_desc desc = {
> > - .mm = vma->vm_mm,
> > - .file = file,
> > - .start = vma->vm_start,
> > - .end = vma->vm_end,
> > -
> > - .pgoff = vma->vm_pgoff,
> > - .vm_file = vma->vm_file,
> > - .vma_flags = vma->flags,
> > - .page_prot = vma->vm_page_prot,
> > -
> > - .action.type = MMAP_NOTHING, /* Default */
> > - };
> > int err;
> >
> > - err = vfs_mmap_prepare(file, &desc);
> > - if (err)
> > - return err;
> > -
> > - err = mmap_action_prepare(&desc, &desc.action);
> > + err = __compat_vma_mmap(file, vma);
> > if (err)
> > return err;
> >
> > - set_vma_from_desc(vma, &desc);
> > - return mmap_action_complete(vma, &desc.action);
> > + return __compat_vma_mapped(file, vma);
> > }
> > EXPORT_SYMBOL(compat_vma_mmap);
> >
> > +int __vma_check_mmap_hook(struct vm_area_struct *vma)
> > +{
> > + /* vm_ops->mapped is not valid if mmap() is specified. */
> > + if (WARN_ON_ONCE(vma->vm_ops->mapped))
> > + return -EINVAL;
>
> I think vma->vm_ops can be NULL here. Should be:
>
> if (vma->vm_ops && WARN_ON_ONCE(vma->vm_ops->mapped))
> return -EINVAL;

I think again you'd probably only invoke this on file-backed so be ok, but again
as a matter of principle we should check it so will fix, thanks!

>
> > +
> > + return 0;
> > +}
> > +EXPORT_SYMBOL(__vma_check_mmap_hook);
> > +
> > static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
> > const struct page *page)
> > {
> > @@ -1316,10 +1357,7 @@ static int mmap_action_finish(struct vm_area_struct *vma,
> > * invoked if we do NOT merge, so we only clean up the VMA we created.
> > */
> > if (err) {
> > - const size_t len = vma_pages(vma) << PAGE_SHIFT;
> > -
> > - do_munmap(current->mm, vma->vm_start, len, NULL);
> > -
> > + unmap_vma_locked(vma);
> > if (action->error_hook) {
> > /* We may want to filter the error. */
> > err = action->error_hook(err);
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 054cf1d262fb..ef9f5a5365d1 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -2705,21 +2705,35 @@ static bool can_set_ksm_flags_early(struct mmap_state *map)
> > return false;
> > }
> >
> > -static int call_action_complete(struct mmap_state *map,
> > - struct mmap_action *action,
> > - struct vm_area_struct *vma)
> > +static int call_mapped_hook(struct vm_area_struct *vma)
> > {
> > - int ret;
> > + const struct vm_operations_struct *vm_ops = vma->vm_ops;
> > + void *vm_private_data = vma->vm_private_data;
> > + int err;
> >
> > - ret = mmap_action_complete(vma, action);
> > + if (!vm_ops || !vm_ops->mapped)
> > + return 0;
> > + err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff,
> > + vma->vm_file, &vm_private_data);
> > + if (err) {
> > + unmap_vma_locked(vma);
> > + return err;
> > + }
> > + /* Update private data if changed. */
> > + if (vm_private_data != vma->vm_private_data)
> > + vma->vm_private_data = vm_private_data;
> > + return 0;
> > +}
> >
> > - /* If we held the file rmap we need to release it. */
> > - if (map->hold_file_rmap_lock) {
> > - struct file *file = vma->vm_file;
> > +static void maybe_drop_file_rmap_lock(struct mmap_state *map,
> > + struct vm_area_struct *vma)
> > +{
> > + struct file *file;
> >
> > - i_mmap_unlock_write(file->f_mapping);
> > - }
> > - return ret;
> > + if (!map->hold_file_rmap_lock)
> > + return;
> > + file = vma->vm_file;
> > + i_mmap_unlock_write(file->f_mapping);
> > }
> >
> > static unsigned long __mmap_region(struct file *file, unsigned long addr,
> > @@ -2773,8 +2787,11 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
> > __mmap_complete(&map, vma);
> >
> > if (have_mmap_prepare && allocated_new) {
> > - error = call_action_complete(&map, &desc.action, vma);
> > + error = mmap_action_complete(vma, &desc.action);
> > + if (!error)
> > + error = call_mapped_hook(vma);
> >
> > + maybe_drop_file_rmap_lock(&map, vma);
> > if (error)
> > return error;
> > }
> > diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
> > index 908beb263307..47d8db809f31 100644
> > --- a/tools/testing/vma/include/dup.h
> > +++ b/tools/testing/vma/include/dup.h
> > @@ -606,12 +606,34 @@ struct vm_area_struct {
> > } __randomize_layout;
> >
> > struct vm_operations_struct {
> > - void (*open)(struct vm_area_struct * area);
> > + /**
> > + * @open: Called when a VMA is remapped or split. Not called upon first
> > + * mapping a VMA.
> > + * Context: User context. May sleep. Caller holds mmap_lock.
> > + */
> > + void (*open)(struct vm_area_struct *vma);
> > /**
> > * @close: Called when the VMA is being removed from the MM.
> > * Context: User context. May sleep. Caller holds mmap_lock.
> > */
> > - void (*close)(struct vm_area_struct * area);
> > + void (*close)(struct vm_area_struct *vma);
> > + /**
> > + * @mapped: Called when the VMA is first mapped in the MM. Not called if
> > + * the new VMA is merged with an adjacent VMA.
> > + *
> > + * The @vm_private_data field is an output field allowing the user to
> > + * modify vma->vm_private_data as necessary.
> > + *
> > + * ONLY valid if set from f_op->mmap_prepare. Will result in an error if
> > + * set from f_op->mmap.
> > + *
> > + * Returns %0 on success, or an error otherwise. On error, the VMA will
> > + * be unmapped.
> > + *
> > + * Context: User context. May sleep. Caller holds mmap_lock.
> > + */
> > + int (*mapped)(unsigned long start, unsigned long end, pgoff_t pgoff,
> > + const struct file *file, void **vm_private_data);
> > /* Called any time before splitting to check if it's allowed */
> > int (*may_split)(struct vm_area_struct *area, unsigned long addr);
> > int (*mremap)(struct vm_area_struct *area);
> > @@ -1345,3 +1367,11 @@ static inline void vma_set_file(struct vm_area_struct *vma, struct file *file)
> > swap(vma->vm_file, file);
> > fput(file);
> > }
> > +
> > +static inline void unmap_vma_locked(struct vm_area_struct *vma)
> > +{
> > + const size_t len = vma_pages(vma) << PAGE_SHIFT;
> > +
> > + mmap_assert_locked(vma->vm_mm);
> > + do_munmap(vma->vm_mm, vma->vm_start, len, NULL);
> > +}
> > --
> > 2.53.0
> >
> >

Cheers, Lorenzo