[PATCH v16 6/9] mm: rust: add VmaNew for f_ops->mmap()

Alice Ryhl
2025-04-08T09:24:17Z

This type will be used when setting up a new vma in an f_ops->mmap()
hook. Using a separate type from VmaRef allows us to have a separate set
of operations that you are only able to use during the mmap() hook. For
example, the VM_MIXEDMAP flag must not be changed after the initial
setup that happens during the f_ops->mmap() hook.

To avoid setting invalid flag values, the methods for clearing
VM_MAYWRITE and similar involve a check of VM_WRITE, and return an error
if VM_WRITE is set. Trying to use `try_clear_maywrite` without checking
the return value results in a compilation error because the `Result`
type is marked #[must_use].

For now, there's only a method for VM_MIXEDMAP and not VM_PFNMAP. When
we add a VM_PFNMAP method, we will need some way to prevent you from
setting both VM_MIXEDMAP and VM_PFNMAP on the same vma.

Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Jann Horn <jannh@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/kernel/mm/virt.rs | 186 ++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 185 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
index 3e2eabcc21450497a02ffa1ed1f31f3e7e7e1b6b..31803674aecc57408df7960def17cfdc2cebcd6c 100644
--- a/rust/kernel/mm/virt.rs
+++ b/rust/kernel/mm/virt.rs
@@ -16,7 +16,7 @@

use crate::{
bindings,
- error::{to_result, Result},
+ error::{code::EINVAL, to_result, Result},
mm::MmWithUser,
page::Page,
types::Opaque,
@@ -198,6 +198,190 @@ pub fn vm_insert_page(&self, address: usize, page: &Page) -> Result {
}
}

+/// A configuration object for setting up a VMA in an `f_ops->mmap()` hook.
+///
+/// The `f_ops->mmap()` hook is called when a new VMA is being created, and the hook is able to
+/// configure the VMA in various ways to fit the driver that owns it. Using `VmaNew` indicates that
+/// you are allowed to perform operations on the VMA that can only be performed before the VMA is
+/// fully initialized.
+///
+/// # Invariants
+///
+/// For the duration of 'a, the referenced vma must be undergoing initialization in an
+/// `f_ops->mmap()` hook.
+pub struct VmaNew {
+ vma: VmaRef,
+}
+
+// Make all `VmaRef` methods available on `VmaNew`.
+impl Deref for VmaNew {
+ type Target = VmaRef;
+
+ #[inline]
+ fn deref(&self) -> &VmaRef {
+ &self.vma
+ }
+}
+
+impl VmaNew {
+ /// Access a virtual memory area given a raw pointer.
+ ///
+ /// # Safety
+ ///
+ /// Callers must ensure that `vma` is undergoing initial vma setup for the duration of 'a.
+ #[inline]
+ pub unsafe fn from_raw<'a>(vma: *mut bindings::vm_area_struct) -> &'a Self {
+ // SAFETY: The caller ensures that the invariants are satisfied for the duration of 'a.
+ unsafe { &*vma.cast() }
+ }
+
+ /// Internal method for updating the vma flags.
+ ///
+ /// # Safety
+ ///
+ /// This must not be used to set the flags to an invalid value.
+ #[inline]
+ unsafe fn update_flags(&self, set: vm_flags_t, unset: vm_flags_t) {
+ let mut flags = self.flags();
+ flags |= set;
+ flags &= !unset;
+
+ // SAFETY: This is not a data race: the vma is undergoing initial setup, so it's not yet
+ // shared. Additionally, `VmaNew` is `!Sync`, so it cannot be used to write in parallel.
+ // The caller promises that this does not set the flags to an invalid value.
+ unsafe { (*self.as_ptr()).__bindgen_anon_2.__vm_flags = flags };
+ }
+
+ /// Set the `VM_MIXEDMAP` flag on this vma.
+ ///
+ /// This enables the vma to contain both `struct page` and pure PFN pages. Returns a reference
+ /// that can be used to call `vm_insert_page` on the vma.
+ #[inline]
+ pub fn set_mixedmap(&self) -> &VmaMixedMap {
+ // SAFETY: We don't yet provide a way to set VM_PFNMAP, so this cannot put the flags in an
+ // invalid state.
+ unsafe { self.update_flags(flags::MIXEDMAP, 0) };
+
+ // SAFETY: We just set `VM_MIXEDMAP` on the vma.
+ unsafe { VmaMixedMap::from_raw(self.vma.as_ptr()) }
+ }
+
+ /// Set the `VM_IO` flag on this vma.
+ ///
+ /// This is used for memory mapped IO and similar. The flag tells other parts of the kernel to
+ /// avoid looking at the pages. For memory mapped IO this is useful as accesses to the pages
+ /// could have side effects.
+ #[inline]
+ pub fn set_io(&self) {
+ // SAFETY: Setting the VM_IO flag is always okay.
+ unsafe { self.update_flags(flags::IO, 0) };
+ }
+
+ /// Set the `VM_DONTEXPAND` flag on this vma.
+ ///
+ /// This prevents the vma from being expanded with `mremap()`.
+ #[inline]
+ pub fn set_dontexpand(&self) {
+ // SAFETY: Setting the VM_DONTEXPAND flag is always okay.
+ unsafe { self.update_flags(flags::DONTEXPAND, 0) };
+ }
+
+ /// Set the `VM_DONTCOPY` flag on this vma.
+ ///
+ /// This prevents the vma from being copied on fork. This option is only permanent if `VM_IO`
+ /// is set.
+ #[inline]
+ pub fn set_dontcopy(&self) {
+ // SAFETY: Setting the VM_DONTCOPY flag is always okay.
+ unsafe { self.update_flags(flags::DONTCOPY, 0) };
+ }
+
+ /// Set the `VM_DONTDUMP` flag on this vma.
+ ///
+ /// This prevents the vma from being included in core dumps. This option is only permanent if
+ /// `VM_IO` is set.
+ #[inline]
+ pub fn set_dontdump(&self) {
+ // SAFETY: Setting the VM_DONTDUMP flag is always okay.
+ unsafe { self.update_flags(flags::DONTDUMP, 0) };
+ }
+
+ /// Returns whether `VM_READ` is set.
+ ///
+ /// This flag indicates whether userspace is mapping this vma as readable.
+ #[inline]
+ pub fn readable(&self) -> bool {
+ (self.flags() & flags::READ) != 0
+ }
+
+ /// Try to clear the `VM_MAYREAD` flag, failing if `VM_READ` is set.
+ ///
+ /// This flag indicates whether userspace is allowed to make this vma readable with
+ /// `mprotect()`.
+ ///
+ /// Note that this operation is irreversible. Once `VM_MAYREAD` has been cleared, it can never
+ /// be set again.
+ #[inline]
+ pub fn try_clear_mayread(&self) -> Result {
+ if self.readable() {
+ return Err(EINVAL);
+ }
+ // SAFETY: Clearing `VM_MAYREAD` is okay when `VM_READ` is not set.
+ unsafe { self.update_flags(0, flags::MAYREAD) };
+ Ok(())
+ }
+
+ /// Returns whether `VM_WRITE` is set.
+ ///
+ /// This flag indicates whether userspace is mapping this vma as writable.
+ #[inline]
+ pub fn writable(&self) -> bool {
+ (self.flags() & flags::WRITE) != 0
+ }
+
+ /// Try to clear the `VM_MAYWRITE` flag, failing if `VM_WRITE` is set.
+ ///
+ /// This flag indicates whether userspace is allowed to make this vma writable with
+ /// `mprotect()`.
+ ///
+ /// Note that this operation is irreversible. Once `VM_MAYWRITE` has been cleared, it can never
+ /// be set again.
+ #[inline]
+ pub fn try_clear_maywrite(&self) -> Result {
+ if self.writable() {
+ return Err(EINVAL);
+ }
+ // SAFETY: Clearing `VM_MAYWRITE` is okay when `VM_WRITE` is not set.
+ unsafe { self.update_flags(0, flags::MAYWRITE) };
+ Ok(())
+ }
+
+ /// Returns whether `VM_EXEC` is set.
+ ///
+ /// This flag indicates whether userspace is mapping this vma as executable.
+ #[inline]
+ pub fn executable(&self) -> bool {
+ (self.flags() & flags::EXEC) != 0
+ }
+
+ /// Try to clear the `VM_MAYEXEC` flag, failing if `VM_EXEC` is set.
+ ///
+ /// This flag indicates whether userspace is allowed to make this vma executable with
+ /// `mprotect()`.
+ ///
+ /// Note that this operation is irreversible. Once `VM_MAYEXEC` has been cleared, it can never
+ /// be set again.
+ #[inline]
+ pub fn try_clear_mayexec(&self) -> Result {
+ if self.executable() {
+ return Err(EINVAL);
+ }
+ // SAFETY: Clearing `VM_MAYEXEC` is okay when `VM_EXEC` is not set.
+ unsafe { self.update_flags(0, flags::MAYEXEC) };
+ Ok(())
+ }
+}
+
/// The integer type used for vma flags.
#[doc(inline)]
pub use bindings::vm_flags_t;

--
2.49.0.504.g3bcea36a83-goog