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>
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 3e2eabcc2145..31803674aecc 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;

-- 
2.48.1.502.g6dc24dfdaf-goog

On Thu, 13 Feb 2025 11:04:05 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> 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>
> 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 3e2eabcc2145..31803674aecc 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`.

Are there operations that can't be performed when VMA is still being
setup? If so, using typestate might be more preferrable to Deref.

> +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

It is possible to make this API `&mut self` then?

> +        // 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.

Does `VmaRef` has to be `!Sync`? I wonder if we should explicitly mark
`VmaNew` as `!Sync` so this is more explicitly correct.

> +        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()) }
> +    }

On Tue, Feb 25, 2025 at 5:24 PM Gary Guo <gary@garyguo.net> wrote:
>
> On Thu, 13 Feb 2025 11:04:05 +0000
> Alice Ryhl <aliceryhl@google.com> wrote:
>
> > 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>
> > 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 3e2eabcc2145..31803674aecc 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`.
>
> Are there operations that can't be performed when VMA is still being
> setup? If so, using typestate might be more preferrable to Deref.

I don't think so.

> > +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
>
> It is possible to make this API `&mut self` then?

No because of pinning. And taking `self: Pin<&mut Self>` is obnoxious.

> > +        // 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.
>
> Does `VmaRef` has to be `!Sync`? I wonder if we should explicitly mark
> `VmaNew` as `!Sync` so this is more explicitly correct.

See above.

Alice