[PATCH v16 7/9] rust: miscdevice: add mmap support

Alice Ryhl
2025-04-08T09:24:20Z

Add the ability to write a file_operations->mmap hook in Rust when using
the miscdevice abstraction. The `vma` argument to the `mmap` hook uses
the `VmaNew` type from the previous commit; this type provides the
correct set of operations for a file_operations->mmap hook.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/kernel/miscdevice.rs | 45 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 45 insertions(+)

diff --git a/rust/kernel/miscdevice.rs b/rust/kernel/miscdevice.rs
index fa9ecc42602a477328a25b5d357db90b59dc72ae..9d9771247c3865761c4387467de3f1d1a82691b7 100644
--- a/rust/kernel/miscdevice.rs
+++ b/rust/kernel/miscdevice.rs
@@ -14,6 +14,7 @@
error::{to_result, Error, Result, VTABLE_DEFAULT_ERROR},
ffi::{c_int, c_long, c_uint, c_ulong},
fs::File,
+ mm::virt::VmaNew,
prelude::*,
seq_file::SeqFile,
str::CStr,
@@ -119,6 +120,22 @@ fn release(device: Self::Ptr, _file: &File) {
drop(device);
}

+ /// Handle for mmap.
+ ///
+ /// This function is invoked when a user space process invokes the `mmap` system call on
+ /// `file`. The function is a callback that is part of the VMA initializer. The kernel will do
+ /// initial setup of the VMA before calling this function. The function can then interact with
+ /// the VMA initialization by calling methods of `vma`. If the function does not return an
+ /// error, the kernel will complete initialization of the VMA according to the properties of
+ /// `vma`.
+ fn mmap(
+ _device: <Self::Ptr as ForeignOwnable>::Borrowed<'_>,
+ _file: &File,
+ _vma: &VmaNew,
+ ) -> Result {
+ build_error!(VTABLE_DEFAULT_ERROR)
+ }
+
/// Handler for ioctls.
///
/// The `cmd` argument is usually manipulated using the utilties in [`kernel::ioctl`].
@@ -223,6 +240,33 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
0
}

+ /// # Safety
+ ///
+ /// `file` must be a valid file that is associated with a `MiscDeviceRegistration<T>`.
+ /// `vma` must be a vma that is currently being mmap'ed with this file.
+ unsafe extern "C" fn mmap(
+ file: *mut bindings::file,
+ vma: *mut bindings::vm_area_struct,
+ ) -> c_int {
+ // SAFETY: The mmap call of a file can access the private data.
+ let private = unsafe { (*file).private_data };
+ // SAFETY: This is a Rust Miscdevice, so we call `into_foreign` in `open` and
+ // `from_foreign` in `release`, and `fops_mmap` is guaranteed to be called between those
+ // two operations.
+ let device = unsafe { <T::Ptr as ForeignOwnable>::borrow(private) };
+ // SAFETY: The caller provides a vma that is undergoing initial VMA setup.
+ let area = unsafe { VmaNew::from_raw(vma) };
+ // SAFETY:
+ // * The file is valid for the duration of this call.
+ // * There is no active fdget_pos region on the file on this thread.
+ let file = unsafe { File::from_raw_file(file) };
+
+ match T::mmap(device, file, area) {
+ Ok(()) => 0,
+ Err(err) => err.to_errno(),
+ }
+ }
+
/// # Safety
///
/// `file` must be a valid file that is associated with a `MiscDeviceRegistration<T>`.
@@ -291,6 +335,7 @@ impl<T: MiscDevice> MiscdeviceVTable<T> {
const VTABLE: bindings::file_operations = bindings::file_operations {
open: Some(Self::open),
release: Some(Self::release),
+ mmap: if T::HAS_MMAP { Some(Self::mmap) } else { None },
unlocked_ioctl: if T::HAS_IOCTL {
Some(Self::ioctl)
} else {

--
2.49.0.504.g3bcea36a83-goog