mm: rust: add vm_area_struct methods that require read access

This adds a type called VmaRef which is used when referencing a vma that
you have read access to.  Here, read access means that you hold either the
mmap read lock or the vma read lock (or stronger).

Additionally, a vma_lookup method is added to the mmap read guard, which
enables you to obtain a &VmaRef in safe Rust code.

This patch only provides a way to lock the mmap read lock, but a follow-up
patch also provides a way to just lock the vma read lock.

Link: https://lkml.kernel.org/r/20250408-vma-v16-2-d8b446e885d9@google.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
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>
Reviewed-by: Gary Guo <gary@garyguo.net>
Cc: Alex Gaynor <alex.gaynor@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Benno Lossin <benno.lossin@proton.me>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Trevor Gross <tmgross@umich.edu>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
index eda7a479cff7..f1689ccb3740 100644
--- a/rust/kernel/mm.rs
+++ b/rust/kernel/mm.rs
@@ -18,6 +18,8 @@ use crate::{
 };
 use core::{ops::Deref, ptr::NonNull};
 
+pub mod virt;
+
 /// A wrapper for the kernel's `struct mm_struct`.
 ///
 /// This represents the address space of a userspace process, so each process has one `Mm`
@@ -201,6 +203,27 @@ pub struct MmapReadGuard<'a> {
     _nts: NotThreadSafe,
 }
 
+impl<'a> MmapReadGuard<'a> {
+    /// Look up a vma at the given address.
+    #[inline]
+    pub fn vma_lookup(&self, vma_addr: usize) -> Option<&virt::VmaRef> {
+        // SAFETY: By the type invariants we hold the mmap read guard, so we can safely call this
+        // method. Any value is okay for `vma_addr`.
+        let vma = unsafe { bindings::vma_lookup(self.mm.as_raw(), vma_addr) };
+
+        if vma.is_null() {
+            None
+        } else {
+            // SAFETY: We just checked that a vma was found, so the pointer references a valid vma.
+            //
+            // Furthermore, the returned vma is still under the protection of the read lock guard
+            // and can be used while the mmap read lock is still held. That the vma is not used
+            // after the MmapReadGuard gets dropped is enforced by the borrow-checker.
+            unsafe { Some(virt::VmaRef::from_raw(vma)) }
+        }
+    }
+}
+
 impl Drop for MmapReadGuard<'_> {
     #[inline]
     fn drop(&mut self) {