Message-ID: <20241211-vma-v11-6-466640428fc3@google.com>
Date: Wed, 11 Dec 2024 10:37:10 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Miguel Ojeda <ojeda@...nel.org>, Matthew Wilcox <willy@...radead.org>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	John Hubbard <jhubbard@...dia.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Arnd Bergmann <arnd@...db.de>, Christian Brauner <brauner@...nel.org>, Jann Horn <jannh@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>
Cc: Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, rust-for-linux@...r.kernel.org, 
	Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH v11 6/8] mm: rust: add VmAreaNew for f_ops->mmap()

This type will be used when setting up a new vma in an f_ops->mmap()
hook. Using a separate type from VmAreaRef 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@...cle.com> (for mm bits)
Reviewed-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
 rust/kernel/mm/virt.rs | 181 ++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 180 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
index 3a23854e14f4..6d9ba56d4f95 100644
--- a/rust/kernel/mm/virt.rs
+++ b/rust/kernel/mm/virt.rs
@@ -6,7 +6,7 @@
 
 use crate::{
     bindings,
-    error::{to_result, Result},
+    error::{code::EINVAL, to_result, Result},
     mm::MmWithUser,
     page::Page,
     types::Opaque,
@@ -171,6 +171,185 @@ pub fn vm_insert_page(&self, address: usize, page: &Page) -> Result {
     }
 }
 
+/// A builder for setting up a vma in an `f_ops->mmap()` hook.
+///
+/// # Invariants
+///
+/// For the duration of 'a, the referenced vma must be undergoing initialization in an
+/// `f_ops->mmap()` hook.
+pub struct VmAreaNew {
+    vma: VmAreaRef,
+}
+
+// Make all `VmAreaRef` methods available on `VmAreaNew`.
+impl Deref for VmAreaNew {
+    type Target = VmAreaRef;
+
+    #[inline]
+    fn deref(&self) -> &VmAreaRef {
+        &self.vma
+    }
+}
+
+impl VmAreaNew {
+    /// 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: *const 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, `VmAreaNew` 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) -> &VmAreaMixedMap {
+        // 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 { VmAreaMixedMap::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 get_read(&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.get_read() {
+            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 get_write(&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.get_write() {
+            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 get_exec(&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.get_exec() {
+            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;

-- 
2.47.1.613.gc27f4b7a9f-goog

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