[v2,4/4] rust: add abstraction for `struct page`
Commit Message
Adds a new struct called `Page` that wraps a pointer to `struct page`.
This struct is assumed to hold ownership over the page, so that Rust
code can allocate and manage pages directly.
The page type has various methods for reading and writing into the page.
These methods will temporarily map the page to allow the operation. All
of these methods use a helper that takes an offset and length, performs
bounds checks, and returns a pointer to the given offset in the page.
This patch only adds support for pages of order zero, as that is all
Rust Binder needs. However, it is written to make it easy to add support
for higher-order pages in the future. To do that, you would add a const
generic parameter to `Page` that specifies the order. Most of the
methods do not need to be adjusted, as the logic for dealing with
mapping multiple pages at once can be isolated to just the
`with_pointer_into_page` method. Finally, the struct can be renamed to
`Pages<ORDER>`, and the type alias `Page = Pages<0>` can be introduced.
Rust Binder needs to manage pages directly as that is how transactions
are delivered: Each process has an mmap'd region for incoming
transactions. When an incoming transaction arrives, the Binder driver
will choose a region in the mmap, allocate and map the relevant pages
manually, and copy the incoming transaction directly into the page. This
architecture allows the driver to copy transactions directly from the
address space of one process to another, without an intermediate copy
to a kernel buffer.
This code is based on Wedson's page abstractions from the old rust
branch, but it has been modified by Alice by removing the incomplete
support for higher-order pages, and by introducing the `with_*` helpers
to consolidate the bounds checking logic into a single place.
Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
rust/bindings/bindings_helper.h | 1 +
rust/helpers.c | 20 ++++
rust/kernel/lib.rs | 1 +
rust/kernel/page.rs | 209 ++++++++++++++++++++++++++++++++++++++++
4 files changed, 231 insertions(+)
Comments
On 2/8/24 12:47, Alice Ryhl wrote:
> [...]
> + /// Maps the page and reads from it into the given buffer.
> + ///
> + /// This method will perform bounds checks on the page offset. If `offset ..
> + /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
> + ///
> + /// # Safety
> + ///
> + /// * Callers must ensure that `dst` is valid for writing `len` bytes.
> + /// * Callers must ensure that this call does not race with a write to the
> + /// same page that overlaps with this read.
This safety section says that a call mustn't race with a page that
overlaps this read, hmmmmm.
> + pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: usize) -> Result {
> + self.with_pointer_into_page(offset, len, move |src| {
> + // SAFETY: If `with_pointer_into_page` calls into this closure, then
> + // it has performed a bounds check and guarantees that `src` is
> + // valid for `len` bytes.
> + //
> + // There caller guarantees that there is no data race.
> + unsafe { ptr::copy(src, dst, len) };
If `src` and `dst` overlap then wouldn't that be a bad idea? If so then
how about mentioning that callers have to ensure that `dst` does not
overlap with the page that's being read and use
`core::ptr::copy_nonoverlapping` instead, otherwise the doc comment
could mention that `dst` can overlap.
> + Ok(())
> + })
> + }
> +
> + /// Maps the page and writes into it from the given buffer.
> + ///
> + /// This method will perform bounds checks on the page offset. If `offset ..
> + /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
> + ///
> + /// # Safety
> + ///
> + /// * Callers must ensure that `src` is valid for reading `len` bytes.
> + /// * Callers must ensure that this call does not race with a read or write
> + /// to the same page that overlaps with this write.
> + pub unsafe fn write_raw(&self, src: *const u8, offset: usize, len: usize) -> Result {
> + self.with_pointer_into_page(offset, len, move |dst| {
> + // SAFETY: If `with_pointer_into_page` calls into this closure, then
> + // it has performed a bounds check and guarantees that `dst` is
> + // valid for `len` bytes.
> + //
> + // There caller guarantees that there is no data race.
> + unsafe { ptr::copy(src, dst, len) };
Same as above
> + Ok(())
> + })
> + }
> [...]
On Sat, Feb 10, 2024 at 5:23 AM Martin Rodriguez Reboredo
<yakoyoku@gmail.com> wrote:
>
> On 2/8/24 12:47, Alice Ryhl wrote:
> > [...]
> > + /// Maps the page and reads from it into the given buffer.
> > + ///
> > + /// This method will perform bounds checks on the page offset. If `offset ..
> > + /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
> > + ///
> > + /// # Safety
> > + ///
> > + /// * Callers must ensure that `dst` is valid for writing `len` bytes.
> > + /// * Callers must ensure that this call does not race with a write to the
> > + /// same page that overlaps with this read.
>
> This safety section says that a call mustn't race with a page that
> overlaps this read, hmmmmm.
Is there a question here?
> > + pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: usize) -> Result {
> > + self.with_pointer_into_page(offset, len, move |src| {
> > + // SAFETY: If `with_pointer_into_page` calls into this closure, then
> > + // it has performed a bounds check and guarantees that `src` is
> > + // valid for `len` bytes.
> > + //
> > + // There caller guarantees that there is no data race.
> > + unsafe { ptr::copy(src, dst, len) };
>
> If `src` and `dst` overlap then wouldn't that be a bad idea? If so then
> how about mentioning that callers have to ensure that `dst` does not
> overlap with the page that's being read and use
> `core::ptr::copy_nonoverlapping` instead, otherwise the doc comment
> could mention that `dst` can overlap.
I'll use copy_nonoverlapping. Thanks for the suggestion.
Alice
On 2/12/24 06:36, Alice Ryhl wrote:
> On Sat, Feb 10, 2024 at 5:23 AM Martin Rodriguez Reboredo
> <yakoyoku@gmail.com> wrote:
>>
>> On 2/8/24 12:47, Alice Ryhl wrote:
>>> [...]
>>> + /// Maps the page and reads from it into the given buffer.
>>> + ///
>>> + /// This method will perform bounds checks on the page offset. If `offset ..
>>> + /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
>>> + ///
>>> + /// # Safety
>>> + ///
>>> + /// * Callers must ensure that `dst` is valid for writing `len` bytes.
>>> + /// * Callers must ensure that this call does not race with a write to the
>>> + /// same page that overlaps with this read.
>>
>> This safety section says that a call mustn't race with a page that
>> overlaps this read, hmmmmm.
>
> Is there a question here?
I've said more like introducing the next point, but if you want to use
`copy_nonoverlapping` then the safety section should mention that both
`src` and `dst` memory areas are forbidden to be overlapping.
> [...]
> I'll use copy_nonoverlapping. Thanks for the suggestion.
>
> Alice
You're welcome.
Hi Alice,
Alice Ryhl <aliceryhl@google.com> writes:
> Adds a new struct called `Page` that wraps a pointer to `struct page`.
> This struct is assumed to hold ownership over the page, so that Rust
> code can allocate and manage pages directly.
<cut>
> +/// A bitwise shift for the page size.
> +pub const PAGE_SHIFT: usize = bindings::PAGE_SHIFT as usize;
> +/// The number of bytes in a page.
> +pub const PAGE_SIZE: usize = 1 << PAGE_SHIFT;
For consistency, could we get page size from bindings as well? The folio
patches already do this [1].
BR Andreas
[1] https://lore.kernel.org/rust-for-linux/20231018122518.128049-10-wedsonaf@gmail.com/
On Thu, Feb 08, 2024 at 03:47:54PM +0000, Alice Ryhl wrote:
> +impl Page {
> + /// Allocates a new page.
> + pub fn new() -> Result<Self, AllocError> {
> + // SAFETY: These are the correct arguments to allocate a single page.
> + let page = unsafe {
> + bindings::alloc_pages(
> + bindings::GFP_KERNEL | bindings::__GFP_ZERO | bindings::__GFP_HIGHMEM,
I thought I raised this last time, but this is over-specialised for
Binder's purposes. Many places that want to allocate a page want
different GFP flags from this; they shouldn't even be the default flags.
So either what you're defining here is a BinderPage, or new() needs
to take GFP flags.
On Tue, Feb 27, 2024 at 4:37 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, Feb 08, 2024 at 03:47:54PM +0000, Alice Ryhl wrote:
> > +impl Page {
> > + /// Allocates a new page.
> > + pub fn new() -> Result<Self, AllocError> {
> > + // SAFETY: These are the correct arguments to allocate a single page.
> > + let page = unsafe {
> > + bindings::alloc_pages(
> > + bindings::GFP_KERNEL | bindings::__GFP_ZERO | bindings::__GFP_HIGHMEM,
>
> I thought I raised this last time, but this is over-specialised for
> Binder's purposes. Many places that want to allocate a page want
> different GFP flags from this; they shouldn't even be the default flags.
> So either what you're defining here is a BinderPage, or new() needs
> to take GFP flags.
Ah, yeah, sorry. I never got around to figuring that out.
I can change it to take GFP flags.
Thank you for taking another look!
Alice
@@ -22,3 +22,4 @@
const size_t RUST_CONST_HELPER_ARCH_SLAB_MINALIGN = ARCH_SLAB_MINALIGN;
const gfp_t RUST_CONST_HELPER_GFP_KERNEL = GFP_KERNEL;
const gfp_t RUST_CONST_HELPER___GFP_ZERO = __GFP_ZERO;
+const gfp_t RUST_CONST_HELPER___GFP_HIGHMEM = ___GFP_HIGHMEM;
@@ -25,6 +25,8 @@
#include <linux/build_bug.h>
#include <linux/err.h>
#include <linux/errname.h>
+#include <linux/gfp.h>
+#include <linux/highmem.h>
#include <linux/mutex.h>
#include <linux/refcount.h>
#include <linux/sched/signal.h>
@@ -93,6 +95,24 @@ int rust_helper_signal_pending(struct task_struct *t)
}
EXPORT_SYMBOL_GPL(rust_helper_signal_pending);
+struct page *rust_helper_alloc_pages(gfp_t gfp_mask, unsigned int order)
+{
+ return alloc_pages(gfp_mask, order);
+}
+EXPORT_SYMBOL_GPL(rust_helper_alloc_pages);
+
+void *rust_helper_kmap_local_page(struct page *page)
+{
+ return kmap_local_page(page);
+}
+EXPORT_SYMBOL_GPL(rust_helper_kmap_local_page);
+
+void rust_helper_kunmap_local(const void *addr)
+{
+ kunmap_local(addr);
+}
+EXPORT_SYMBOL_GPL(rust_helper_kunmap_local);
+
refcount_t rust_helper_REFCOUNT_INIT(int n)
{
return (refcount_t)REFCOUNT_INIT(n);
@@ -40,6 +40,7 @@
pub mod kunit;
#[cfg(CONFIG_NET)]
pub mod net;
+pub mod page;
pub mod prelude;
pub mod print;
mod static_assert;
new file mode 100644
@@ -0,0 +1,209 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Kernel page allocation and management.
+
+use crate::{bindings, error::code::*, error::Result, uaccess::UserSliceReader};
+use core::{
+ alloc::AllocError,
+ ptr::{self, NonNull},
+};
+
+/// A bitwise shift for the page size.
+pub const PAGE_SHIFT: usize = bindings::PAGE_SHIFT as usize;
+/// The number of bytes in a page.
+pub const PAGE_SIZE: usize = 1 << PAGE_SHIFT;
+
+/// A pointer to a page that owns the page allocation.
+///
+/// # Invariants
+///
+/// The pointer points at a page, and has ownership over the page.
+pub struct Page {
+ page: NonNull<bindings::page>,
+}
+
+// SAFETY: It is safe to transfer page allocations between threads.
+unsafe impl Send for Page {}
+
+// SAFETY: As long as the safety requirements for `&self` methods on this type
+// are followed, there is no problem with calling them in parallel.
+unsafe impl Sync for Page {}
+
+impl Page {
+ /// Allocates a new page.
+ pub fn new() -> Result<Self, AllocError> {
+ // SAFETY: These are the correct arguments to allocate a single page.
+ let page = unsafe {
+ bindings::alloc_pages(
+ bindings::GFP_KERNEL | bindings::__GFP_ZERO | bindings::__GFP_HIGHMEM,
+ 0,
+ )
+ };
+
+ let page = NonNull::new(page).ok_or(AllocError)?;
+ // INVARIANT: We checked that the allocation succeeded.
+ Ok(Self { page })
+ }
+
+ /// Returns a raw pointer to the page.
+ pub fn as_ptr(&self) -> *mut bindings::page {
+ self.page.as_ptr()
+ }
+
+ /// Runs a piece of code with this page mapped to an address.
+ ///
+ /// The page is unmapped when this call returns.
+ ///
+ /// It is up to the caller to use the provided raw pointer correctly.
+ pub fn with_page_mapped<T>(&self, f: impl FnOnce(*mut u8) -> T) -> T {
+ // SAFETY: `page` is valid due to the type invariants on `Page`.
+ let mapped_addr = unsafe { bindings::kmap_local_page(self.as_ptr()) };
+
+ let res = f(mapped_addr.cast());
+
+ // SAFETY: This unmaps the page mapped above.
+ //
+ // Since this API takes the user code as a closure, it can only be used
+ // in a manner where the pages are unmapped in reverse order. This is as
+ // required by `kunmap_local`.
+ //
+ // In other words, if this call to `kunmap_local` happens when a
+ // different page should be unmapped first, then there must necessarily
+ // be a call to `kmap_local_page` other than the call just above in
+ // `with_page_mapped` that made that possible. In this case, it is the
+ // unsafe block that wraps that other call that is incorrect.
+ unsafe { bindings::kunmap_local(mapped_addr) };
+
+ res
+ }
+
+ /// Runs a piece of code with a raw pointer to a slice of this page, with
+ /// bounds checking.
+ ///
+ /// If `f` is called, then it will be called with a pointer that points at
+ /// `off` bytes into the page, and the pointer will be valid for at least
+ /// `len` bytes. The pointer is only valid on this task, as this method uses
+ /// a local mapping.
+ ///
+ /// If `off` and `len` refers to a region outside of this page, then this
+ /// method returns `EINVAL` and does not call `f`.
+ ///
+ /// It is up to the caller to use the provided raw pointer correctly.
+ pub fn with_pointer_into_page<T>(
+ &self,
+ off: usize,
+ len: usize,
+ f: impl FnOnce(*mut u8) -> Result<T>,
+ ) -> Result<T> {
+ let bounds_ok = off <= PAGE_SIZE && len <= PAGE_SIZE && (off + len) <= PAGE_SIZE;
+
+ if bounds_ok {
+ self.with_page_mapped(move |page_addr| {
+ // SAFETY: The `off` integer is at most `PAGE_SIZE`, so this pointer offset will
+ // result in a pointer that is in bounds or one off the end of the page.
+ f(unsafe { page_addr.add(off) })
+ })
+ } else {
+ Err(EINVAL)
+ }
+ }
+
+ /// Maps the page and reads from it into the given buffer.
+ ///
+ /// This method will perform bounds checks on the page offset. If `offset ..
+ /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
+ ///
+ /// # Safety
+ ///
+ /// * Callers must ensure that `dst` is valid for writing `len` bytes.
+ /// * Callers must ensure that this call does not race with a write to the
+ /// same page that overlaps with this read.
+ pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: usize) -> Result {
+ self.with_pointer_into_page(offset, len, move |src| {
+ // SAFETY: If `with_pointer_into_page` calls into this closure, then
+ // it has performed a bounds check and guarantees that `src` is
+ // valid for `len` bytes.
+ //
+ // There caller guarantees that there is no data race.
+ unsafe { ptr::copy(src, dst, len) };
+ Ok(())
+ })
+ }
+
+ /// Maps the page and writes into it from the given buffer.
+ ///
+ /// This method will perform bounds checks on the page offset. If `offset ..
+ /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
+ ///
+ /// # Safety
+ ///
+ /// * Callers must ensure that `src` is valid for reading `len` bytes.
+ /// * Callers must ensure that this call does not race with a read or write
+ /// to the same page that overlaps with this write.
+ pub unsafe fn write_raw(&self, src: *const u8, offset: usize, len: usize) -> Result {
+ self.with_pointer_into_page(offset, len, move |dst| {
+ // SAFETY: If `with_pointer_into_page` calls into this closure, then
+ // it has performed a bounds check and guarantees that `dst` is
+ // valid for `len` bytes.
+ //
+ // There caller guarantees that there is no data race.
+ unsafe { ptr::copy(src, dst, len) };
+ Ok(())
+ })
+ }
+
+ /// Maps the page and zeroes the given slice.
+ ///
+ /// This method will perform bounds checks on the page offset. If `offset ..
+ /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
+ ///
+ /// # Safety
+ ///
+ /// Callers must ensure that this call does not race with a read or write to
+ /// the same page that overlaps with this write.
+ pub unsafe fn fill_zero(&self, offset: usize, len: usize) -> Result {
+ self.with_pointer_into_page(offset, len, move |dst| {
+ // SAFETY: If `with_pointer_into_page` calls into this closure, then
+ // it has performed a bounds check and guarantees that `dst` is
+ // valid for `len` bytes.
+ //
+ // There caller guarantees that there is no data race.
+ unsafe { ptr::write_bytes(dst, 0u8, len) };
+ Ok(())
+ })
+ }
+
+ /// Copies data from userspace into this page.
+ ///
+ /// This method will perform bounds checks on the page offset. If `offset ..
+ /// offset+len` goes outside ot the page, then this call returns `EINVAL`.
+ ///
+ /// # Safety
+ ///
+ /// Callers must ensure that this call does not race with a read or write to
+ /// the same page that overlaps with this write.
+ pub unsafe fn copy_from_user_slice(
+ &self,
+ reader: &mut UserSliceReader,
+ offset: usize,
+ len: usize,
+ ) -> Result {
+ self.with_pointer_into_page(offset, len, move |dst| {
+ // SAFETY: If `with_pointer_into_page` calls into this closure, then
+ // it has performed a bounds check and guarantees that `dst` is
+ // valid for `len` bytes.
+ //
+ // There caller guarantees that there is no data race when writing
+ // to `dst`.
+ unsafe { reader.read_raw(dst, len) }
+ })
+ }
+}
+
+impl Drop for Page {
+ fn drop(&mut self) {
+ // SAFETY: By the type invariants, we have ownership of the page and can
+ // free it.
+ unsafe { bindings::__free_pages(self.page.as_ptr(), 0) };
+ }
+}