[PATCH v8 0/8] File abstractions needed by Rust Binder

[PATCH v8 0/8] File abstractions needed by Rust Binder

[Alice Ryhl]

This patchset contains the file abstractions needed by the Rust
implementation of the Binder driver.

Please see the Rust Binder RFC for usage examples:
https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-0-08ba9197f637@google.com/

Users of "rust: types: add `NotThreadSafe`":
	[PATCH 5/9] rust: file: add `FileDescriptorReservation`

Users of "rust: task: add `Task::current_raw`":
	[PATCH 7/9] rust: file: add `Kuid` wrapper
	[PATCH 8/9] rust: file: add `DeferredFdCloser`

Users of "rust: file: add Rust abstraction for `struct file`":
	[PATCH RFC 02/20] rust_binder: add binderfs support to Rust binder
	[PATCH RFC 03/20] rust_binder: add threading support

Users of "rust: cred: add Rust abstraction for `struct cred`":
	[PATCH RFC 05/20] rust_binder: add nodes and context managers
	[PATCH RFC 06/20] rust_binder: add oneway transactions
	[PATCH RFC 11/20] rust_binder: send nodes in transaction
	[PATCH RFC 13/20] rust_binder: add BINDER_TYPE_FD support

Users of "rust: security: add abstraction for secctx":
	[PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: file: add `FileDescriptorReservation`":
	[PATCH RFC 13/20] rust_binder: add BINDER_TYPE_FD support
	[PATCH RFC 14/20] rust_binder: add BINDER_TYPE_FDA support

Users of "rust: file: add `Kuid` wrapper":
	[PATCH RFC 05/20] rust_binder: add nodes and context managers
	[PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: file: add abstraction for `poll_table`":
	[PATCH RFC 07/20] rust_binder: add epoll support

This patchset has some uses of read_volatile in place of READ_ONCE.
Please see the following rfc for context on this:
https://lore.kernel.org/all/20231025195339.1431894-1-boqun.feng@gmail.com/

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v8:
- Rename File::from_ptr to File::from_raw_file.
- Mention that NotThreadSafe also affects Sync.
- Fix copyright lines.
- Move rust/kernel/file.rs to rust/kernel/fs/file.rs to reduce conflicts
  with Wedson's vfs patches.
- Link to v7: https://lore.kernel.org/r/20240628-alice-file-v7-0-4d701f6335f3@google.com

Changes in v7:
- Replace file sharing modes with File / LocalFile.
- Link to v6: https://lore.kernel.org/r/20240517-alice-file-v6-0-b25bafdc9b97@google.com

Changes in v6:
- Introduce file sharing modes.
- Rewrite most documentation for `struct file` wrapper.
- Drop `DeferredFdCloser`. It will be sent later when it can be placed
  somewhere where only Rust Binder can use it.
- Rebase on top of rust-next: 97ab3e8eec0c ("rust: alloc: fix dangling pointer in VecExt<T>::reserve()")
- Link to v5: https://lore.kernel.org/r/20240209-alice-file-v5-0-a37886783025@google.com

Changes in v5:
- Pass a null pointer to task_tgid_nr_ns.
- Fix some typos and other formatting issues.
- Add Reviewed-by where appropriate.
- Link to v4: https://lore.kernel.org/r/20240202-alice-file-v4-0-fc9c2080663b@google.com

Changes in v4:
- Moved the two really simple patches to the beginning of the patchset.
- Update Send safety comments.
- Use srctree relative links.
- Mention that `Credential::euid` is immutable.
- Update some safety comments to mention the invariant on Self.
- Use new name for close_fd_get_file.
- Move safety comments on DeferredFdCloser around and be more explicit
  about how many refcounts we own.
- Reword safety comments related to _qproc.
- Add Reviewed-by where appropriate.
- Link to v3: https://lore.kernel.org/r/20240118-alice-file-v3-0-9694b6f9580c@google.com

Changes in v3:
- Completely rewrite comments about refcounting in the first patch.
  - And add a note to the documentation in fs/file.c.
- Discuss speculation gadgets in commit message for the Kuid wrapper.
- Introduce NotThreadSafe and Task::current_raw patches and use them in
  later patches.
- Improve safety comments in DeferredFdCloser.
- Some other minor changes.
- Link to v2: https://lore.kernel.org/r/20231206-alice-file-v2-0-af617c0d9d94@google.com

Changes in v2:
- Update various docs and safety comments.
- Rename method names to match the C name.
- Use ordinary read instead of READ_ONCE in File::cred.
- Changed null check in secctx.
- Add type alias for PhantomData in FileDescriptorReservation.
- Use Kuid::from_raw in Kuid::current_euid.
- Make DeferredFdCloser fallible if it is unable to schedule a task
  work. And also schedule the task work *before* closing the file.
- Moved PollCondVar to rust/kernel/sync.
- Updated PollCondVar to use wake_up_pollfree.
- Link to v1: https://lore.kernel.org/all/20231129-alice-file-v1-0-f81afe8c7261@google.com/

Link to RFC:
https://lore.kernel.org/all/20230720152820.3566078-1-aliceryhl@google.com/

---
Alice Ryhl (5):
      rust: types: add `NotThreadSafe`
      rust: task: add `Task::current_raw`
      rust: security: add abstraction for secctx
      rust: file: add `Kuid` wrapper
      rust: file: add abstraction for `poll_table`

Wedson Almeida Filho (3):
      rust: file: add Rust abstraction for `struct file`
      rust: cred: add Rust abstraction for `struct cred`
      rust: file: add `FileDescriptorReservation`

 fs/file.c                       |   7 +
 rust/bindings/bindings_helper.h |   6 +
 rust/helpers.c                  |  86 ++++++++
 rust/kernel/cred.rs             |  85 ++++++++
 rust/kernel/fs.rs               |   8 +
 rust/kernel/fs/file.rs          | 461 ++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs              |   3 +
 rust/kernel/security.rs         |  74 +++++++
 rust/kernel/sync.rs             |   1 +
 rust/kernel/sync/lock.rs        |  13 +-
 rust/kernel/sync/poll.rs        | 121 +++++++++++
 rust/kernel/task.rs             |  91 +++++++-
 rust/kernel/types.rs            |  29 +++
 13 files changed, 973 insertions(+), 12 deletions(-)
---
base-commit: b1263411112305acf2af728728591465becb45b0
change-id: 20231123-alice-file-525b98e8a724

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>

[PATCH v8 1/8] rust: types: add `NotThreadSafe`

[Alice Ryhl]

This introduces a new marker type for types that shouldn't be thread
safe. By adding a field of this type to a struct, it becomes non-Send
and non-Sync, which means that it cannot be accessed in any way from
threads other than the one it was created on.

This is useful for APIs that require globals such as `current` to remain
constant while the value exists.

We update two existing users in the Kernel to use this helper:

 * `Task::current()` - moving the return type of this value to a
   different thread would not be safe as you can no longer be guaranteed
   that the `current` pointer remains valid.
 * Lock guards. Mutexes and spinlocks should be unlocked on the same
   thread as where they were locked, so we enforce this using the Send
   trait.

There are also additional users in later patches of this patchset. See
[1] and [2] for the discussion that led to the introduction of this
patch.

Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [1]
Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [2]
Suggested-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/sync/lock.rs | 13 +++++++++----
 rust/kernel/task.rs      | 10 ++++++----
 rust/kernel/types.rs     | 21 +++++++++++++++++++++
 3 files changed, 36 insertions(+), 8 deletions(-)

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index f6c34ca4d819..d6e9bab114b8 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -6,8 +6,13 @@
 //! spinlocks, raw spinlocks) to be provided with minimal effort.
 
 use super::LockClassKey;
-use crate::{init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
-use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
+use crate::{
+    init::PinInit,
+    pin_init,
+    str::CStr,
+    types::{NotThreadSafe, Opaque, ScopeGuard},
+};
+use core::{cell::UnsafeCell, marker::PhantomPinned};
 use macros::pin_data;
 
 pub mod mutex;
@@ -139,7 +144,7 @@ pub fn lock(&self) -> Guard<'_, T, B> {
 pub struct Guard<'a, T: ?Sized, B: Backend> {
     pub(crate) lock: &'a Lock<T, B>,
     pub(crate) state: B::GuardState,
-    _not_send: PhantomData<*mut ()>,
+    _not_send: NotThreadSafe,
 }
 
 // SAFETY: `Guard` is sync when the data protected by the lock is also sync.
@@ -191,7 +196,7 @@ pub(crate) unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
         Self {
             lock,
             state,
-            _not_send: PhantomData,
+            _not_send: NotThreadSafe,
         }
     }
 }
diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 55dff7e088bf..278c623de0c6 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -4,10 +4,12 @@
 //!
 //! C header: [`include/linux/sched.h`](srctree/include/linux/sched.h).
 
-use crate::types::Opaque;
+use crate::{
+    bindings,
+    types::{NotThreadSafe, Opaque},
+};
 use core::{
     ffi::{c_int, c_long, c_uint},
-    marker::PhantomData,
     ops::Deref,
     ptr,
 };
@@ -106,7 +108,7 @@ impl Task {
     pub unsafe fn current() -> impl Deref<Target = Task> {
         struct TaskRef<'a> {
             task: &'a Task,
-            _not_send: PhantomData<*mut ()>,
+            _not_send: NotThreadSafe,
         }
 
         impl Deref for TaskRef<'_> {
@@ -125,7 +127,7 @@ fn deref(&self) -> &Self::Target {
             // that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
             // (where it could potentially outlive the caller).
             task: unsafe { &*ptr.cast() },
-            _not_send: PhantomData,
+            _not_send: NotThreadSafe,
         }
     }
 
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index bd189d646adb..bb115d730ebb 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -473,3 +473,24 @@ unsafe impl AsBytes for str {}
 // does not have any uninitialized portions either.
 unsafe impl<T: AsBytes> AsBytes for [T] {}
 unsafe impl<T: AsBytes, const N: usize> AsBytes for [T; N] {}
+
+/// Zero-sized type to mark types not [`Send`].
+///
+/// Add this type as a field to your struct if your type should not be sent to a different task.
+/// Since [`Send`] is an auto trait, adding a single field that is `!Send` will ensure that the
+/// whole type is `!Send`.
+///
+/// If a type is `!Send` it is impossible to give control over an instance of the type to another
+/// task. This is useful to include in types that store or reference task-local information. A file
+/// descriptor is an example of such task-local information.
+///
+/// This type also makes the type `!Sync`, which prevents immutable access to the value from
+/// several threads in parallel.
+pub type NotThreadSafe = PhantomData<*mut ()>;
+
+/// Used to construct instances of type [`NotThreadSafe`] similar to how `PhantomData` is
+/// constructed.
+///
+/// [`NotThreadSafe`]: type@NotThreadSafe
+#[allow(non_upper_case_globals)]
+pub const NotThreadSafe: NotThreadSafe = PhantomData;

-- 
2.45.2.1089.g2a221341d9-goog

Re: [PATCH v8 1/8] rust: types: add `NotThreadSafe`

[Peter Zijlstra]

On Thu, Jul 25, 2024 at 02:27:34PM +0000, Alice Ryhl wrote:
> This introduces a new marker type for types that shouldn't be thread
> safe. By adding a field of this type to a struct, it becomes non-Send
> and non-Sync, which means that it cannot be accessed in any way from
> threads other than the one it was created on.
> 
> This is useful for APIs that require globals such as `current` to remain
> constant while the value exists.
> 
> We update two existing users in the Kernel to use this helper:
> 
>  * `Task::current()` - moving the return type of this value to a
>    different thread would not be safe as you can no longer be guaranteed
>    that the `current` pointer remains valid.
>  * Lock guards. Mutexes and spinlocks should be unlocked on the same
>    thread as where they were locked, so we enforce this using the Send
>    trait.
> 
> There are also additional users in later patches of this patchset. See
> [1] and [2] for the discussion that led to the introduction of this
> patch.
> 
> Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [1]
> Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [2]
> Suggested-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Trevor Gross <tmgross@umich.edu>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/sync/lock.rs | 13 +++++++++----
>  rust/kernel/task.rs      | 10 ++++++----
>  rust/kernel/types.rs     | 21 +++++++++++++++++++++
>  3 files changed, 36 insertions(+), 8 deletions(-)
> 
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index f6c34ca4d819..d6e9bab114b8 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -6,8 +6,13 @@
>  //! spinlocks, raw spinlocks) to be provided with minimal effort.
>  
>  use super::LockClassKey;
> -use crate::{init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
> -use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
> +use crate::{
> +    init::PinInit,
> +    pin_init,
> +    str::CStr,
> +    types::{NotThreadSafe, Opaque, ScopeGuard},
> +};
> +use core::{cell::UnsafeCell, marker::PhantomPinned};
>  use macros::pin_data;
>  
>  pub mod mutex;
> @@ -139,7 +144,7 @@ pub fn lock(&self) -> Guard<'_, T, B> {
>  pub struct Guard<'a, T: ?Sized, B: Backend> {
>      pub(crate) lock: &'a Lock<T, B>,
>      pub(crate) state: B::GuardState,
> -    _not_send: PhantomData<*mut ()>,
> +    _not_send: NotThreadSafe,
>  }
>  
>  // SAFETY: `Guard` is sync when the data protected by the lock is also sync.
> @@ -191,7 +196,7 @@ pub(crate) unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
>          Self {
>              lock,
>              state,
> -            _not_send: PhantomData,
> +            _not_send: NotThreadSafe,
>          }
>      }
>  }
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 55dff7e088bf..278c623de0c6 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -4,10 +4,12 @@
>  //!
>  //! C header: [`include/linux/sched.h`](srctree/include/linux/sched.h).
>  
> -use crate::types::Opaque;
> +use crate::{
> +    bindings,
> +    types::{NotThreadSafe, Opaque},
> +};
>  use core::{
>      ffi::{c_int, c_long, c_uint},
> -    marker::PhantomData,
>      ops::Deref,
>      ptr,
>  };
> @@ -106,7 +108,7 @@ impl Task {
>      pub unsafe fn current() -> impl Deref<Target = Task> {
>          struct TaskRef<'a> {
>              task: &'a Task,
> -            _not_send: PhantomData<*mut ()>,
> +            _not_send: NotThreadSafe,
>          }
>  
>          impl Deref for TaskRef<'_> {
> @@ -125,7 +127,7 @@ fn deref(&self) -> &Self::Target {
>              // that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
>              // (where it could potentially outlive the caller).
>              task: unsafe { &*ptr.cast() },
> -            _not_send: PhantomData,
> +            _not_send: NotThreadSafe,
>          }
>      }

As per always for not being able to read rust; how does this extend to
get_task_struct()? Once you've taken a reference on current, you should
be free to pass it along to whomever.

Re: [PATCH v8 1/8] rust: types: add `NotThreadSafe`

[Alice Ryhl]

On Thu, Jul 25, 2024 at 4:37 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Jul 25, 2024 at 02:27:34PM +0000, Alice Ryhl wrote:
> > This introduces a new marker type for types that shouldn't be thread
> > safe. By adding a field of this type to a struct, it becomes non-Send
> > and non-Sync, which means that it cannot be accessed in any way from
> > threads other than the one it was created on.
> >
> > This is useful for APIs that require globals such as `current` to remain
> > constant while the value exists.
> >
> > We update two existing users in the Kernel to use this helper:
> >
> >  * `Task::current()` - moving the return type of this value to a
> >    different thread would not be safe as you can no longer be guaranteed
> >    that the `current` pointer remains valid.
> >  * Lock guards. Mutexes and spinlocks should be unlocked on the same
> >    thread as where they were locked, so we enforce this using the Send
> >    trait.
> >
> > There are also additional users in later patches of this patchset. See
> > [1] and [2] for the discussion that led to the introduction of this
> > patch.
> >
> > Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [1]
> > Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [2]
> > Suggested-by: Benno Lossin <benno.lossin@proton.me>
> > Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> > Reviewed-by: Trevor Gross <tmgross@umich.edu>
> > Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> > Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
> > Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> > ---
> >  rust/kernel/sync/lock.rs | 13 +++++++++----
> >  rust/kernel/task.rs      | 10 ++++++----
> >  rust/kernel/types.rs     | 21 +++++++++++++++++++++
> >  3 files changed, 36 insertions(+), 8 deletions(-)
> >
> > diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> > index f6c34ca4d819..d6e9bab114b8 100644
> > --- a/rust/kernel/sync/lock.rs
> > +++ b/rust/kernel/sync/lock.rs
> > @@ -6,8 +6,13 @@
> >  //! spinlocks, raw spinlocks) to be provided with minimal effort.
> >
> >  use super::LockClassKey;
> > -use crate::{init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
> > -use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
> > +use crate::{
> > +    init::PinInit,
> > +    pin_init,
> > +    str::CStr,
> > +    types::{NotThreadSafe, Opaque, ScopeGuard},
> > +};
> > +use core::{cell::UnsafeCell, marker::PhantomPinned};
> >  use macros::pin_data;
> >
> >  pub mod mutex;
> > @@ -139,7 +144,7 @@ pub fn lock(&self) -> Guard<'_, T, B> {
> >  pub struct Guard<'a, T: ?Sized, B: Backend> {
> >      pub(crate) lock: &'a Lock<T, B>,
> >      pub(crate) state: B::GuardState,
> > -    _not_send: PhantomData<*mut ()>,
> > +    _not_send: NotThreadSafe,
> >  }
> >
> >  // SAFETY: `Guard` is sync when the data protected by the lock is also sync.
> > @@ -191,7 +196,7 @@ pub(crate) unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
> >          Self {
> >              lock,
> >              state,
> > -            _not_send: PhantomData,
> > +            _not_send: NotThreadSafe,
> >          }
> >      }
> >  }
> > diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> > index 55dff7e088bf..278c623de0c6 100644
> > --- a/rust/kernel/task.rs
> > +++ b/rust/kernel/task.rs
> > @@ -4,10 +4,12 @@
> >  //!
> >  //! C header: [`include/linux/sched.h`](srctree/include/linux/sched.h).
> >
> > -use crate::types::Opaque;
> > +use crate::{
> > +    bindings,
> > +    types::{NotThreadSafe, Opaque},
> > +};
> >  use core::{
> >      ffi::{c_int, c_long, c_uint},
> > -    marker::PhantomData,
> >      ops::Deref,
> >      ptr,
> >  };
> > @@ -106,7 +108,7 @@ impl Task {
> >      pub unsafe fn current() -> impl Deref<Target = Task> {
> >          struct TaskRef<'a> {
> >              task: &'a Task,
> > -            _not_send: PhantomData<*mut ()>,
> > +            _not_send: NotThreadSafe,
> >          }
> >
> >          impl Deref for TaskRef<'_> {
> > @@ -125,7 +127,7 @@ fn deref(&self) -> &Self::Target {
> >              // that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
> >              // (where it could potentially outlive the caller).
> >              task: unsafe { &*ptr.cast() },
> > -            _not_send: PhantomData,
> > +            _not_send: NotThreadSafe,
> >          }
> >      }
>
> As per always for not being able to read rust; how does this extend to
> get_task_struct()? Once you've taken a reference on current, you should
> be free to pass it along to whomever.

Once you take a reference on current, it becomes thread-safe. This is
because taking a reference creates a value of type ARef<Task> rather
than TaskRef, and ARef<Task> is considered thread-safe.

Alice

Re: [PATCH v8 1/8] rust: types: add `NotThreadSafe`

[Peter Zijlstra]

On Thu, Jul 25, 2024 at 05:09:14PM +0200, Alice Ryhl wrote:

> > As per always for not being able to read rust; how does this extend to
> > get_task_struct()? Once you've taken a reference on current, you should
> > be free to pass it along to whomever.
> 
> Once you take a reference on current, it becomes thread-safe. This is
> because taking a reference creates a value of type ARef<Task> rather
> than TaskRef, and ARef<Task> is considered thread-safe.

Ignoring comments, there isn't a single mention of ARef there. Where
does it come from?

Re: [PATCH v8 1/8] rust: types: add `NotThreadSafe`

[Alice Ryhl]

On Thu, Jul 25, 2024 at 5:30 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Jul 25, 2024 at 05:09:14PM +0200, Alice Ryhl wrote:
>
> > > As per always for not being able to read rust; how does this extend to
> > > get_task_struct()? Once you've taken a reference on current, you should
> > > be free to pass it along to whomever.
> >
> > Once you take a reference on current, it becomes thread-safe. This is
> > because taking a reference creates a value of type ARef<Task> rather
> > than TaskRef, and ARef<Task> is considered thread-safe.
>
> Ignoring comments, there isn't a single mention of ARef there. Where
> does it come from?

That's because the conversion to ARef<Task> is a two-step process. The
TaskRef type just provides a conversion from &TaskRef to &Task via the
`impl Deref` block. There is a conversion from &Task to ARef<Task>
elsewhere, and this conversion is where the call to get_task_struct()
happens.

Alice

Re: [PATCH v8 1/8] rust: types: add `NotThreadSafe`

[Gary Guo]

On Thu, 25 Jul 2024 14:27:34 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> This introduces a new marker type for types that shouldn't be thread
> safe. By adding a field of this type to a struct, it becomes non-Send
> and non-Sync, which means that it cannot be accessed in any way from
> threads other than the one it was created on.
> 
> This is useful for APIs that require globals such as `current` to remain
> constant while the value exists.
> 
> We update two existing users in the Kernel to use this helper:
> 
>  * `Task::current()` - moving the return type of this value to a
>    different thread would not be safe as you can no longer be guaranteed
>    that the `current` pointer remains valid.
>  * Lock guards. Mutexes and spinlocks should be unlocked on the same
>    thread as where they were locked, so we enforce this using the Send
>    trait.
> 
> There are also additional users in later patches of this patchset. See
> [1] and [2] for the discussion that led to the introduction of this
> patch.
> 
> Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [1]
> Link: https://lore.kernel.org/all/nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me/ [2]
> Suggested-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Trevor Gross <tmgross@umich.edu>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/sync/lock.rs | 13 +++++++++----
>  rust/kernel/task.rs      | 10 ++++++----
>  rust/kernel/types.rs     | 21 +++++++++++++++++++++
>  3 files changed, 36 insertions(+), 8 deletions(-)

[PATCH v8 2/8] rust: task: add `Task::current_raw`

[Alice Ryhl]

Introduces a safe function for getting a raw pointer to the current
task.

When writing bindings that need to access the current task, it is often
more convenient to call a method that directly returns a raw pointer
than to use the existing `Task::current` method. However, the only way
to do that is `bindings::get_current()` which is unsafe since it calls
into C. By introducing `Task::current_raw()`, it becomes possible to
obtain a pointer to the current task without using unsafe.

Link: https://lore.kernel.org/all/CAH5fLgjT48X-zYtidv31mox3C4_Ogoo_2cBOCmX0Ang3tAgGHA@mail.gmail.com/
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/task.rs | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 278c623de0c6..367b4bbddd9f 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -97,6 +97,15 @@ unsafe impl Sync for Task {}
 type Pid = bindings::pid_t;
 
 impl Task {
+    /// Returns a raw pointer to the current task.
+    ///
+    /// It is up to the user to use the pointer correctly.
+    #[inline]
+    pub fn current_raw() -> *mut bindings::task_struct {
+        // SAFETY: Getting the current pointer is always safe.
+        unsafe { bindings::get_current() }
+    }
+
     /// Returns a task reference for the currently executing task/thread.
     ///
     /// The recommended way to get the current task/thread is to use the
@@ -119,14 +128,12 @@ fn deref(&self) -> &Self::Target {
             }
         }
 
-        // SAFETY: Just an FFI call with no additional safety requirements.
-        let ptr = unsafe { bindings::get_current() };
-
+        let current = Task::current_raw();
         TaskRef {
             // SAFETY: If the current thread is still running, the current task is valid. Given
             // that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
             // (where it could potentially outlive the caller).
-            task: unsafe { &*ptr.cast() },
+            task: unsafe { &*current.cast() },
             _not_send: NotThreadSafe,
         }
     }

-- 
2.45.2.1089.g2a221341d9-goog

Re: [PATCH v8 2/8] rust: task: add `Task::current_raw`

[Gary Guo]

On Thu, 25 Jul 2024 14:27:35 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> Introduces a safe function for getting a raw pointer to the current
> task.
> 
> When writing bindings that need to access the current task, it is often
> more convenient to call a method that directly returns a raw pointer
> than to use the existing `Task::current` method. However, the only way
> to do that is `bindings::get_current()` which is unsafe since it calls
> into C. By introducing `Task::current_raw()`, it becomes possible to
> obtain a pointer to the current task without using unsafe.
> 
> Link: https://lore.kernel.org/all/CAH5fLgjT48X-zYtidv31mox3C4_Ogoo_2cBOCmX0Ang3tAgGHA@mail.gmail.com/
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Reviewed-by: Trevor Gross <tmgross@umich.edu>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/task.rs | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)

[PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

[Alice Ryhl]

From: Wedson Almeida Filho <wedsonaf@gmail.com>

This abstraction makes it possible to manipulate the open files for a
process. The new `File` struct wraps the C `struct file`. When accessing
it using the smart pointer `ARef<File>`, the pointer will own a
reference count to the file. When accessing it as `&File`, then the
reference does not own a refcount, but the borrow checker will ensure
that the reference count does not hit zero while the `&File` is live.

Since this is intended to manipulate the open files of a process, we
introduce an `fget` constructor that corresponds to the C `fget`
method. In future patches, it will become possible to create a new fd in
a process and bind it to a `File`. Rust Binder will use these to send
fds from one process to another.

We also provide a method for accessing the file's flags. Rust Binder
will use this to access the flags of the Binder fd to check whether the
non-blocking flag is set, which affects what the Binder ioctl does.

This introduces a struct for the EBADF error type, rather than just
using the Error type directly. This has two advantages:
* `File::fget` returns a `Result<ARef<File>, BadFdError>`, which the
  compiler will represent as a single pointer, with null being an error.
  This is possible because the compiler understands that `BadFdError`
  has only one possible value, and it also understands that the
  `ARef<File>` smart pointer is guaranteed non-null.
* Additionally, we promise to users of the method that the method can
  only fail with EBADF, which means that they can rely on this promise
  without having to inspect its implementation.
That said, there are also two disadvantages:
* Defining additional error types involves boilerplate.
* The question mark operator will only utilize the `From` trait once,
  which prevents you from using the question mark operator on
  `BadFdError` in methods that return some third error type that the
  kernel `Error` is convertible into. (However, it works fine in methods
  that return `Error`.)

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 fs/file.c                       |   7 +
 rust/bindings/bindings_helper.h |   2 +
 rust/helpers.c                  |   7 +
 rust/kernel/fs.rs               |   8 +
 rust/kernel/fs/file.rs          | 375 ++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs              |   1 +
 rust/kernel/types.rs            |   8 +
 7 files changed, 408 insertions(+)

diff --git a/fs/file.c b/fs/file.c
index a3b72aa64f11..1d910f96ec2e 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1127,6 +1127,13 @@ EXPORT_SYMBOL(task_lookup_next_fdget_rcu);
  *
  * The fput_needed flag returned by fget_light should be passed to the
  * corresponding fput_light.
+ *
+ * (As an exception to rule 2, you can call filp_close between fget_light and
+ * fput_light provided that you capture a real refcount with get_file before
+ * the call to filp_close, and ensure that this real refcount is fput *after*
+ * the fput_light call.)
+ *
+ * See also the documentation in rust/kernel/file.rs.
  */
 static unsigned long __fget_light(unsigned int fd, fmode_t mask)
 {
diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index f2bafb10f181..e2d22f151ec9 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -9,6 +9,8 @@
 #include <kunit/test.h>
 #include <linux/errname.h>
 #include <linux/ethtool.h>
+#include <linux/file.h>
+#include <linux/fs.h>
 #include <linux/jiffies.h>
 #include <linux/mdio.h>
 #include <linux/phy.h>
diff --git a/rust/helpers.c b/rust/helpers.c
index 305f0577fae9..5ba1f6de0251 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -25,6 +25,7 @@
 #include <linux/build_bug.h>
 #include <linux/err.h>
 #include <linux/errname.h>
+#include <linux/fs.h>
 #include <linux/gfp.h>
 #include <linux/highmem.h>
 #include <linux/mutex.h>
@@ -199,6 +200,12 @@ rust_helper_krealloc(const void *objp, size_t new_size, gfp_t flags)
 }
 EXPORT_SYMBOL_GPL(rust_helper_krealloc);
 
+struct file *rust_helper_get_file(struct file *f)
+{
+	return get_file(f);
+}
+EXPORT_SYMBOL_GPL(rust_helper_get_file);
+
 /*
  * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
  * use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/fs.rs b/rust/kernel/fs.rs
new file mode 100644
index 000000000000..0121b38c59e6
--- /dev/null
+++ b/rust/kernel/fs.rs
@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Kernel file systems.
+//!
+//! C headers: [`include/linux/fs.h`](srctree/include/linux/fs.h)
+
+pub mod file;
+pub use self::file::{File, LocalFile};
diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
new file mode 100644
index 000000000000..f1a52814b2da
--- /dev/null
+++ b/rust/kernel/fs/file.rs
@@ -0,0 +1,375 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Files and file descriptors.
+//!
+//! C headers: [`include/linux/fs.h`](srctree/include/linux/fs.h) and
+//! [`include/linux/file.h`](srctree/include/linux/file.h)
+
+use crate::{
+    bindings,
+    error::{code::*, Error, Result},
+    types::{ARef, AlwaysRefCounted, Opaque},
+};
+use core::ptr;
+
+/// Flags associated with a [`File`].
+pub mod flags {
+    /// File is opened in append mode.
+    pub const O_APPEND: u32 = bindings::O_APPEND;
+
+    /// Signal-driven I/O is enabled.
+    pub const O_ASYNC: u32 = bindings::FASYNC;
+
+    /// Close-on-exec flag is set.
+    pub const O_CLOEXEC: u32 = bindings::O_CLOEXEC;
+
+    /// File was created if it didn't already exist.
+    pub const O_CREAT: u32 = bindings::O_CREAT;
+
+    /// Direct I/O is enabled for this file.
+    pub const O_DIRECT: u32 = bindings::O_DIRECT;
+
+    /// File must be a directory.
+    pub const O_DIRECTORY: u32 = bindings::O_DIRECTORY;
+
+    /// Like [`O_SYNC`] except metadata is not synced.
+    pub const O_DSYNC: u32 = bindings::O_DSYNC;
+
+    /// Ensure that this file is created with the `open(2)` call.
+    pub const O_EXCL: u32 = bindings::O_EXCL;
+
+    /// Large file size enabled (`off64_t` over `off_t`).
+    pub const O_LARGEFILE: u32 = bindings::O_LARGEFILE;
+
+    /// Do not update the file last access time.
+    pub const O_NOATIME: u32 = bindings::O_NOATIME;
+
+    /// File should not be used as process's controlling terminal.
+    pub const O_NOCTTY: u32 = bindings::O_NOCTTY;
+
+    /// If basename of path is a symbolic link, fail open.
+    pub const O_NOFOLLOW: u32 = bindings::O_NOFOLLOW;
+
+    /// File is using nonblocking I/O.
+    pub const O_NONBLOCK: u32 = bindings::O_NONBLOCK;
+
+    /// File is using nonblocking I/O.
+    ///
+    /// This is effectively the same flag as [`O_NONBLOCK`] on all architectures
+    /// except SPARC64.
+    pub const O_NDELAY: u32 = bindings::O_NDELAY;
+
+    /// Used to obtain a path file descriptor.
+    pub const O_PATH: u32 = bindings::O_PATH;
+
+    /// Write operations on this file will flush data and metadata.
+    pub const O_SYNC: u32 = bindings::O_SYNC;
+
+    /// This file is an unnamed temporary regular file.
+    pub const O_TMPFILE: u32 = bindings::O_TMPFILE;
+
+    /// File should be truncated to length 0.
+    pub const O_TRUNC: u32 = bindings::O_TRUNC;
+
+    /// Bitmask for access mode flags.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::fs::file;
+    /// # fn do_something() {}
+    /// # let flags = 0;
+    /// if (flags & file::flags::O_ACCMODE) == file::flags::O_RDONLY {
+    ///     do_something();
+    /// }
+    /// ```
+    pub const O_ACCMODE: u32 = bindings::O_ACCMODE;
+
+    /// File is read only.
+    pub const O_RDONLY: u32 = bindings::O_RDONLY;
+
+    /// File is write only.
+    pub const O_WRONLY: u32 = bindings::O_WRONLY;
+
+    /// File can be both read and written.
+    pub const O_RDWR: u32 = bindings::O_RDWR;
+}
+
+/// Wraps the kernel's `struct file`. Thread safe.
+///
+/// This represents an open file rather than a file on a filesystem. Processes generally reference
+/// open files using file descriptors. However, file descriptors are not the same as files. A file
+/// descriptor is just an integer that corresponds to a file, and a single file may be referenced
+/// by multiple file descriptors.
+///
+/// # Refcounting
+///
+/// Instances of this type are reference-counted. The reference count is incremented by the
+/// `fget`/`get_file` functions and decremented by `fput`. The Rust type `ARef<File>` represents a
+/// pointer that owns a reference count on the file.
+///
+/// Whenever a process opens a file descriptor (fd), it stores a pointer to the file in its fd
+/// table (`struct files_struct`). This pointer owns a reference count to the file, ensuring the
+/// file isn't prematurely deleted while the file descriptor is open. In Rust terminology, the
+/// pointers in `struct files_struct` are `ARef<File>` pointers.
+///
+/// ## Light refcounts
+///
+/// Whenever a process has an fd to a file, it may use something called a "light refcount" as a
+/// performance optimization. Light refcounts are acquired by calling `fdget` and released with
+/// `fdput`. The idea behind light refcounts is that if the fd is not closed between the calls to
+/// `fdget` and `fdput`, then the refcount cannot hit zero during that time, as the `struct
+/// files_struct` holds a reference until the fd is closed. This means that it's safe to access the
+/// file even if `fdget` does not increment the refcount.
+///
+/// The requirement that the fd is not closed during a light refcount applies globally across all
+/// threads - not just on the thread using the light refcount. For this reason, light refcounts are
+/// only used when the `struct files_struct` is not shared with other threads, since this ensures
+/// that other unrelated threads cannot suddenly start using the fd and close it. Therefore,
+/// calling `fdget` on a shared `struct files_struct` creates a normal refcount instead of a light
+/// refcount.
+///
+/// Light reference counts must be released with `fdput` before the system call returns to
+/// userspace. This means that if you wait until the current system call returns to userspace, then
+/// all light refcounts that existed at the time have gone away.
+///
+/// ### The file position
+///
+/// Each `struct file` has a position integer, which is protected by the `f_pos_lock` mutex.
+/// However, if the `struct file` is not shared, then the kernel may avoid taking the lock as a
+/// performance optimization.
+///
+/// The condition for avoiding the `f_pos_lock` mutex is different from the condition for using
+/// `fdget`. With `fdget`, you may avoid incrementing the refcount as long as the current fd table
+/// is not shared; it is okay if there are other fd tables that also reference the same `struct
+/// file`. However, `fdget_pos` can only avoid taking the `f_pos_lock` if the entire `struct file`
+/// is not shared, as different processes with an fd to the same `struct file` share the same
+/// position.
+///
+/// To represent files that are not thread safe due to this optimization, the [`LocalFile`] type is
+/// used.
+///
+/// ## Rust references
+///
+/// The reference type `&File` is similar to light refcounts:
+///
+/// * `&File` references don't own a reference count. They can only exist as long as the reference
+///   count stays positive, and can only be created when there is some mechanism in place to ensure
+///   this.
+///
+/// * The Rust borrow-checker normally ensures this by enforcing that the `ARef<File>` from which
+///   a `&File` is created outlives the `&File`.
+///
+/// * Using the unsafe [`File::from_raw_file`] means that it is up to the caller to ensure that the
+///   `&File` only exists while the reference count is positive.
+///
+/// * You can think of `fdget` as using an fd to look up an `ARef<File>` in the `struct
+///   files_struct` and create an `&File` from it. The "fd cannot be closed" rule is like the Rust
+///   rule "the `ARef<File>` must outlive the `&File`".
+///
+/// # Invariants
+///
+/// * All instances of this type are refcounted using the `f_count` field.
+/// * There must not be any active calls to `fdget_pos` on this file that did not take the
+///   `f_pos_lock` mutex.
+#[repr(transparent)]
+pub struct File {
+    inner: Opaque<bindings::file>,
+}
+
+/// Wraps the kernel's `struct file`. Not thread safe.
+///
+/// This type represents a file that is not known to be safe to transfer across thread boundaries.
+/// To obtain a thread-safe [`File`], use the [`assume_no_fdget_pos`] conversion.
+///
+/// See the documentation for [`File`] for more information.
+///
+/// # Invariants
+///
+/// * All instances of this type are refcounted using the `f_count` field.
+/// * If there is an active call to `fdget_pos` that did not take the `f_pos_lock` mutex, then it
+///   must be on the same thread as this `File`.
+///
+/// [`assume_no_fdget_pos`]: LocalFile::assume_no_fdget_pos
+pub struct LocalFile {
+    inner: Opaque<bindings::file>,
+}
+
+// SAFETY: This file is known to not have any active `fdget_pos` calls that did not take the
+// `f_pos_lock` mutex, so it is safe to transfer it between threads.
+unsafe impl Send for File {}
+
+// SAFETY: This file is known to not have any active `fdget_pos` calls that did not take the
+// `f_pos_lock` mutex, so it is safe to access its methods from several threads in parallel.
+unsafe impl Sync for File {}
+
+impl LocalFile {
+    /// Constructs a new `struct file` wrapper from a file descriptor.
+    ///
+    /// The file descriptor belongs to the current process, and there might be active local calls
+    /// to `fdget_pos` on the same file.
+    ///
+    /// To obtain an `ARef<File>`, use the [`assume_no_fdget_pos`] function to convert.
+    ///
+    /// [`assume_no_fdget_pos`]: LocalFile::assume_no_fdget_pos
+    #[inline]
+    pub fn fget(fd: u32) -> Result<ARef<LocalFile>, BadFdError> {
+        // SAFETY: FFI call, there are no requirements on `fd`.
+        let ptr = ptr::NonNull::new(unsafe { bindings::fget(fd) }).ok_or(BadFdError)?;
+
+        // SAFETY: `bindings::fget` created a refcount, and we pass ownership of it to the `ARef`.
+        //
+        // INVARIANT: This file is in the fd table on this thread, so either all `fdget_pos` calls
+        // are on this thread, or the file is shared, in which case `fdget_pos` calls took the
+        // `f_pos_lock` mutex.
+        Ok(unsafe { ARef::from_raw(ptr.cast()) })
+    }
+
+    /// Creates a reference to a [`LocalFile`] from a valid pointer.
+    ///
+    /// # Safety
+    ///
+    /// * The caller must ensure that `ptr` points at a valid file and that the file's refcount is
+    ///   positive for the duration of 'a.
+    /// * The caller must ensure that if there is an active call to `fdget_pos` that did not take
+    ///   the `f_pos_lock` mutex, then that call is on the current thread.
+    #[inline]
+    pub unsafe fn from_raw_file<'a>(ptr: *const bindings::file) -> &'a LocalFile {
+        // SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the
+        // duration of 'a. The cast is okay because `File` is `repr(transparent)`.
+        //
+        // INVARIANT: The caller guarantees that there are no problematic `fdget_pos` calls.
+        unsafe { &*ptr.cast() }
+    }
+
+    /// Assume that there are no active `fdget_pos` calls that prevent us from sharing this file.
+    ///
+    /// This makes it safe to transfer this file to other threads. No checks are performed, and
+    /// using it incorrectly may lead to a data race on the file position if the file is shared
+    /// with another thread.
+    ///
+    /// This method is intended to be used together with [`LocalFile::fget`] when the caller knows
+    /// statically that there are no `fdget_pos` calls on the current thread. For example, you
+    /// might use it when calling `fget` from an ioctl, since ioctls usually do not touch the file
+    /// position.
+    ///
+    /// # Safety
+    ///
+    /// There must not be any active `fdget_pos` calls on the current thread.
+    #[inline]
+    pub unsafe fn assume_no_fdget_pos(me: ARef<LocalFile>) -> ARef<File> {
+        // INVARIANT: There are no `fdget_pos` calls on the current thread, and by the type
+        // invariants, if there is a `fdget_pos` call on another thread, then it took the
+        // `f_pos_lock` mutex.
+        //
+        // SAFETY: `LocalFile` and `File` have the same layout.
+        unsafe { ARef::from_raw(ARef::into_raw(me).cast()) }
+    }
+
+    /// Returns a raw pointer to the inner C struct.
+    #[inline]
+    pub fn as_ptr(&self) -> *mut bindings::file {
+        self.inner.get()
+    }
+
+    /// Returns the flags associated with the file.
+    ///
+    /// The flags are a combination of the constants in [`flags`].
+    #[inline]
+    pub fn flags(&self) -> u32 {
+        // This `read_volatile` is intended to correspond to a READ_ONCE call.
+        //
+        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
+        //
+        // FIXME(read_once): Replace with `read_once` when available on the Rust side.
+        unsafe { core::ptr::addr_of!((*self.as_ptr()).f_flags).read_volatile() }
+    }
+}
+
+impl File {
+    /// Creates a reference to a [`File`] from a valid pointer.
+    ///
+    /// # Safety
+    ///
+    /// * The caller must ensure that `ptr` points at a valid file and that the file's refcount is
+    ///   positive for the duration of 'a.
+    /// * The caller must ensure that if there are active `fdget_pos` calls on this file, then they
+    ///   took the `f_pos_lock` mutex.
+    #[inline]
+    pub unsafe fn from_raw_file<'a>(ptr: *const bindings::file) -> &'a File {
+        // SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the
+        // duration of 'a. The cast is okay because `File` is `repr(transparent)`.
+        //
+        // INVARIANT: The caller guarantees that there are no problematic `fdget_pos` calls.
+        unsafe { &*ptr.cast() }
+    }
+}
+
+// Make LocalFile methods available on File.
+impl core::ops::Deref for File {
+    type Target = LocalFile;
+    #[inline]
+    fn deref(&self) -> &LocalFile {
+        // SAFETY: The caller provides a `&File`, and since it is a reference, it must point at a
+        // valid file for the desired duration.
+        //
+        // By the type invariants, there are no `fdget_pos` calls that did not take the
+        // `f_pos_lock` mutex.
+        unsafe { LocalFile::from_raw_file(self as *const File as *const bindings::file) }
+    }
+}
+
+// SAFETY: The type invariants guarantee that `LocalFile` is always ref-counted. This implementation
+// makes `ARef<File>` own a normal refcount.
+unsafe impl AlwaysRefCounted for LocalFile {
+    #[inline]
+    fn inc_ref(&self) {
+        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
+        unsafe { bindings::get_file(self.as_ptr()) };
+    }
+
+    #[inline]
+    unsafe fn dec_ref(obj: ptr::NonNull<LocalFile>) {
+        // SAFETY: To call this method, the caller passes us ownership of a normal refcount, so we
+        // may drop it. The cast is okay since `File` has the same representation as `struct file`.
+        unsafe { bindings::fput(obj.cast().as_ptr()) }
+    }
+}
+
+// SAFETY: The type invariants guarantee that `File` is always ref-counted. This implementation
+// makes `ARef<File>` own a normal refcount.
+unsafe impl AlwaysRefCounted for File {
+    #[inline]
+    fn inc_ref(&self) {
+        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
+        unsafe { bindings::get_file(self.as_ptr()) };
+    }
+
+    #[inline]
+    unsafe fn dec_ref(obj: ptr::NonNull<File>) {
+        // SAFETY: To call this method, the caller passes us ownership of a normal refcount, so we
+        // may drop it. The cast is okay since `File` has the same representation as `struct file`.
+        unsafe { bindings::fput(obj.cast().as_ptr()) }
+    }
+}
+
+/// Represents the `EBADF` error code.
+///
+/// Used for methods that can only fail with `EBADF`.
+#[derive(Copy, Clone, Eq, PartialEq)]
+pub struct BadFdError;
+
+impl From<BadFdError> for Error {
+    #[inline]
+    fn from(_: BadFdError) -> Error {
+        EBADF
+    }
+}
+
+impl core::fmt::Debug for BadFdError {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.pad("EBADF")
+    }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 5d310e79485f..c364893e13a2 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -29,6 +29,7 @@
 pub mod alloc;
 mod build_assert;
 pub mod error;
+pub mod fs;
 pub mod init;
 pub mod ioctl;
 #[cfg(CONFIG_KUNIT)]
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index bb115d730ebb..cb3e178b2000 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -366,6 +366,14 @@ pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
             _p: PhantomData,
         }
     }
+
+    /// Convert this [`ARef`] into a raw pointer.
+    ///
+    /// The caller retains ownership of the refcount that this `ARef` used to own.
+    pub fn into_raw(me: Self) -> NonNull<T> {
+        let me = core::mem::ManuallyDrop::new(me);
+        me.ptr
+    }
 }
 
 impl<T: AlwaysRefCounted> Clone for ARef<T> {

-- 
2.45.2.1089.g2a221341d9-goog

Re: [PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

[Benno Lossin]

On 25.07.24 16:27, Alice Ryhl wrote:
> +/// Wraps the kernel's `struct file`. Not thread safe.
> +///
> +/// This type represents a file that is not known to be safe to transfer across thread boundaries.
> +/// To obtain a thread-safe [`File`], use the [`assume_no_fdget_pos`] conversion.
> +///
> +/// See the documentation for [`File`] for more information.
> +///
> +/// # Invariants
> +///
> +/// * All instances of this type are refcounted using the `f_count` field.
> +/// * If there is an active call to `fdget_pos` that did not take the `f_pos_lock` mutex, then it
> +///   must be on the same thread as this `File`.

Do you mean `LocalFile`?

> +///
> +/// [`assume_no_fdget_pos`]: LocalFile::assume_no_fdget_pos
> +pub struct LocalFile {
> +    inner: Opaque<bindings::file>,
> +}

[...]

> +    /// Returns the flags associated with the file.
> +    ///
> +    /// The flags are a combination of the constants in [`flags`].
> +    #[inline]
> +    pub fn flags(&self) -> u32 {
> +        // This `read_volatile` is intended to correspond to a READ_ONCE call.
> +        //
> +        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> +        //
> +        // FIXME(read_once): Replace with `read_once` when available on the Rust side.

Do you know the status of this?

> +        unsafe { core::ptr::addr_of!((*self.as_ptr()).f_flags).read_volatile() }
> +    }
> +}
> +
> +impl File {
> +    /// Creates a reference to a [`File`] from a valid pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// * The caller must ensure that `ptr` points at a valid file and that the file's refcount is
> +    ///   positive for the duration of 'a.
> +    /// * The caller must ensure that if there are active `fdget_pos` calls on this file, then they
> +    ///   took the `f_pos_lock` mutex.
> +    #[inline]
> +    pub unsafe fn from_raw_file<'a>(ptr: *const bindings::file) -> &'a File {
> +        // SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the
> +        // duration of 'a. The cast is okay because `File` is `repr(transparent)`.
> +        //
> +        // INVARIANT: The caller guarantees that there are no problematic `fdget_pos` calls.
> +        unsafe { &*ptr.cast() }
> +    }
> +}
> +
> +// Make LocalFile methods available on File.
> +impl core::ops::Deref for File {
> +    type Target = LocalFile;
> +    #[inline]
> +    fn deref(&self) -> &LocalFile {
> +        // SAFETY: The caller provides a `&File`, and since it is a reference, it must point at a
> +        // valid file for the desired duration.
> +        //
> +        // By the type invariants, there are no `fdget_pos` calls that did not take the
> +        // `f_pos_lock` mutex.
> +        unsafe { LocalFile::from_raw_file(self as *const File as *const bindings::file) }
> +    }
> +}
> +
> +// SAFETY: The type invariants guarantee that `LocalFile` is always ref-counted. This implementation
> +// makes `ARef<File>` own a normal refcount.
> +unsafe impl AlwaysRefCounted for LocalFile {
> +    #[inline]
> +    fn inc_ref(&self) {
> +        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> +        unsafe { bindings::get_file(self.as_ptr()) };
> +    }
> +
> +    #[inline]
> +    unsafe fn dec_ref(obj: ptr::NonNull<LocalFile>) {
> +        // SAFETY: To call this method, the caller passes us ownership of a normal refcount, so we
> +        // may drop it. The cast is okay since `File` has the same representation as `struct file`.
> +        unsafe { bindings::fput(obj.cast().as_ptr()) }
> +    }
> +}

Can you move these `AlwaysRefCounted` impls towards the struct
definitions?

With those two comments fixed:

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

---
Cheers,
Benno

Re: [PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

[Alice Ryhl]

On Tue, Aug 6, 2024 at 10:44 AM Benno Lossin <benno.lossin@proton.me> wrote:
>
> On 25.07.24 16:27, Alice Ryhl wrote:
> > +/// Wraps the kernel's `struct file`. Not thread safe.
> > +///
> > +/// This type represents a file that is not known to be safe to transfer across thread boundaries.
> > +/// To obtain a thread-safe [`File`], use the [`assume_no_fdget_pos`] conversion.
> > +///
> > +/// See the documentation for [`File`] for more information.
> > +///
> > +/// # Invariants
> > +///
> > +/// * All instances of this type are refcounted using the `f_count` field.
> > +/// * If there is an active call to `fdget_pos` that did not take the `f_pos_lock` mutex, then it
> > +///   must be on the same thread as this `File`.
>
> Do you mean `LocalFile`?

I guess. Perhaps I should just say "file" as a general concept?

> > +///
> > +/// [`assume_no_fdget_pos`]: LocalFile::assume_no_fdget_pos
> > +pub struct LocalFile {
> > +    inner: Opaque<bindings::file>,
> > +}
>
> [...]
>
> > +    /// Returns the flags associated with the file.
> > +    ///
> > +    /// The flags are a combination of the constants in [`flags`].
> > +    #[inline]
> > +    pub fn flags(&self) -> u32 {
> > +        // This `read_volatile` is intended to correspond to a READ_ONCE call.
> > +        //
> > +        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> > +        //
> > +        // FIXME(read_once): Replace with `read_once` when available on the Rust side.
>
> Do you know the status of this?

It's still unavailable.

> > +        unsafe { core::ptr::addr_of!((*self.as_ptr()).f_flags).read_volatile() }
> > +    }
> > +}
> > +
> > +impl File {
> > +    /// Creates a reference to a [`File`] from a valid pointer.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// * The caller must ensure that `ptr` points at a valid file and that the file's refcount is
> > +    ///   positive for the duration of 'a.
> > +    /// * The caller must ensure that if there are active `fdget_pos` calls on this file, then they
> > +    ///   took the `f_pos_lock` mutex.
> > +    #[inline]
> > +    pub unsafe fn from_raw_file<'a>(ptr: *const bindings::file) -> &'a File {
> > +        // SAFETY: The caller guarantees that the pointer is not dangling and stays valid for the
> > +        // duration of 'a. The cast is okay because `File` is `repr(transparent)`.
> > +        //
> > +        // INVARIANT: The caller guarantees that there are no problematic `fdget_pos` calls.
> > +        unsafe { &*ptr.cast() }
> > +    }
> > +}
> > +
> > +// Make LocalFile methods available on File.
> > +impl core::ops::Deref for File {
> > +    type Target = LocalFile;
> > +    #[inline]
> > +    fn deref(&self) -> &LocalFile {
> > +        // SAFETY: The caller provides a `&File`, and since it is a reference, it must point at a
> > +        // valid file for the desired duration.
> > +        //
> > +        // By the type invariants, there are no `fdget_pos` calls that did not take the
> > +        // `f_pos_lock` mutex.
> > +        unsafe { LocalFile::from_raw_file(self as *const File as *const bindings::file) }
> > +    }
> > +}
> > +
> > +// SAFETY: The type invariants guarantee that `LocalFile` is always ref-counted. This implementation
> > +// makes `ARef<File>` own a normal refcount.
> > +unsafe impl AlwaysRefCounted for LocalFile {
> > +    #[inline]
> > +    fn inc_ref(&self) {
> > +        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> > +        unsafe { bindings::get_file(self.as_ptr()) };
> > +    }
> > +
> > +    #[inline]
> > +    unsafe fn dec_ref(obj: ptr::NonNull<LocalFile>) {
> > +        // SAFETY: To call this method, the caller passes us ownership of a normal refcount, so we
> > +        // may drop it. The cast is okay since `File` has the same representation as `struct file`.
> > +        unsafe { bindings::fput(obj.cast().as_ptr()) }
> > +    }
> > +}
>
> Can you move these `AlwaysRefCounted` impls towards the struct
> definitions?
>
> With those two comments fixed:
>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>

Thanks!

Alice

Re: [PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

[Boqun Feng]

On Tue, Aug 06, 2024 at 10:48:11AM +0200, Alice Ryhl wrote:
[...]
> > > +    /// Returns the flags associated with the file.
> > > +    ///
> > > +    /// The flags are a combination of the constants in [`flags`].
> > > +    #[inline]
> > > +    pub fn flags(&self) -> u32 {
> > > +        // This `read_volatile` is intended to correspond to a READ_ONCE call.
> > > +        //
> > > +        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> > > +        //
> > > +        // FIXME(read_once): Replace with `read_once` when available on the Rust side.
> >
> > Do you know the status of this?
> 
> It's still unavailable.
> 

I think with our own Atomic API, we can just use atomic_read() here:
yes, I know that to make this is not a UB, we need the C side to also do
atomic write on this `f_flags`, however, my reading of C code seems to
suggest that FS relies on writes to this field is atomic, therefore
unless someone is willing to convert all writes to `f_flags` in C into
a WRITE_ONCE(), nothing more we can do on Rust side. So using
atomic_read() is the correct thing to begin with.

Regards,
Boqun

> > > +        unsafe { core::ptr::addr_of!((*self.as_ptr()).f_flags).read_volatile() }
> > > +    }
> > > +}
[...]

Re: [PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

[Alice Ryhl]

On Tue, Aug 6, 2024 at 9:30 PM Boqun Feng <boqun.feng@gmail.com> wrote:
>
> On Tue, Aug 06, 2024 at 10:48:11AM +0200, Alice Ryhl wrote:
> [...]
> > > > +    /// Returns the flags associated with the file.
> > > > +    ///
> > > > +    /// The flags are a combination of the constants in [`flags`].
> > > > +    #[inline]
> > > > +    pub fn flags(&self) -> u32 {
> > > > +        // This `read_volatile` is intended to correspond to a READ_ONCE call.
> > > > +        //
> > > > +        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> > > > +        //
> > > > +        // FIXME(read_once): Replace with `read_once` when available on the Rust side.
> > >
> > > Do you know the status of this?
> >
> > It's still unavailable.
> >
>
> I think with our own Atomic API, we can just use atomic_read() here:
> yes, I know that to make this is not a UB, we need the C side to also do
> atomic write on this `f_flags`, however, my reading of C code seems to
> suggest that FS relies on writes to this field is atomic, therefore
> unless someone is willing to convert all writes to `f_flags` in C into
> a WRITE_ONCE(), nothing more we can do on Rust side. So using
> atomic_read() is the correct thing to begin with.

Huh? The C side uses atomic reads for this?

Alice

Re: [PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

[Boqun Feng]

On Wed, Aug 07, 2024 at 10:50:32AM +0200, Alice Ryhl wrote:
> On Tue, Aug 6, 2024 at 9:30 PM Boqun Feng <boqun.feng@gmail.com> wrote:
> >
> > On Tue, Aug 06, 2024 at 10:48:11AM +0200, Alice Ryhl wrote:
> > [...]
> > > > > +    /// Returns the flags associated with the file.
> > > > > +    ///
> > > > > +    /// The flags are a combination of the constants in [`flags`].
> > > > > +    #[inline]
> > > > > +    pub fn flags(&self) -> u32 {
> > > > > +        // This `read_volatile` is intended to correspond to a READ_ONCE call.
> > > > > +        //
> > > > > +        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> > > > > +        //
> > > > > +        // FIXME(read_once): Replace with `read_once` when available on the Rust side.
> > > >
> > > > Do you know the status of this?
> > >
> > > It's still unavailable.
> > >
> >
> > I think with our own Atomic API, we can just use atomic_read() here:
> > yes, I know that to make this is not a UB, we need the C side to also do
> > atomic write on this `f_flags`, however, my reading of C code seems to
> > suggest that FS relies on writes to this field is atomic, therefore
> > unless someone is willing to convert all writes to `f_flags` in C into
> > a WRITE_ONCE(), nothing more we can do on Rust side. So using
> > atomic_read() is the correct thing to begin with.
> 
> Huh? The C side uses atomic reads for this?
> 

Well, READ_ONCE(->f_flags) is atomic, so I thought you want to use
atomic here. However, after a quick look of `->f_flags` accesses, I find
out they should be protected by `->f_lock` (a few cases rely on
data race accesses, see p4_fd_open()), so I think what you should really
do here is the similar: make sure Rust code only accesses `->f_flags`
if `->f_lock` is held. Unless that's not the case for binder?

Regards,
Boqun

> Alice

Re: [PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

[Alice Ryhl]

On 8/7/24 4:46 PM, Boqun Feng wrote:
> On Wed, Aug 07, 2024 at 10:50:32AM +0200, Alice Ryhl wrote:
>> On Tue, Aug 6, 2024 at 9:30 PM Boqun Feng <boqun.feng@gmail.com> wrote:
>>>
>>> On Tue, Aug 06, 2024 at 10:48:11AM +0200, Alice Ryhl wrote:
>>> [...]
>>>>>> +    /// Returns the flags associated with the file.
>>>>>> +    ///
>>>>>> +    /// The flags are a combination of the constants in [`flags`].
>>>>>> +    #[inline]
>>>>>> +    pub fn flags(&self) -> u32 {
>>>>>> +        // This `read_volatile` is intended to correspond to a READ_ONCE call.
>>>>>> +        //
>>>>>> +        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
>>>>>> +        //
>>>>>> +        // FIXME(read_once): Replace with `read_once` when available on the Rust side.
>>>>>
>>>>> Do you know the status of this?
>>>>
>>>> It's still unavailable.
>>>>
>>>
>>> I think with our own Atomic API, we can just use atomic_read() here:
>>> yes, I know that to make this is not a UB, we need the C side to also do
>>> atomic write on this `f_flags`, however, my reading of C code seems to
>>> suggest that FS relies on writes to this field is atomic, therefore
>>> unless someone is willing to convert all writes to `f_flags` in C into
>>> a WRITE_ONCE(), nothing more we can do on Rust side. So using
>>> atomic_read() is the correct thing to begin with.
>>
>> Huh? The C side uses atomic reads for this?
>>
> 
> Well, READ_ONCE(->f_flags) is atomic, so I thought you want to use
> atomic here. However, after a quick look of `->f_flags` accesses, I find
> out they should be protected by `->f_lock` (a few cases rely on
> data race accesses, see p4_fd_open()), so I think what you should really
> do here is the similar: make sure Rust code only accesses `->f_flags`
> if `->f_lock` is held. Unless that's not the case for binder?


Binder just has an `if (filp->f_flags & O_NONBLOCK)` block somewhere in 
the ioctl, where filp is the `struct file *` passed to the ioctl. Binder 
doesn't take the lock.

Alice

Re: [PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

[Boqun Feng]

On Wed, Aug 07, 2024 at 11:59:47PM +0200, Alice Ryhl wrote:
> On 8/7/24 4:46 PM, Boqun Feng wrote:
> > On Wed, Aug 07, 2024 at 10:50:32AM +0200, Alice Ryhl wrote:
> > > On Tue, Aug 6, 2024 at 9:30 PM Boqun Feng <boqun.feng@gmail.com> wrote:
> > > > 
> > > > On Tue, Aug 06, 2024 at 10:48:11AM +0200, Alice Ryhl wrote:
> > > > [...]
> > > > > > > +    /// Returns the flags associated with the file.
> > > > > > > +    ///
> > > > > > > +    /// The flags are a combination of the constants in [`flags`].
> > > > > > > +    #[inline]
> > > > > > > +    pub fn flags(&self) -> u32 {
> > > > > > > +        // This `read_volatile` is intended to correspond to a READ_ONCE call.
> > > > > > > +        //
> > > > > > > +        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> > > > > > > +        //
> > > > > > > +        // FIXME(read_once): Replace with `read_once` when available on the Rust side.
> > > > > > 
> > > > > > Do you know the status of this?
> > > > > 
> > > > > It's still unavailable.
> > > > > 
> > > > 
> > > > I think with our own Atomic API, we can just use atomic_read() here:
> > > > yes, I know that to make this is not a UB, we need the C side to also do
> > > > atomic write on this `f_flags`, however, my reading of C code seems to
> > > > suggest that FS relies on writes to this field is atomic, therefore
> > > > unless someone is willing to convert all writes to `f_flags` in C into
> > > > a WRITE_ONCE(), nothing more we can do on Rust side. So using
> > > > atomic_read() is the correct thing to begin with.
> > > 
> > > Huh? The C side uses atomic reads for this?
> > > 
> > 
> > Well, READ_ONCE(->f_flags) is atomic, so I thought you want to use
> > atomic here. However, after a quick look of `->f_flags` accesses, I find
> > out they should be protected by `->f_lock` (a few cases rely on
> > data race accesses, see p4_fd_open()), so I think what you should really
> > do here is the similar: make sure Rust code only accesses `->f_flags`
> > if `->f_lock` is held. Unless that's not the case for binder?
> 
> 
> Binder just has an `if (filp->f_flags & O_NONBLOCK)` block somewhere in the
> ioctl, where filp is the `struct file *` passed to the ioctl. Binder doesn't
> take the lock.
> 

Yep, that's my point, I think binder C driver relies on the behaviors of
data race today (or probably all `->f_flags`s accessed by binder don't
have any concurrent write to them). Either way, what you do here is
better than C code if there was a data race. I was simply suggesting
instead of `read_once`, we could just do a `atomic_read` on `->f_flags`
once we support *unsafe` usage of doing atomic accesses on normal data
fields (of course, such a usage will be limited).

In other words, nothing needs to be changed here right now.

Regards,
Boqun

> Alice

[PATCH v8 4/8] rust: cred: add Rust abstraction for `struct cred`

[Alice Ryhl]

From: Wedson Almeida Filho <wedsonaf@gmail.com>

Add a wrapper around `struct cred` called `Credential`, and provide
functionality to get the `Credential` associated with a `File`.

Rust Binder must check the credentials of processes when they attempt to
perform various operations, and these checks usually take a
`&Credential` as parameter. The security_binder_set_context_mgr function
would be one example. This patch is necessary to access these security_*
methods from Rust.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/bindings/bindings_helper.h |  1 +
 rust/helpers.c                  | 13 +++++++
 rust/kernel/cred.rs             | 76 +++++++++++++++++++++++++++++++++++++++++
 rust/kernel/fs/file.rs          | 13 +++++++
 rust/kernel/lib.rs              |  1 +
 5 files changed, 104 insertions(+)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index e2d22f151ec9..22e62b3b34b0 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -7,6 +7,7 @@
  */
 
 #include <kunit/test.h>
+#include <linux/cred.h>
 #include <linux/errname.h>
 #include <linux/ethtool.h>
 #include <linux/file.h>
diff --git a/rust/helpers.c b/rust/helpers.c
index 5ba1f6de0251..4831abb5a438 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -23,6 +23,7 @@
 #include <kunit/test-bug.h>
 #include <linux/bug.h>
 #include <linux/build_bug.h>
+#include <linux/cred.h>
 #include <linux/err.h>
 #include <linux/errname.h>
 #include <linux/fs.h>
@@ -206,6 +207,18 @@ struct file *rust_helper_get_file(struct file *f)
 }
 EXPORT_SYMBOL_GPL(rust_helper_get_file);
 
+const struct cred *rust_helper_get_cred(const struct cred *cred)
+{
+	return get_cred(cred);
+}
+EXPORT_SYMBOL_GPL(rust_helper_get_cred);
+
+void rust_helper_put_cred(const struct cred *cred)
+{
+	put_cred(cred);
+}
+EXPORT_SYMBOL_GPL(rust_helper_put_cred);
+
 /*
  * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
  * use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
new file mode 100644
index 000000000000..acee04768927
--- /dev/null
+++ b/rust/kernel/cred.rs
@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Credentials management.
+//!
+//! C header: [`include/linux/cred.h`](srctree/include/linux/cred.h).
+//!
+//! Reference: <https://www.kernel.org/doc/html/latest/security/credentials.html>
+
+use crate::{
+    bindings,
+    types::{AlwaysRefCounted, Opaque},
+};
+
+/// Wraps the kernel's `struct cred`.
+///
+/// Credentials are used for various security checks in the kernel.
+///
+/// Most fields of credentials are immutable. When things have their credentials changed, that
+/// happens by replacing the credential instead of changing an existing credential. See the [kernel
+/// documentation][ref] for more info on this.
+///
+/// # Invariants
+///
+/// Instances of this type are always ref-counted, that is, a call to `get_cred` ensures that the
+/// allocation remains valid at least until the matching call to `put_cred`.
+///
+/// [ref]: https://www.kernel.org/doc/html/latest/security/credentials.html
+#[repr(transparent)]
+pub struct Credential(Opaque<bindings::cred>);
+
+// SAFETY:
+// - `Credential::dec_ref` can be called from any thread.
+// - It is okay to send ownership of `Credential` across thread boundaries.
+unsafe impl Send for Credential {}
+
+// SAFETY: It's OK to access `Credential` through shared references from other threads because
+// we're either accessing properties that don't change or that are properly synchronised by C code.
+unsafe impl Sync for Credential {}
+
+impl Credential {
+    /// Creates a reference to a [`Credential`] from a valid pointer.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
+    /// returned [`Credential`] reference.
+    pub unsafe fn from_ptr<'a>(ptr: *const bindings::cred) -> &'a Credential {
+        // SAFETY: The safety requirements guarantee the validity of the dereference, while the
+        // `Credential` type being transparent makes the cast ok.
+        unsafe { &*ptr.cast() }
+    }
+
+    /// Returns the effective UID of the given credential.
+    pub fn euid(&self) -> bindings::kuid_t {
+        // SAFETY: By the type invariant, we know that `self.0` is valid. Furthermore, the `euid`
+        // field of a credential is never changed after initialization, so there is no potential
+        // for data races.
+        unsafe { (*self.0.get()).euid }
+    }
+}
+
+// SAFETY: The type invariants guarantee that `Credential` is always ref-counted.
+unsafe impl AlwaysRefCounted for Credential {
+    fn inc_ref(&self) {
+        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
+        unsafe { bindings::get_cred(self.0.get()) };
+    }
+
+    unsafe fn dec_ref(obj: core::ptr::NonNull<Credential>) {
+        // SAFETY: The safety requirements guarantee that the refcount is nonzero. The cast is okay
+        // because `Credential` has the same representation as `struct cred`.
+        unsafe { bindings::put_cred(obj.cast().as_ptr()) };
+    }
+}
diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
index f1a52814b2da..8bed7bebcc43 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -9,6 +9,7 @@
 
 use crate::{
     bindings,
+    cred::Credential,
     error::{code::*, Error, Result},
     types::{ARef, AlwaysRefCounted, Opaque},
 };
@@ -274,6 +275,18 @@ pub fn as_ptr(&self) -> *mut bindings::file {
         self.inner.get()
     }
 
+    /// Returns the credentials of the task that originally opened the file.
+    pub fn cred(&self) -> &Credential {
+        // SAFETY: It's okay to read the `f_cred` field without synchronization because `f_cred` is
+        // never changed after initialization of the file.
+        let ptr = unsafe { (*self.as_ptr()).f_cred };
+
+        // SAFETY: The signature of this function ensures that the caller will only access the
+        // returned credential while the file is still valid, and the C side ensures that the
+        // credential stays valid at least as long as the file.
+        unsafe { Credential::from_ptr(ptr) }
+    }
+
     /// Returns the flags associated with the file.
     ///
     /// The flags are a combination of the constants in [`flags`].
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index c364893e13a2..8fb57ec20867 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -28,6 +28,7 @@
 
 pub mod alloc;
 mod build_assert;
+pub mod cred;
 pub mod error;
 pub mod fs;
 pub mod init;

-- 
2.45.2.1089.g2a221341d9-goog

Re: [PATCH v8 4/8] rust: cred: add Rust abstraction for `struct cred`

[Gary Guo]

On Thu, 25 Jul 2024 14:27:37 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> From: Wedson Almeida Filho <wedsonaf@gmail.com>
> 
> Add a wrapper around `struct cred` called `Credential`, and provide
> functionality to get the `Credential` associated with a `File`.
> 
> Rust Binder must check the credentials of processes when they attempt to
> perform various operations, and these checks usually take a
> `&Credential` as parameter. The security_binder_set_context_mgr function
> would be one example. This patch is necessary to access these security_*
> methods from Rust.
> 
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Co-developed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Trevor Gross <tmgross@umich.edu>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/bindings/bindings_helper.h |  1 +
>  rust/helpers.c                  | 13 +++++++
>  rust/kernel/cred.rs             | 76 +++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/fs/file.rs          | 13 +++++++
>  rust/kernel/lib.rs              |  1 +
>  5 files changed, 104 insertions(+)

[PATCH v8 5/8] rust: security: add abstraction for secctx

[Alice Ryhl]

Add an abstraction for viewing the string representation of a security
context.

This is needed by Rust Binder because it has a feature where a process
can view the string representation of the security context for incoming
transactions. The process can use that to authenticate incoming
transactions, and since the feature is provided by the kernel, the
process can trust that the security context is legitimate.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/bindings/bindings_helper.h |  1 +
 rust/helpers.c                  | 21 ++++++++++++
 rust/kernel/cred.rs             |  8 +++++
 rust/kernel/lib.rs              |  1 +
 rust/kernel/security.rs         | 74 +++++++++++++++++++++++++++++++++++++++++
 5 files changed, 105 insertions(+)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 22e62b3b34b0..afa24d54c1a0 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -17,6 +17,7 @@
 #include <linux/phy.h>
 #include <linux/refcount.h>
 #include <linux/sched.h>
+#include <linux/security.h>
 #include <linux/slab.h>
 #include <linux/wait.h>
 #include <linux/workqueue.h>
diff --git a/rust/helpers.c b/rust/helpers.c
index 4831abb5a438..b61f5a8ce1da 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -32,6 +32,7 @@
 #include <linux/mutex.h>
 #include <linux/refcount.h>
 #include <linux/sched/signal.h>
+#include <linux/security.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
 #include <linux/wait.h>
@@ -219,6 +220,26 @@ void rust_helper_put_cred(const struct cred *cred)
 }
 EXPORT_SYMBOL_GPL(rust_helper_put_cred);
 
+#ifndef CONFIG_SECURITY
+void rust_helper_security_cred_getsecid(const struct cred *c, u32 *secid)
+{
+	security_cred_getsecid(c, secid);
+}
+EXPORT_SYMBOL_GPL(rust_helper_security_cred_getsecid);
+
+int rust_helper_security_secid_to_secctx(u32 secid, char **secdata, u32 *seclen)
+{
+	return security_secid_to_secctx(secid, secdata, seclen);
+}
+EXPORT_SYMBOL_GPL(rust_helper_security_secid_to_secctx);
+
+void rust_helper_security_release_secctx(char *secdata, u32 seclen)
+{
+	security_release_secctx(secdata, seclen);
+}
+EXPORT_SYMBOL_GPL(rust_helper_security_release_secctx);
+#endif
+
 /*
  * `bindgen` binds the C `size_t` type as the Rust `usize` type, so we can
  * use it in contexts where Rust expects a `usize` like slice (array) indices.
diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
index acee04768927..92659649e932 100644
--- a/rust/kernel/cred.rs
+++ b/rust/kernel/cred.rs
@@ -52,6 +52,14 @@ pub unsafe fn from_ptr<'a>(ptr: *const bindings::cred) -> &'a Credential {
         unsafe { &*ptr.cast() }
     }
 
+    /// Get the id for this security context.
+    pub fn get_secid(&self) -> u32 {
+        let mut secid = 0;
+        // SAFETY: The invariants of this type ensures that the pointer is valid.
+        unsafe { bindings::security_cred_getsecid(self.0.get(), &mut secid) };
+        secid
+    }
+
     /// Returns the effective UID of the given credential.
     pub fn euid(&self) -> bindings::kuid_t {
         // SAFETY: By the type invariant, we know that `self.0` is valid. Furthermore, the `euid`
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 8fb57ec20867..a8de8293376e 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -40,6 +40,7 @@
 pub mod page;
 pub mod prelude;
 pub mod print;
+pub mod security;
 mod static_assert;
 #[doc(hidden)]
 pub mod std_vendor;
diff --git a/rust/kernel/security.rs b/rust/kernel/security.rs
new file mode 100644
index 000000000000..2522868862a1
--- /dev/null
+++ b/rust/kernel/security.rs
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Linux Security Modules (LSM).
+//!
+//! C header: [`include/linux/security.h`](srctree/include/linux/security.h).
+
+use crate::{
+    bindings,
+    error::{to_result, Result},
+};
+
+/// A security context string.
+///
+/// # Invariants
+///
+/// The `secdata` and `seclen` fields correspond to a valid security context as returned by a
+/// successful call to `security_secid_to_secctx`, that has not yet been destroyed by calling
+/// `security_release_secctx`.
+pub struct SecurityCtx {
+    secdata: *mut core::ffi::c_char,
+    seclen: usize,
+}
+
+impl SecurityCtx {
+    /// Get the security context given its id.
+    pub fn from_secid(secid: u32) -> Result<Self> {
+        let mut secdata = core::ptr::null_mut();
+        let mut seclen = 0u32;
+        // SAFETY: Just a C FFI call. The pointers are valid for writes.
+        to_result(unsafe { bindings::security_secid_to_secctx(secid, &mut secdata, &mut seclen) })?;
+
+        // INVARIANT: If the above call did not fail, then we have a valid security context.
+        Ok(Self {
+            secdata,
+            seclen: seclen as usize,
+        })
+    }
+
+    /// Returns whether the security context is empty.
+    pub fn is_empty(&self) -> bool {
+        self.seclen == 0
+    }
+
+    /// Returns the length of this security context.
+    pub fn len(&self) -> usize {
+        self.seclen
+    }
+
+    /// Returns the bytes for this security context.
+    pub fn as_bytes(&self) -> &[u8] {
+        let ptr = self.secdata;
+        if ptr.is_null() {
+            debug_assert_eq!(self.seclen, 0);
+            // We can't pass a null pointer to `slice::from_raw_parts` even if the length is zero.
+            return &[];
+        }
+
+        // SAFETY: The call to `security_secid_to_secctx` guarantees that the pointer is valid for
+        // `seclen` bytes. Furthermore, if the length is zero, then we have ensured that the
+        // pointer is not null.
+        unsafe { core::slice::from_raw_parts(ptr.cast(), self.seclen) }
+    }
+}
+
+impl Drop for SecurityCtx {
+    fn drop(&mut self) {
+        // SAFETY: By the invariant of `Self`, this frees a pointer that came from a successful
+        // call to `security_secid_to_secctx` and has not yet been destroyed by
+        // `security_release_secctx`.
+        unsafe { bindings::security_release_secctx(self.secdata, self.seclen as u32) };
+    }
+}

-- 
2.45.2.1089.g2a221341d9-goog

Re: [PATCH v8 5/8] rust: security: add abstraction for secctx

[Gary Guo]

On Thu, 25 Jul 2024 14:27:38 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> Add an abstraction for viewing the string representation of a security
> context.
> 
> This is needed by Rust Binder because it has a feature where a process
> can view the string representation of the security context for incoming
> transactions. The process can use that to authenticate incoming
> transactions, and since the feature is provided by the kernel, the
> process can trust that the security context is legitimate.
> 
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Reviewed-by: Trevor Gross <tmgross@umich.edu>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/bindings/bindings_helper.h |  1 +
>  rust/helpers.c                  | 21 ++++++++++++
>  rust/kernel/cred.rs             |  8 +++++
>  rust/kernel/lib.rs              |  1 +
>  rust/kernel/security.rs         | 74 +++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 105 insertions(+)

[PATCH v8 6/8] rust: file: add `FileDescriptorReservation`

[Alice Ryhl]

From: Wedson Almeida Filho <wedsonaf@gmail.com>

Allow for the creation of a file descriptor in two steps: first, we
reserve a slot for it, then we commit or drop the reservation. The first
step may fail (e.g., the current process ran out of available slots),
but commit and drop never fail (and are mutually exclusive).

This is needed by Rust Binder when fds are sent from one process to
another. It has to be a two-step process to properly handle the case
where multiple fds are sent: The operation must fail or succeed
atomically, which we achieve by first reserving the fds we need, and
only installing the files once we have reserved enough fds to send the
files.

Fd reservations assume that the value of `current` does not change
between the call to get_unused_fd_flags and the call to fd_install (or
put_unused_fd). By not implementing the Send trait, this abstraction
ensures that the `FileDescriptorReservation` cannot be moved into a
different process.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/fs/file.rs | 75 +++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 74 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
index 8bed7bebcc43..af98513bb2ee 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -11,7 +11,7 @@
     bindings,
     cred::Credential,
     error::{code::*, Error, Result},
-    types::{ARef, AlwaysRefCounted, Opaque},
+    types::{ARef, AlwaysRefCounted, NotThreadSafe, Opaque},
 };
 use core::ptr;
 
@@ -368,6 +368,79 @@ unsafe fn dec_ref(obj: ptr::NonNull<File>) {
     }
 }
 
+/// A file descriptor reservation.
+///
+/// This allows the creation of a file descriptor in two steps: first, we reserve a slot for it,
+/// then we commit or drop the reservation. The first step may fail (e.g., the current process ran
+/// out of available slots), but commit and drop never fail (and are mutually exclusive).
+///
+/// Dropping the reservation happens in the destructor of this type.
+///
+/// # Invariants
+///
+/// The fd stored in this struct must correspond to a reserved file descriptor of the current task.
+pub struct FileDescriptorReservation {
+    fd: u32,
+    /// Prevent values of this type from being moved to a different task.
+    ///
+    /// The `fd_install` and `put_unused_fd` functions assume that the value of `current` is
+    /// unchanged since the call to `get_unused_fd_flags`. By adding this marker to this type, we
+    /// prevent it from being moved across task boundaries, which ensures that `current` does not
+    /// change while this value exists.
+    _not_send: NotThreadSafe,
+}
+
+impl FileDescriptorReservation {
+    /// Creates a new file descriptor reservation.
+    pub fn get_unused_fd_flags(flags: u32) -> Result<Self> {
+        // SAFETY: FFI call, there are no safety requirements on `flags`.
+        let fd: i32 = unsafe { bindings::get_unused_fd_flags(flags) };
+        if fd < 0 {
+            return Err(Error::from_errno(fd));
+        }
+        Ok(Self {
+            fd: fd as u32,
+            _not_send: NotThreadSafe,
+        })
+    }
+
+    /// Returns the file descriptor number that was reserved.
+    pub fn reserved_fd(&self) -> u32 {
+        self.fd
+    }
+
+    /// Commits the reservation.
+    ///
+    /// The previously reserved file descriptor is bound to `file`. This method consumes the
+    /// [`FileDescriptorReservation`], so it will not be usable after this call.
+    pub fn fd_install(self, file: ARef<File>) {
+        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`. We have not yet used
+        // the fd, so it is still valid, and `current` still refers to the same task, as this type
+        // cannot be moved across task boundaries.
+        //
+        // Furthermore, the file pointer is guaranteed to own a refcount by its type invariants,
+        // and we take ownership of that refcount by not running the destructor below.
+        // Additionally, the file is known to not have any non-shared `fdget_pos` calls, so even if
+        // this process starts using the file position, this will not result in a data race on the
+        // file position.
+        unsafe { bindings::fd_install(self.fd, file.as_ptr()) };
+
+        // `fd_install` consumes both the file descriptor and the file reference, so we cannot run
+        // the destructors.
+        core::mem::forget(self);
+        core::mem::forget(file);
+    }
+}
+
+impl Drop for FileDescriptorReservation {
+    fn drop(&mut self) {
+        // SAFETY: By the type invariants of this type, `self.fd` was previously returned by
+        // `get_unused_fd_flags`. We have not yet used the fd, so it is still valid, and `current`
+        // still refers to the same task, as this type cannot be moved across task boundaries.
+        unsafe { bindings::put_unused_fd(self.fd) };
+    }
+}
+
 /// Represents the `EBADF` error code.
 ///
 /// Used for methods that can only fail with `EBADF`.

-- 
2.45.2.1089.g2a221341d9-goog

Re: [PATCH v8 6/8] rust: file: add `FileDescriptorReservation`

[Gary Guo]

On Thu, 25 Jul 2024 14:27:39 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> From: Wedson Almeida Filho <wedsonaf@gmail.com>
> 
> Allow for the creation of a file descriptor in two steps: first, we
> reserve a slot for it, then we commit or drop the reservation. The first
> step may fail (e.g., the current process ran out of available slots),
> but commit and drop never fail (and are mutually exclusive).
> 
> This is needed by Rust Binder when fds are sent from one process to
> another. It has to be a two-step process to properly handle the case
> where multiple fds are sent: The operation must fail or succeed
> atomically, which we achieve by first reserving the fds we need, and
> only installing the files once we have reserved enough fds to send the
> files.
> 
> Fd reservations assume that the value of `current` does not change
> between the call to get_unused_fd_flags and the call to fd_install (or
> put_unused_fd). By not implementing the Send trait, this abstraction
> ensures that the `FileDescriptorReservation` cannot be moved into a
> different process.
> 
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Co-developed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Reviewed-by: Trevor Gross <tmgross@umich.edu>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/fs/file.rs | 75 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 74 insertions(+), 1 deletion(-)

[PATCH v8 7/8] rust: file: add `Kuid` wrapper

[Alice Ryhl]

Adds a wrapper around `kuid_t` called `Kuid`. This allows us to define
various operations on kuids such as equality and current_euid. It also
lets us provide conversions from kuid into userspace values.

Rust Binder needs these operations because it needs to compare kuids for
equality, and it needs to tell userspace about the pid and uid of
incoming transactions.

To read kuids from a `struct task_struct`, you must currently use
various #defines that perform the appropriate field access under an RCU
read lock. Currently, we do not have a Rust wrapper for rcu_read_lock,
which means that for this patch, there are two ways forward:

 1. Inline the methods into Rust code, and use __rcu_read_lock directly
    rather than the rcu_read_lock wrapper. This gives up lockdep for
    these usages of RCU.

 2. Wrap the various #defines in helpers and call the helpers from Rust.

This patch uses the second option. One possible disadvantage of the
second option is the possible introduction of speculation gadgets, but
as discussed in [1], the risk appears to be acceptable.

Of course, once a wrapper for rcu_read_lock is available, it is
preferable to use that over either of the two above approaches.

Link: https://lore.kernel.org/all/202312080947.674CD2DC7@keescook/ [1]
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/bindings/bindings_helper.h |  1 +
 rust/helpers.c                  | 45 ++++++++++++++++++++++++++++
 rust/kernel/cred.rs             |  5 ++--
 rust/kernel/task.rs             | 66 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 115 insertions(+), 2 deletions(-)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index afa24d54c1a0..d7f7ae109e6f 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -15,6 +15,7 @@
 #include <linux/jiffies.h>
 #include <linux/mdio.h>
 #include <linux/phy.h>
+#include <linux/pid_namespace.h>
 #include <linux/refcount.h>
 #include <linux/sched.h>
 #include <linux/security.h>
diff --git a/rust/helpers.c b/rust/helpers.c
index b61f5a8ce1da..dda7d26ccf8f 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -177,6 +177,51 @@ void rust_helper_put_task_struct(struct task_struct *t)
 }
 EXPORT_SYMBOL_GPL(rust_helper_put_task_struct);
 
+kuid_t rust_helper_task_uid(struct task_struct *task)
+{
+	return task_uid(task);
+}
+EXPORT_SYMBOL_GPL(rust_helper_task_uid);
+
+kuid_t rust_helper_task_euid(struct task_struct *task)
+{
+	return task_euid(task);
+}
+EXPORT_SYMBOL_GPL(rust_helper_task_euid);
+
+#ifndef CONFIG_USER_NS
+uid_t rust_helper_from_kuid(struct user_namespace *to, kuid_t uid)
+{
+	return from_kuid(to, uid);
+}
+EXPORT_SYMBOL_GPL(rust_helper_from_kuid);
+#endif /* CONFIG_USER_NS */
+
+bool rust_helper_uid_eq(kuid_t left, kuid_t right)
+{
+	return uid_eq(left, right);
+}
+EXPORT_SYMBOL_GPL(rust_helper_uid_eq);
+
+kuid_t rust_helper_current_euid(void)
+{
+	return current_euid();
+}
+EXPORT_SYMBOL_GPL(rust_helper_current_euid);
+
+struct user_namespace *rust_helper_current_user_ns(void)
+{
+	return current_user_ns();
+}
+EXPORT_SYMBOL_GPL(rust_helper_current_user_ns);
+
+pid_t rust_helper_task_tgid_nr_ns(struct task_struct *tsk,
+				  struct pid_namespace *ns)
+{
+	return task_tgid_nr_ns(tsk, ns);
+}
+EXPORT_SYMBOL_GPL(rust_helper_task_tgid_nr_ns);
+
 struct kunit *rust_helper_kunit_get_current_test(void)
 {
 	return kunit_get_current_test();
diff --git a/rust/kernel/cred.rs b/rust/kernel/cred.rs
index 92659649e932..81d67789b16f 100644
--- a/rust/kernel/cred.rs
+++ b/rust/kernel/cred.rs
@@ -10,6 +10,7 @@
 
 use crate::{
     bindings,
+    task::Kuid,
     types::{AlwaysRefCounted, Opaque},
 };
 
@@ -61,11 +62,11 @@ pub fn get_secid(&self) -> u32 {
     }
 
     /// Returns the effective UID of the given credential.
-    pub fn euid(&self) -> bindings::kuid_t {
+    pub fn euid(&self) -> Kuid {
         // SAFETY: By the type invariant, we know that `self.0` is valid. Furthermore, the `euid`
         // field of a credential is never changed after initialization, so there is no potential
         // for data races.
-        unsafe { (*self.0.get()).euid }
+        Kuid::from_raw(unsafe { (*self.0.get()).euid })
     }
 }
 
diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 367b4bbddd9f..1a36a9f19368 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -9,6 +9,7 @@
     types::{NotThreadSafe, Opaque},
 };
 use core::{
+    cmp::{Eq, PartialEq},
     ffi::{c_int, c_long, c_uint},
     ops::Deref,
     ptr,
@@ -96,6 +97,12 @@ unsafe impl Sync for Task {}
 /// The type of process identifiers (PIDs).
 type Pid = bindings::pid_t;
 
+/// The type of user identifiers (UIDs).
+#[derive(Copy, Clone)]
+pub struct Kuid {
+    kuid: bindings::kuid_t,
+}
+
 impl Task {
     /// Returns a raw pointer to the current task.
     ///
@@ -157,12 +164,31 @@ pub fn pid(&self) -> Pid {
         unsafe { *ptr::addr_of!((*self.0.get()).pid) }
     }
 
+    /// Returns the UID of the given task.
+    pub fn uid(&self) -> Kuid {
+        // SAFETY: By the type invariant, we know that `self.0` is valid.
+        Kuid::from_raw(unsafe { bindings::task_uid(self.0.get()) })
+    }
+
+    /// Returns the effective UID of the given task.
+    pub fn euid(&self) -> Kuid {
+        // SAFETY: By the type invariant, we know that `self.0` is valid.
+        Kuid::from_raw(unsafe { bindings::task_euid(self.0.get()) })
+    }
+
     /// Determines whether the given task has pending signals.
     pub fn signal_pending(&self) -> bool {
         // SAFETY: By the type invariant, we know that `self.0` is valid.
         unsafe { bindings::signal_pending(self.0.get()) != 0 }
     }
 
+    /// Returns the given task's pid in the current pid namespace.
+    pub fn pid_in_current_ns(&self) -> Pid {
+        // SAFETY: We know that `self.0.get()` is valid by the type invariant, and passing a null
+        // pointer as the namespace is correct for using the current namespace.
+        unsafe { bindings::task_tgid_nr_ns(self.0.get(), ptr::null_mut()) }
+    }
+
     /// Wakes up the task.
     pub fn wake_up(&self) {
         // SAFETY: By the type invariant, we know that `self.0.get()` is non-null and valid.
@@ -184,3 +210,43 @@ unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
         unsafe { bindings::put_task_struct(obj.cast().as_ptr()) }
     }
 }
+
+impl Kuid {
+    /// Get the current euid.
+    #[inline]
+    pub fn current_euid() -> Kuid {
+        // SAFETY: Just an FFI call.
+        Self::from_raw(unsafe { bindings::current_euid() })
+    }
+
+    /// Create a `Kuid` given the raw C type.
+    #[inline]
+    pub fn from_raw(kuid: bindings::kuid_t) -> Self {
+        Self { kuid }
+    }
+
+    /// Turn this kuid into the raw C type.
+    #[inline]
+    pub fn into_raw(self) -> bindings::kuid_t {
+        self.kuid
+    }
+
+    /// Converts this kernel UID into a userspace UID.
+    ///
+    /// Uses the namespace of the current task.
+    #[inline]
+    pub fn into_uid_in_current_ns(self) -> bindings::uid_t {
+        // SAFETY: Just an FFI call.
+        unsafe { bindings::from_kuid(bindings::current_user_ns(), self.kuid) }
+    }
+}
+
+impl PartialEq for Kuid {
+    #[inline]
+    fn eq(&self, other: &Kuid) -> bool {
+        // SAFETY: Just an FFI call.
+        unsafe { bindings::uid_eq(self.kuid, other.kuid) }
+    }
+}
+
+impl Eq for Kuid {}

-- 
2.45.2.1089.g2a221341d9-goog

[PATCH v8 8/8] rust: file: add abstraction for `poll_table`

[Alice Ryhl]

The existing `CondVar` abstraction is a wrapper around
`wait_queue_head`, but it does not support all use-cases of the C
`wait_queue_head` type. To be specific, a `CondVar` cannot be registered
with a `struct poll_table`. This limitation has the advantage that you
do not need to call `synchronize_rcu` when destroying a `CondVar`.

However, we need the ability to register a `poll_table` with a
`wait_queue_head` in Rust Binder. To enable this, introduce a type
called `PollCondVar`, which is like `CondVar` except that you can
register a `poll_table`. We also introduce `PollTable`, which is a safe
wrapper around `poll_table` that is intended to be used with
`PollCondVar`.

The destructor of `PollCondVar` unconditionally calls `synchronize_rcu`
to ensure that the removal of epoll waiters has fully completed before
the `wait_queue_head` is destroyed.

That said, `synchronize_rcu` is rather expensive and is not needed in
all cases: If we have never registered a `poll_table` with the
`wait_queue_head`, then we don't need to call `synchronize_rcu`. (And
this is a common case in Binder - not all processes use Binder with
epoll.) The current implementation does not account for this, but if we
find that it is necessary to improve this, a future patch could store a
boolean next to the `wait_queue_head` to keep track of whether a
`poll_table` has ever been registered.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/bindings/bindings_helper.h |   1 +
 rust/kernel/sync.rs             |   1 +
 rust/kernel/sync/poll.rs        | 121 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 123 insertions(+)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index d7f7ae109e6f..809112fd73d3 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -16,6 +16,7 @@
 #include <linux/mdio.h>
 #include <linux/phy.h>
 #include <linux/pid_namespace.h>
+#include <linux/poll.h>
 #include <linux/refcount.h>
 #include <linux/sched.h>
 #include <linux/security.h>
diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index 0ab20975a3b5..bae4a5179c72 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -11,6 +11,7 @@
 mod condvar;
 pub mod lock;
 mod locked_by;
+pub mod poll;
 
 pub use arc::{Arc, ArcBorrow, UniqueArc};
 pub use condvar::{new_condvar, CondVar, CondVarTimeoutResult};
diff --git a/rust/kernel/sync/poll.rs b/rust/kernel/sync/poll.rs
new file mode 100644
index 000000000000..d5f17153b424
--- /dev/null
+++ b/rust/kernel/sync/poll.rs
@@ -0,0 +1,121 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Utilities for working with `struct poll_table`.
+
+use crate::{
+    bindings,
+    fs::File,
+    prelude::*,
+    sync::{CondVar, LockClassKey},
+    types::Opaque,
+};
+use core::ops::Deref;
+
+/// Creates a [`PollCondVar`] initialiser with the given name and a newly-created lock class.
+#[macro_export]
+macro_rules! new_poll_condvar {
+    ($($name:literal)?) => {
+        $crate::sync::poll::PollCondVar::new(
+            $crate::optional_name!($($name)?), $crate::static_lock_class!()
+        )
+    };
+}
+
+/// Wraps the kernel's `struct poll_table`.
+///
+/// # Invariants
+///
+/// This struct contains a valid `struct poll_table`.
+///
+/// For a `struct poll_table` to be valid, its `_qproc` function must follow the safety
+/// requirements of `_qproc` functions:
+///
+/// * The `_qproc` function is given permission to enqueue a waiter to the provided `poll_table`
+///   during the call. Once the waiter is removed and an rcu grace period has passed, it must no
+///   longer access the `wait_queue_head`.
+#[repr(transparent)]
+pub struct PollTable(Opaque<bindings::poll_table>);
+
+impl PollTable {
+    /// Creates a reference to a [`PollTable`] from a valid pointer.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that for the duration of 'a, the pointer will point at a valid poll
+    /// table (as defined in the type invariants).
+    ///
+    /// The caller must also ensure that the `poll_table` is only accessed via the returned
+    /// reference for the duration of 'a.
+    pub unsafe fn from_ptr<'a>(ptr: *mut bindings::poll_table) -> &'a mut PollTable {
+        // SAFETY: The safety requirements guarantee the validity of the dereference, while the
+        // `PollTable` type being transparent makes the cast ok.
+        unsafe { &mut *ptr.cast() }
+    }
+
+    fn get_qproc(&self) -> bindings::poll_queue_proc {
+        let ptr = self.0.get();
+        // SAFETY: The `ptr` is valid because it originates from a reference, and the `_qproc`
+        // field is not modified concurrently with this call since we have an immutable reference.
+        unsafe { (*ptr)._qproc }
+    }
+
+    /// Register this [`PollTable`] with the provided [`PollCondVar`], so that it can be notified
+    /// using the condition variable.
+    pub fn register_wait(&mut self, file: &File, cv: &PollCondVar) {
+        if let Some(qproc) = self.get_qproc() {
+            // SAFETY: The pointers to `file` and `self` need to be valid for the duration of this
+            // call to `qproc`, which they are because they are references.
+            //
+            // The `cv.wait_queue_head` pointer must be valid until an rcu grace period after the
+            // waiter is removed. The `PollCondVar` is pinned, so before `cv.wait_queue_head` can
+            // be destroyed, the destructor must run. That destructor first removes all waiters,
+            // and then waits for an rcu grace period. Therefore, `cv.wait_queue_head` is valid for
+            // long enough.
+            unsafe { qproc(file.as_ptr() as _, cv.wait_queue_head.get(), self.0.get()) };
+        }
+    }
+}
+
+/// A wrapper around [`CondVar`] that makes it usable with [`PollTable`].
+///
+/// [`CondVar`]: crate::sync::CondVar
+#[pin_data(PinnedDrop)]
+pub struct PollCondVar {
+    #[pin]
+    inner: CondVar,
+}
+
+impl PollCondVar {
+    /// Constructs a new condvar initialiser.
+    pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
+        pin_init!(Self {
+            inner <- CondVar::new(name, key),
+        })
+    }
+}
+
+// Make the `CondVar` methods callable on `PollCondVar`.
+impl Deref for PollCondVar {
+    type Target = CondVar;
+
+    fn deref(&self) -> &CondVar {
+        &self.inner
+    }
+}
+
+#[pinned_drop]
+impl PinnedDrop for PollCondVar {
+    fn drop(self: Pin<&mut Self>) {
+        // Clear anything registered using `register_wait`.
+        //
+        // SAFETY: The pointer points at a valid `wait_queue_head`.
+        unsafe { bindings::__wake_up_pollfree(self.inner.wait_queue_head.get()) };
+
+        // Wait for epoll items to be properly removed.
+        //
+        // SAFETY: Just an FFI call.
+        unsafe { bindings::synchronize_rcu() };
+    }
+}

-- 
2.45.2.1089.g2a221341d9-goog