The C function stream_open() has no Rust equivalent, requiring Rust
drivers to manipulate f_mode directly through unsafe bindings. This adds
the necessary FMODE constants to the bindings and implements stream_open()
natively on LocalFile, giving Rust character device drivers a safe way to
mark a file as a stream.

Add the FMODE_STREAM, FMODE_LSEEK, FMODE_PREAD, FMODE_PWRITE and
FMODE_ATOMIC_POS constants to the bindings. These constants were not
previously accessible to Rust code because bindgen cannot handle the
__force cast used in their C macro definitions. They are exposed here
using the RUST_CONST_HELPER pattern already established in
bindings_helper.h.

stream_open() should be called from the open() handler of a character
device that produces data as a stream rather than supporting random
access. It clears the seek-related file mode flags and sets FMODE_STREAM,
telling the VFS layer not to validate seek positions on read_iter and
write_iter calls. This mirrors the behavior of stream_open() in
fs/open.c, which does not use the inode parameter and only manipulates
f_mode on the file struct.

The method is placed on LocalFile rather than File because stream_open()
must be called during open(), before the file descriptor is installed
into the process fd table. LocalFile is !Send, which statically
guarantees that no other thread can concurrently access f_mode at the
point this method is called, making the unsafe f_mode manipulation sound.

Signed-off-by: Christian Benton <t1bur0n.kernel.org@protonmail.ch>
---
 rust/bindings/bindings_helper.h |  5 +++++
 rust/kernel/fs/file.rs          | 30 ++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 083cc44aa..21421296c 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -113,6 +113,11 @@ const gfp_t RUST_CONST_HELPER___GFP_ZERO = __GFP_ZERO;
 const gfp_t RUST_CONST_HELPER___GFP_HIGHMEM = ___GFP_HIGHMEM;
 const gfp_t RUST_CONST_HELPER___GFP_NOWARN = ___GFP_NOWARN;
 const blk_features_t RUST_CONST_HELPER_BLK_FEAT_ROTATIONAL = BLK_FEAT_ROTATIONAL;
+const fmode_t RUST_CONST_HELPER_FMODE_ATOMIC_POS = FMODE_ATOMIC_POS;
+const fmode_t RUST_CONST_HELPER_FMODE_LSEEK = FMODE_LSEEK;
+const fmode_t RUST_CONST_HELPER_FMODE_PREAD = FMODE_PREAD;
+const fmode_t RUST_CONST_HELPER_FMODE_PWRITE = FMODE_PWRITE;
+const fmode_t RUST_CONST_HELPER_FMODE_STREAM = FMODE_STREAM;
 const fop_flags_t RUST_CONST_HELPER_FOP_UNSIGNED_OFFSET = FOP_UNSIGNED_OFFSET;
 
 const xa_mark_t RUST_CONST_HELPER_XA_PRESENT = XA_PRESENT;
diff --git a/rust/kernel/fs/file.rs b/rust/kernel/fs/file.rs
index 23ee689bd..a22200c56 100644
--- a/rust/kernel/fs/file.rs
+++ b/rust/kernel/fs/file.rs
@@ -342,6 +342,36 @@ pub fn flags(&self) -> u32 {
         // 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() }
     }
+
+    /// Marks this file as a stream, disabling seek position validation.
+    ///
+    /// This should be called from the `open` handler of a character device that
+    /// produces data as a stream rather than supporting random access. It clears
+    /// the seek-related file mode flags and sets `FMODE_STREAM`, which tells the
+    /// VFS layer not to validate seek positions on `read_iter` and `write_iter`
+    /// calls.
+    ///
+    /// Must only be called during `open()`, before the file descriptor is
+    /// installed into the process fd table and becomes accessible to other
+    /// threads.
+    pub fn stream_open(&self) {
+        // SAFETY: The file pointer is valid for the duration of this call
+        // because `LocalFile` can only exist while the underlying `struct file`
+        // is alive. Since `LocalFile` is not `Send`, this method can only be
+        // called before the file is shared across threads, which means no other
+        // thread can be concurrently modifying `f_mode`. The pointer is
+        // guaranteed non-null by the type invariants of `LocalFile`.
+        unsafe {
+            let file = self.as_ptr();
+            (*file).f_mode &= !(
+                bindings::FMODE_LSEEK
+                    | bindings::FMODE_PREAD
+                    | bindings::FMODE_PWRITE
+                    | bindings::FMODE_ATOMIC_POS
+            );
+            (*file).f_mode |= bindings::FMODE_STREAM;
+        }
+    }
 }
 
 impl File {
-- 
2.53.0

On Fri, Apr 3, 2026 at 10:20 PM Christian Benton
<t1bur0n.kernel.org@protonmail.ch> wrote:
>
> The C function stream_open() has no Rust equivalent, requiring Rust
> drivers to manipulate f_mode directly through unsafe bindings. This adds
> the necessary FMODE constants to the bindings and implements stream_open()
> natively on LocalFile, giving Rust character device drivers a safe way to
> mark a file as a stream.
>
> Add the FMODE_STREAM, FMODE_LSEEK, FMODE_PREAD, FMODE_PWRITE and
> FMODE_ATOMIC_POS constants to the bindings. These constants were not
> previously accessible to Rust code because bindgen cannot handle the
> __force cast used in their C macro definitions. They are exposed here
> using the RUST_CONST_HELPER pattern already established in
> bindings_helper.h.
>
> stream_open() should be called from the open() handler of a character
> device that produces data as a stream rather than supporting random
> access. It clears the seek-related file mode flags and sets FMODE_STREAM,
> telling the VFS layer not to validate seek positions on read_iter and
> write_iter calls. This mirrors the behavior of stream_open() in
> fs/open.c, which does not use the inode parameter and only manipulates
> f_mode on the file struct.
>
> The method is placed on LocalFile rather than File because stream_open()
> must be called during open(), before the file descriptor is installed
> into the process fd table. LocalFile is !Send, which statically
> guarantees that no other thread can concurrently access f_mode at the
> point this method is called, making the unsafe f_mode manipulation sound.
>
> Signed-off-by: Christian Benton <t1bur0n.kernel.org@protonmail.ch>

Please use `scripts/get_maintainer.pl` to get a list of people to Cc,
i.e. VFS maintainers and reviewers should be included at least, plus
usually the RUST entry. Cc'ing them here for you. :)

I think one thing they may ask is whether you have an expected user of
this new API to land.

Thanks!

Cheers,
Miguel