Re: [PATCH v2] rust: seq_file: route seq_print! directly to seq_write

[donplat]

Thanks for the reminder.Got it. I'll make sure to send future versions 
as completely separate threads.

在 2026/5/16 15:57, Onur Özkan 写道:
> On Sat, 16 May 2026 12:08:12 +0800
> Yifei Yao <donplat@xxxxxxxxxxxxx> wrote:
>
> > Currently, the `seq_print!` macro formats output by passing a Rust
> > `fmt::Arguments` object to the C `seq_printf` function using the `%pA`.
> > This involves crossing the FFI boundary, parsing the
> > format string in C via `vsnprintf`, and triggering a callback back
> > into Rust to perform the actual buffer write.
> >
> > This patch implements `core::fmt::Write` for `&SeqFile` and updates
> > `call_printf` to use it, routing Rust's formatting directly to
> > `bindings::seq_write`. This approach leverages the bounded string
> > slices naturally produced by the Rust formatting machinery and copies
> > them directly into the `seq_file` buffer.
> >
> > By mapping to `seq_write`, we bypass the C string parsing state machine
> > and the `%pA` FFI callback, streamlining the execution path. Since
> > `seq_write` uses a bounded `memcpy` relying on the explicitly passed
> > length, it perfectly matches Rust's `&str` semantics and operates
> > safely without requiring null-termination.
> >
> > Note that `fmt::Write` chunks the output into multiple `seq_write` calls.
> > If an overflow occurs during a partial write, `seq_write` sets the
> > overflow flag. The `seq_file` subsystem inherently handles this by
> > discarding the current record and retrying with a larger buffer.
> > Therefore, this optimization introduces no observable semantic change
> > to user-space.
> >
> > Signed-off-by: Yifei Yao <donplat@xxxxxxxxxxxxx>
> > ---
> >
> > Changes in v2:
> > - Add `#[inline]` annotation to the `write_str` trait method.
> > - Fix author name and signed-off name to use real name per the kernel's
> >    DCO policy.
> > - Correctly check the return value of `seq_write` and propagates errors on overflow.
> >
> >   rust/kernel/seq_file.rs | 32 ++++++++++++++++++++++++--------
> >   1 file changed, 24 insertions(+), 8 deletions(-)
> >
> > diff --git a/rust/kernel/seq_file.rs b/rust/kernel/seq_file.rs
> > index 518265558d6..d7dbdadde60 100644
> > --- a/rust/kernel/seq_file.rs
> > +++ b/rust/kernel/seq_file.rs
> > @@ -4,7 +4,7 @@
> >   //!
> >   //! C header: [`include/linux/seq_file.h`](srctree/include/linux/seq_file.h)
> >   
> > -use crate::{bindings, fmt, str::CStrExt as _, types::NotThreadSafe, types::Opaque};
> > +use crate::{bindings, ffi::c_void, fmt, types::NotThreadSafe, types::Opaque};
> >   
> >   /// A utility for generating the contents of a seq file.
> >   #[repr(transparent)]
> > @@ -32,13 +32,29 @@ pub unsafe fn from_raw<'a>(ptr: *mut bindings::seq_file) -> &'a SeqFile {
> >       /// Used by the [`seq_print`] macro.
> >       #[inline]
> >       pub fn call_printf(&self, args: fmt::Arguments<'_>) {
> > -        // SAFETY: Passing a void pointer to `Arguments` is valid for `%pA`.
> > -        unsafe {
> > -            bindings::seq_printf(
> > -                self.inner.get(),
> > -                c"%pA".as_char_ptr(),
> > -                core::ptr::from_ref(&args).cast::<crate::ffi::c_void>(),
> > -            );
> > +        let mut this = self;
> > +        let _ = fmt::Write::write_fmt(&mut this, args);
> > +    }
> > +}
> > +
> > +impl fmt::Write for &SeqFile {
> > +    #[inline]
> > +    fn write_str(&mut self, s: &str) -> fmt::Result {
> > +        // SAFETY: `self` is a valid reference, ensuring `self.inner.get()` is a valid pointer
> > +        // to `struct seq_file`. `s` is a valid string slice, guaranteeing `s.as_ptr()` is
> > +        // readable for `s.len()` bytes. `seq_write` handles bounds checking and does not
> > +        // require a null-terminated string.
> > +        //
> > +        // CAST: `s.as_ptr()` (a `*const u8`) is cast to `*const c_void` because `seq_write`
> > +        // only reads the buffer via `memcpy` and does not care about the underlying type.
> > +
> > +        let res =
> > +            unsafe { bindings::seq_write(self.inner.get(), s.as_ptr().cast::<c_void>(), s.len()) };
> > +
> > +        if res < 0 {
> > +            Err(fmt::Error)
> > +        } else {
> > +            Ok(())
> >           }
> >       }
> >   }
> > --
> > 2.53.0
> FYI new versions are usually sent separately.
>
> Onur