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

[donplat <donplat@barrensea.org>]

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@barrensea.org> 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@barrensea.org>
>> ---
>>
>> 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