From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C494C279DB8; Fri, 11 Jul 2025 09:29:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752226196; cv=none; b=jwMXnN61KVHwi4O7/QlxbAjatrBRd+9TuB/ciKzEhXzrXW8XzCzdcm06wo1U2Ugakyx7EOXSYSVB0fgRNVlbcVwksv4MuhTt35tkqCR0IgxJEobQXrZsI/AHcdg4rwYFDUEksJSgCqGmGVogYeTf/k2sXzvXNa39gZYWR7n7JdI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752226196; c=relaxed/simple; bh=g635AuROiIJUsnlRuPb0NpgIXG9FJEv+7lx2i4prxN8=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=DAiWFeVyk14VUyv2uMY+D4Lu5n/duvhneEkyGSQPlOLFbtkgUVJtshKm1bm4UT44104fPpbIvwOS461SrVaML0nwZp328hnVhl+OSVNRwAq+ugC+TGNYVtHEpiUBTHqbegQssyNADj17f6GQCGT8cTvH622RvVJW7U90do6AMVs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=lE9NDK9O; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="lE9NDK9O" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D190FC4CEF5; Fri, 11 Jul 2025 09:29:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1752226196; bh=g635AuROiIJUsnlRuPb0NpgIXG9FJEv+7lx2i4prxN8=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=lE9NDK9Oqx9H593Fkd6BUGxrwcePtgfvnCJ4HjUbRKYaTOlnHH4m7zdDIVhH7pJU0 bjC70qQg+2NhJvwfgAsO9aGW2BsMASEhzugg6Ysy2RCCBfuXsmZ7tKtYVepA9ADVf4 paI5r7yCC4wmNrdH6yvyASgWKIrIjoHqU58pqHcuugsPhVoDYdgkkBz07tFZgZk004 eWcQLgZ7+IGRPCfCn+x1tg4vu6Nx7MuiHV3HNu8a09D8ITF3jo8ywkjnw+p3W1urS+ jTgCHvo+YD3dzx/JDGZl51bEMGHpgxUnuOBVAFMdq1BrxFOknyxkWNqoLA58EGpFFS JKbeTyJ5VWD1Q== From: Andreas Hindborg To: "Alice Ryhl" Cc: "Boqun Feng" , "Miguel Ojeda" , "Alex Gaynor" , "Gary Guo" , =?utf-8?Q?Bj=C3=B6rn?= Roy Baron , "Benno Lossin" , "Trevor Gross" , "Danilo Krummrich" , "Jens Axboe" , , , Subject: Re: [PATCH v2 05/14] rust: block: use `NullBorrowFormatter` In-Reply-To: (Alice Ryhl's message of "Wed, 09 Jul 2025 13:25:08 +0000") References: <20250708-rnull-up-v6-16-v2-0-ab93c0ff429b@kernel.org> <20250708-rnull-up-v6-16-v2-5-ab93c0ff429b@kernel.org> <1RnkSkM82_BGKhOM4PKNTPqEQdSFQhpr6UlkVOD7EDhmTJxZ_hlNFhVuiqpUtKKW1uFUFSB7Ow3LJ31nvHUnDQ==@protonmail.internalid> User-Agent: mu4e 1.12.9; emacs 30.1 Date: Fri, 11 Jul 2025 11:29:46 +0200 Message-ID: <878qkvhy7p.fsf@kernel.org> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain "Alice Ryhl" writes: > On Tue, Jul 08, 2025 at 09:45:00PM +0200, Andreas Hindborg wrote: >> Use the new `NullBorrowFormatter` to write the name of a `GenDisk` to the >> name buffer. This new formatter automatically adds a trailing null marker >> after the written characters, so we don't need to append that at the call >> site any longer. >> >> Signed-off-by: Andreas Hindborg >> --- >> rust/kernel/block/mq/gen_disk.rs | 8 ++++---- >> rust/kernel/block/mq/raw_writer.rs | 1 + >> rust/kernel/str.rs | 7 ------- >> 3 files changed, 5 insertions(+), 11 deletions(-) >> >> diff --git a/rust/kernel/block/mq/gen_disk.rs b/rust/kernel/block/mq/gen_disk.rs >> index 679ee1bb21950..e0e42f7028276 100644 >> --- a/rust/kernel/block/mq/gen_disk.rs >> +++ b/rust/kernel/block/mq/gen_disk.rs >> @@ -7,9 +7,10 @@ >> >> use crate::{ >> bindings, >> - block::mq::{raw_writer::RawWriter, Operations, TagSet}, >> + block::mq::{Operations, TagSet}, >> error::{self, from_err_ptr, Result}, >> static_lock_class, >> + str::NullBorrowFormatter, >> sync::Arc, >> }; >> use core::fmt::{self, Write}; >> @@ -143,14 +144,13 @@ pub fn build( >> // SAFETY: `gendisk` is a valid pointer as we initialized it above >> unsafe { (*gendisk).fops = &TABLE }; >> >> - let mut raw_writer = RawWriter::from_array( >> + let mut writer = NullBorrowFormatter::from_array( >> // SAFETY: `gendisk` points to a valid and initialized instance. We >> // have exclusive access, since the disk is not added to the VFS >> // yet. >> unsafe { &mut (*gendisk).disk_name }, >> )?; >> - raw_writer.write_fmt(name)?; >> - raw_writer.write_char('\0')?; >> + writer.write_fmt(name)?; > > Although this is nicer than the existing code, I wonder if it should > just be a function rather than a whole NullBorrowFormatter struct? Take > a slice and a fmt::Arguments and write it with a nul-terminator. Do you > need anything more complex than what you have here? I don't need anything more complex right now. But I think the `NullTerminatedFormatter` could be useful anyway: +/// A mutable reference to a byte buffer where a string can be written into. +/// +/// The buffer will be automatically null terminated after the last written character. +/// +/// # Invariants +/// +/// `buffer` is always null terminated. +pub(crate) struct NullTerminatedFormatter<'a> { + buffer: &'a mut [u8], +} + +impl<'a> NullTerminatedFormatter<'a> { + /// Create a new [`Self`] instance. + pub(crate) fn new(buffer: &'a mut [u8]) -> Option> { + *(buffer.first_mut()?) = 0; + + // INVARIANT: We null terminated the buffer above. + Some(Self { buffer }) + } + + pub(crate) fn from_array( + buffer: &'a mut [crate::ffi::c_char; N], + ) -> Option> { + Self::new(buffer) + } +} + +impl Write for NullTerminatedFormatter<'_> { + fn write_str(&mut self, s: &str) -> fmt::Result { + let bytes = s.as_bytes(); + let len = bytes.len(); + + // We want space for a null terminator. Buffer length is always at least 1, so no overflow. + if len > self.buffer.len() - 1 { + return Err(fmt::Error); + } + + let buffer = core::mem::take(&mut self.buffer); + // We break the null termination invariant for a short while. + buffer[..len].copy_from_slice(bytes); + self.buffer = &mut buffer[len..]; + + // INVARIANT: We null terminate the buffer. + self.buffer[0] = 0; + + Ok(()) + } +} + If you insist, I can write something like fn format_to_buffer(buffer: &mut [u8], args: fmt::Arguments) -> fmt::Result although I am not sure I see the point of this change. Best regards, Andreas Hindborg