linux-kernel - Re: [PATCH net-next v3 5/8] rust: time: Add wrapper for fsleep function

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Message-ID: <CAH5fLgjTGmD0=9wJRP+aNtHC2ab7e9tuRwnPZZt8RN3wpmZHBg@mail.gmail.com>
Date: Wed, 16 Oct 2024 10:29:12 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: netdev@...r.kernel.org, rust-for-linux@...r.kernel.org, andrew@...n.ch, 
	hkallweit1@...il.com, tmgross@...ch.edu, ojeda@...nel.org, 
	alex.gaynor@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com, 
	benno.lossin@...ton.me, a.hindborg@...sung.com, anna-maria@...utronix.de, 
	frederic@...nel.org, tglx@...utronix.de, arnd@...db.de, jstultz@...gle.com, 
	sboyd@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v3 5/8] rust: time: Add wrapper for fsleep function

On Wed, Oct 16, 2024 at 5:54 AM FUJITA Tomonori
<fujita.tomonori@...il.com> wrote:
>
> Add a wrapper for fsleep, flexible sleep functions in
> `include/linux/delay.h` which typically deals with hardware delays.
>
> The kernel supports several `sleep` functions to handle various
> lengths of delay. This adds fsleep, automatically chooses the best
> sleep method based on a duration.
>
> `sleep` functions including `fsleep` belongs to TIMERS, not
> TIMEKEEPING. They are maintained separately. rust/kernel/time.rs is an
> abstraction for TIMEKEEPING. To make Rust abstractions match the C
> side, add rust/kernel/time/delay.rs for this wrapper.
>
> fsleep() can only be used in a nonatomic context. This requirement is
> not checked by these abstractions, but it is intended that klint [1]
> or a similar tool will be used to check it in the future.
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
> Link: https://rust-for-linux.com/klint [1]
> ---
>  rust/helpers/helpers.c    |  1 +
>  rust/helpers/time.c       |  8 ++++++++
>  rust/kernel/time.rs       |  4 +++-
>  rust/kernel/time/delay.rs | 31 +++++++++++++++++++++++++++++++
>  4 files changed, 43 insertions(+), 1 deletion(-)
>  create mode 100644 rust/helpers/time.c
>  create mode 100644 rust/kernel/time/delay.rs
>
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 30f40149f3a9..c274546bcf78 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -21,6 +21,7 @@
>  #include "slab.c"
>  #include "spinlock.c"
>  #include "task.c"
> +#include "time.c"
>  #include "uaccess.c"
>  #include "wait.c"
>  #include "workqueue.c"
> diff --git a/rust/helpers/time.c b/rust/helpers/time.c
> new file mode 100644
> index 000000000000..7ae64ad8141d
> --- /dev/null
> +++ b/rust/helpers/time.c
> @@ -0,0 +1,8 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/delay.h>
> +
> +void rust_helper_fsleep(unsigned long usecs)
> +{
> +       fsleep(usecs);
> +}
> diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
> index 9b0537b63cf7..d58daff6f928 100644
> --- a/rust/kernel/time.rs
> +++ b/rust/kernel/time.rs
> @@ -2,12 +2,14 @@
>
>  //! Time related primitives.
>  //!
> -//! This module contains the kernel APIs related to time and timers that
> +//! This module contains the kernel APIs related to time that
>  //! have been ported or wrapped for usage by Rust code in the kernel.
>  //!
>  //! C header: [`include/linux/jiffies.h`](srctree/include/linux/jiffies.h).
>  //! C header: [`include/linux/ktime.h`](srctree/include/linux/ktime.h).
>
> +pub mod delay;
> +
>  /// The number of nanoseconds per microsecond.
>  pub const NSEC_PER_USEC: i64 = bindings::NSEC_PER_USEC as i64;
>
> diff --git a/rust/kernel/time/delay.rs b/rust/kernel/time/delay.rs
> new file mode 100644
> index 000000000000..dc7e2b3a0ab2
> --- /dev/null
> +++ b/rust/kernel/time/delay.rs
> @@ -0,0 +1,31 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Delay and sleep primitives.
> +//!
> +//! This module contains the kernel APIs related to delay and sleep that
> +//! have been ported or wrapped for usage by Rust code in the kernel.
> +//!
> +//! C header: [`include/linux/delay.h`](srctree/include/linux/delay.h).
> +
> +use crate::time;
> +use core::ffi::c_ulong;
> +
> +/// Sleeps for a given duration at least.
> +///
> +/// Equivalent to the kernel's [`fsleep`], flexible sleep function,
> +/// which automatically chooses the best sleep method based on a duration.
> +///
> +/// `Delta` must be longer than one microsecond.

Why is this required? Right now you just round up to one microsecond,
which seems okay.

> +/// This function can only be used in a nonatomic context.
> +pub fn fsleep(delta: time::Delta) {
> +    // SAFETY: FFI call.
> +    unsafe {
> +        // Convert the duration to microseconds and round up to preserve
> +        // the guarantee; fsleep sleeps for at least the provided duration,
> +        // but that it may sleep for longer under some circumstances.
> +        bindings::fsleep(
> +            ((delta.as_nanos() + time::NSEC_PER_USEC - 1) / time::NSEC_PER_USEC) as c_ulong,

You probably want this:

delta.as_nanos().saturating_add(time::NSEC_PER_USEC - 1) / time::NSEC_PER_USEC

This would avoid a crash if someone passes i64::MAX nanoseconds and
CONFIG_RUST_OVERFLOW_CHECKS is enabled.

Alice