linux-kernel - Re: [PATCH v3 0/5] rust: Add bug/warn abstractions

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Message-Id: <20250227.155417.1610168537890385926.fujita.tomonori@gmail.com>
Date: Thu, 27 Feb 2025 15:54:17 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: a.hindborg@...nel.org
Cc: fujita.tomonori@...il.com, linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org, x86@...nel.org,
 linux-riscv@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [PATCH v3 0/5] rust: Add bug/warn abstractions

On Wed, 26 Feb 2025 20:39:45 +0100
Andreas Hindborg <a.hindborg@...nel.org> wrote:

> How does this series compare/overlap with [1] ?
> 
> [1] https://lore.kernel.org/all/20241126-pr_once_macros-v4-0-410b8ca9643e@tuta.io/

No overlap. Each solves a different problem. Both are necessary.

This patchset enables Rust code to call C's BUG/WARN properly.

Currently, Rust's BUG() is a simple wrapper for C's BUG()
(rust/helpers/bug.c). I added BUG() to rnull's init() and got the
following:

# insmod /root/rnull_mod.ko
rnull_mod: Rust null_blk loaded
------------[ cut here ]------------
kernel BUG at rust/helpers/bug.c:7!
Oops: invalid opcode: 0000 [#1] SMP
CPU: 0 UID: 0 PID: 31 Comm: insmod Not tainted 6.14.0-rc1+ #103
RIP: 0010:rust_helper_BUG+0x8/0x10
(snip)

This is NOT debug information that we expect. The problem is that
BUG/WARN feature (lib/bug.c) can only be used from assembly.

This patchset includes only warn() but with bug() implementation on
top of this patchset, I got:

# insmod /root/rnull_mod.ko
rnull_mod: Rust null_blk loaded
------------[ cut here ]------------
WARNING: CPU: 0 PID: 31 at /home/fujita/git/linux-rust/drivers/block/rnull.rs:46 _RNvXCsafUg3oOYix8_5rnullNtB2_13NullBlkModu]
Modules linked in: rnull_mod(+)
CPU: 0 UID: 0 PID: 31 Comm: insmod Not tainted 6.14.0-rc1+ #104
RIP: 0010:_RNvXCsafUg3oOYix8_5rnullNtB2_13NullBlkModuleNtCsaYBeKL739Xz_6kernel13InPlaceModule4init+0x71/0x4f0 [rnull_mod]

The [1] patchset adds an abstraciton for include/linux/once_lite.h,
'call a function once' feature, with pr_*_once() implementation.

pr_*_once() just calls printk() once. Unlike BUG/WARN, no debug
information (call place, registers, stack trace, etc).

The only connection between two patchset is that WARN_ONCE() can be
built on top of both like the C side.