linux-kernel - Re: lock in vsprintf(): was: Re: [PATCH] of: Add printf '%pOFm' for generating modalias

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Message-ID: <84wmfxm6em.fsf@jogness.linutronix.de>
Date: Wed, 18 Dec 2024 15:13:13 +0106
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>, "Rob Herring (Arm)" <robh@...nel.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, Andy Shevchenko
 <andriy.shevchenko@...ux.intel.com>, Rasmus Villemoes
 <linux@...musvillemoes.dk>, Sergey Senozhatsky <senozhatsky@...omium.org>,
 Jonathan Corbet <corbet@....net>, Saravana Kannan <saravanak@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Peter Zijlstra
 <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Zijun Hu
 <quic_zijuhu@...cinc.com>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: lock in vsprintf(): was: Re: [PATCH] of: Add printf '%pOFm' for
 generating modalias

On 2024-12-18, Petr Mladek <pmladek@...e.com> wrote:
> My concern is taking a lock when processing a printf format, see
> below for more details.

Your concern is valid! printk() uses vsnprintf() to format records for
the kernel log. printk() may be called from contexts where locking is
forbidden (such as NMI). If vsnprintf() can take a lock, then either:

vsnprintf() must be made to be lockless

  or

printk() must take another approach to string formatting

  or

we accept that printk() can deadlock for certain format types in certain
contexts.

John Ogness