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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Z2Q2TcM6QPUIIyLC@pathway.suse.cz>
Date: Thu, 19 Dec 2024 16:05:49 +0100
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: "Rob Herring (Arm)" <robh@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Wed 2024-12-18 15:13:13, John Ogness wrote:
> 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

There were the ideas to introduce a %pX[1] or %pf[2] formats. They would
allow to pass pointer to a custom callback and parameters which
then would be used by vsprintf(). This might allow to make it more
obvious that the given vsprintf()/printk() might do some locking.

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

I see that %pOFm does not add any new locking dependency.
The same lock (devtree_lock) is already taken, for example,
by %pOFC format.

I do not want to revert everything now just because of theoretical
problems. It somehow works because people use these formats only
in dedicated subsystems. Also lockdep is able to catch a lot of
problems.

Well, it would be nice to document the lock dependency in
Documentation/core-api/printk-formats.rst

Best Regards,
Petr