linux-kernel - Re: [PATCH] twist: allow converting pr_devel()/pr_debug() into printk(KERN

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Date:   Mon, 25 May 2020 18:11:57 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] twist: allow converting pr_devel()/pr_debug() into
 printk(KERN_DEBUG)

On (20/05/25 10:42), Petr Mladek wrote:
> On Sun 2020-05-24 23:50:34, Tetsuo Handa wrote:
> > syzbot found a NULL pointer dereference bug inside mptcp_recvmsg() due to
> > ssock == NULL, but this bug manifested inside selinux_socket_recvmsg()
> > because pr_debug() was no-op [1].
> > 
> >   pr_debug("fallback-read subflow=%p",
> >            mptcp_subflow_ctx(ssock->sk));
> >   copied = sock_recvmsg(ssock, msg, flags);
> 
> The NULL pointer deference was found even without this patch.
> This patch would just cause that it will manifest itself on another
> place. What is the benefit, please?

Right, I don't get this patch. A NULL-deref is still a NULL pointer deref.
pr_debug() will fault reading one byte from the address and print something
like "fallback-read subflow=(efault)" to printk-safe buffer, but then
sock_recvmsg() is still going to do its thing.

	-ss