linux-kernel - Re: Writing to a const pointer: is this supposed to happen?

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Message-ID: <db0-5ef34880-ab-10c623c0@12577330>
Date:   Wed, 24 Jun 2020 14:34:45 +0200
From:   "Kars Mulder" <kerneldev@...smulder.nl>
To:     "Pavel Machek" <pavel@....cz>
Cc:     linux-kernel@...r.kernel.org,
        "Kai-Heng Feng" <kai.heng.feng@...onical.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Subject: Re: Writing to a const pointer: is this 
 supposed to happen?

On Tuesday, June 23, 2020 21:55 CEST, Pavel Machek wrote: 
> Odd, indeed... but not likely to cause immediate problems.
> 
> You may want to cc relevant maintainers, or even run git
> blame and contact author.

Thank you for your response.

The code was written by Kai-Heng Feng, whom I shall CC. The code is
part of the usbcore module, which does not have a maintainer listed in
MAINTAINERS, but the patch and most other recent patches to usbcore
were signed off exclusively by Greg Kroah-Hartman, so I guess that
makes him the de facto maintainer? I'll CC him as well.

I'm not sure whether it is easy to read the previous messages of this
thread if you got CC'ed just now, so I'll repeat/paraphrase the
important part of my initial mail for your convenience:

> In the file drivers/usb/core/quirks.c, I noticed that the function
> quirks_param_set writes to a const pointer, and would like to check
> whether this is ok with the kernel programming practices. Here are
> the relevant lines from the function (several lines omitted):
> 
> 	static int quirks_param_set(const char *val, const struct kernel_param *kp) {
> 		char *p, *field;
> 		for (i = 0, p = (char *)val; p && *p;) {
> 			field = strsep(&p, ":");
>
> In here a const pointer *val is cast into a non-const pointer and
> then written to by the function strsep, which replaces the first
> occurrence of the ':' token with a null-byte. Is this allowed?

CC: Kai-Heng Feng <kai.heng.feng@...onical.com>
CC: Greg Kroah-Hartman <gregkh@...uxfoundation.org>