linux-kernel - Re: [PATCH 1/3] kernel/sysctl: support setting sysctl parameters from kernel command line

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Date:   Wed, 1 Apr 2020 13:01:47 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-mm@...ck.org, Ivan Teterevkov <ivan.teterevkov@...anix.com>,
        Michal Hocko <mhocko@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Guilherme G . Piccoli" <gpiccoli@...onical.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH 1/3] kernel/sysctl: support setting sysctl parameters from
 kernel command line

On 3/31/20 12:44 AM, Luis Chamberlain wrote:
>> +	} else if (wret != len) {
>> +		pr_err("Wrote only %ld bytes of %d writing to proc file %s to set sysctl parameter '%s=%s'",
>> +			wret, len, path, param, val);
>> +	}
>> +
>> +	err = filp_close(file, NULL);
>> +	if (err)
>> +		pr_err("Error %pe closing proc file to set sysctl parameter '%s=%s'",
>> +			ERR_PTR(err), param, val);
>> +out:
>> +	kfree(path);
>> +	return 0;
>> +}
>> +
>> +void do_sysctl_args(void)
>> +{
>> +	char *command_line;
>> +	struct vfsmount *proc_mnt = NULL;
>> +
>> +	command_line = kstrdup(saved_command_line, GFP_KERNEL);
> 
> can you use kstrndup() ? And then use kfree_const()? Yes, feel free to

I don't follow, what am I missing? Do you mean this?

size_t len = strlen(saved_command_line);
command_line = kstrndup(saved_command_line, len, GFP_KERNEL);

What would be the advantage over plain kstrdup()?
As for kfree_const(), when would command_line be .rodata? I don't see using
kstrndup() resulting in that.

> move __kstrncpy() to a generic kstrncpy().
> 
>> +	if (!command_line)
>> +		panic("%s: Failed to allocate copy of command line\n", __func__);
>> +
>> +	parse_args("Setting sysctl args", command_line,
>> +		   NULL, 0, -1, -1, &proc_mnt, process_sysctl_arg);
>> +
>> +	if (proc_mnt)
>> +		kern_unmount(proc_mnt);
>> +
>> +	kfree(command_line);
>> +}
> 
> Then, can we get this tested as part of lib/test_sysctl.c with its
> respective tools/testing/selftests/sysctl/sysctl.sh ?

Hmm so I add some sysctl to the test "module" (in fact the 'config' file says it
should be build with 'y', which would be needed anyway) and expand the test
instructions so that the test kernel boot has to include it on the command line,
and then I verify it has been set? Or do you see a better way?

Thanks,
Vlastimil

>   Luis
>