linux-kernel - Re: [PATCH v2] Add kernel parameter to blacklist modules

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Date:	Wed, 15 Jun 2016 09:38:49 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>, linux-kernel@...r.kernel.org
CC:	Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2] Add kernel parameter to blacklist modules



On 06/14/2016 05:20 PM, Rusty Russell wrote:
> Prarit Bhargava <prarit@...hat.com> writes:
>> Blacklisting a module in linux has long been a problem.  The current
>> procedure is to use rd.blacklist=module_name, however, that doesn't
>> cover the case after the initramfs and before a boot prompt (where one
>> is supposed to use /etc/modprobe.d/blacklist.conf to blacklist
>> runtime loading). Using rd.shell to get an early prompt is hit-or-miss,
>> and doesn't cover all situations AFAICT.
>>
>> This patch adds this functionality of permanently blacklisting a module
>> by its name via the kernel parameter module_blacklist=module_name.
>>
>> [v2]: Rusty, use core_param() instead of __setup(), and drop struct which
>> simplifies things.
>>
>> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
>> Cc: Jonathan Corbet <corbet@....net>
>> Cc: Rusty Russell <rusty@...tcorp.com.au>
>> Cc: linux-doc@...r.kernel.org
>> ---
>>  Documentation/kernel-parameters.txt |    3 +++
>>  kernel/module.c                     |   25 +++++++++++++++++++++++++
>>  2 files changed, 28 insertions(+)
>>
>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>> index 82b42c958d1c..c720b96f2efc 100644
>> --- a/Documentation/kernel-parameters.txt
>> +++ b/Documentation/kernel-parameters.txt
>> @@ -2295,6 +2295,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>  			Note that if CONFIG_MODULE_SIG_FORCE is set, that
>>  			is always true, so this option does nothing.
>>  
>> +	module_blacklist=  [KNL] Do not load a comma-separated list of
>> +			modules.  Useful for debugging problem modules.
>> +
>>  	mousedev.tap_time=
>>  			[MOUSE] Maximum time between finger touching and
>>  			leaving touchpad surface for touch to be considered
>> diff --git a/kernel/module.c b/kernel/module.c
>> index 5f71aa63ed2a..5ff5287b19a8 100644
>> --- a/kernel/module.c
>> +++ b/kernel/module.c
>> @@ -3155,6 +3155,28 @@ int __weak module_frob_arch_sections(Elf_Ehdr *hdr,
>>  	return 0;
>>  }
>>  
>> +/* module_blacklist is a comma-separated list of module names */
>> +static char *module_blacklist;
>> +static bool blacklisted(char *module_name)
>> +{
>> +	char *str, *entry;
>> +
>> +	if (!module_blacklist)
>> +		return false;
>> +
>> +	str = module_blacklist;
>> +	do {
>> +		entry = strsep(&str, ",");
>> +		if (!strcmp(module_name, entry)) {
>> +			pr_info("module %s is blacklisted\n", module_name);
>> +			return true;
>> +		}
> 
> strsep mangles the string; this will only work once :)
> 
> This is untested, and a little ugly:
> 
>        len = strlen(module_name);
>        
>        while ((p = strstr(p, module_name)) != NULL) {
>               if ((p == module_blacklist || p[-1] == ',') &&
>                   (p[len] == ',' || p[len] == '\0'))
>                         return true;
>               p += len;
>        }
>        return false;

Hmm ... yeah, a bit ugly.  I could also easily do:

        str = module_blacklist;
        do {
                if (str != module_blacklist)
                        module_blacklist[strlen(str) - 1] = ',';
                entry = strsep(&str, ",");
                if (!strcmp(module_name, entry)) {
                        pr_info("module %s is blacklisted\n", module_name);
                        if (str != module_blacklist)
                                module_blacklist[strlen(str) - 1] = ',';
                        return true;
                }
        } while (str);

which results in the correct behavior AFAICT with
"module_blacklist=dummy_module,prarit_module":

On boot:

[   23.737613] blacklist: comparing |dns_resolver| to |dummy_module|
[   23.743713] blacklist: comparing |dns_resolver| to |prarit_module|

when attempting to load dummy_module:

[   43.938798] blacklist: comparing |dummy_module| to |dummy_module|
[   43.944915] module dummy_module is blacklisted

and attempting to load another module after that (in this case ixgbe):

[   47.572557] blacklist: comparing |mdio| to |dummy_module|
[   47.577961] blacklist: comparing |mdio| to |prarit_module|
[   47.684713] blacklist: comparing |ixgbe| to |dummy_module|
[   47.690202] blacklist: comparing |ixgbe| to |prarit_module|

Any objection to that version?  Admittedly yours is shorter but I feel like mine
might be "easier" to read...

P.

> 
> Cheers,
> Rusty.
>