linux-kernel - Re: [RFC PATCH] Allow optional module parameters

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Message-ID: <87a9m6sri3.fsf@rustcorp.com.au>
Date:	Mon, 01 Jul 2013 16:20:44 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Lucas De Marchi <lucas.demarchi@...fusion.mobi>
Cc:	Andy Lutomirski <luto@...capital.net>,
	linux-kernel@...r.kernel.org,
	Ben Hutchings <bhutchings@...arflare.com>,
	linux-kbuild@...r.kernel.org, Jon Masters <jcm@...hat.com>
Subject: Re: [RFC PATCH] Allow optional module parameters

Rusty Russell <rusty@...tcorp.com.au> writes:
> Lucas De Marchi <lucas.demarchi@...fusion.mobi> writes:
>> Hi Rusty,
>>
>> On Mon, Mar 18, 2013 at 11:32 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
>>> Andy Lutomirski <luto@...capital.net> writes:
>>>> On Sun, Mar 17, 2013 at 7:24 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
>>>>> Andy Lutomirski <luto@...capital.net> writes:
>>>>>> On Thu, Mar 14, 2013 at 10:03 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
>>>>>>> Err, yes.  Don't remove module parameters, they're part of the API.  Do
>>>>>>> you have a particular example?
>>>>>>
>>>>>> So things like i915.i915_enable_ppgtt, which is there to enable
>>>>>> something experimental, needs to stay forever once the relevant
>>>>>> feature becomes non-experimental and non-optional?  This seems silly.
>>> ...
>>>>>> Having the module parameter go away while still allowing the module to
>>>>>> load seems like a good solution (possibly with a warning in the logs
>>>>>> so the user can eventually delete the parameter).
>>>>>
>>>>> Why not do that for *every* missing parameter then?  Why have this weird
>>>>> notation where the user must know that the parameter might one day go
>>>>> away?
>>>>
>>>> Fair enough.  What about the other approach, then?  Always warn if an
>>>> option doesn't match (built-in or otherwise) but load the module
>>>> anyways.
>>>
>>> What does everyone think of this?  Jon, Lucas, does this match your
>>> experience?
>>>
>>> Thanks,
>>> Rusty.
>>>
>>> Subject: modules: don't fail to load on unknown parameters.
>>>
>>> Although parameters are supposed to be part of the kernel API, experimental
>>> parameters are often removed.  In addition, downgrading a kernel might cause
>>> previously-working modules to fail to load.
>>
>> I agree with this reasoning
>>
>>>
>>> On balance, it's probably better to warn, and load the module anyway.
>>
>> However loading the module anyway would bring at least one drawback:
>> if the user made a typo when passing the option the module would load
>> anyway and he will probably not even look in the log, since there's
>> was no errors from modprobe.

OK, so I've had this patch on the backburner, but noone has come up with
anything better so I'll queue it into modules-next now.

Thanks,
Rusty.

modules: don't fail to load on unknown parameters.

Although parameters are supposed to be part of the kernel API, experimental
parameters are often removed.  In addition, downgrading a kernel might cause
previously-working modules to fail to load.

On balance, it's probably better to warn, and load the module anyway.
This may let through a typo, but at least the logs will show it.

Reported-by: Andy Lutomirski <luto@...capital.net>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>

diff --git a/kernel/module.c b/kernel/module.c
index 3c2c72d..46db10a 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3206,6 +3206,17 @@ out:
 	return err;
 }
 
+static int unknown_module_param_cb(char *param, char *val, const char *modname)
+{
+	/* Check for magic 'dyndbg' arg */ 
+	int ret = ddebug_dyndbg_module_param_cb(param, val, modname);
+	if (ret != 0) {
+		printk(KERN_WARNING "%s: unknown parameter '%s' ignored\n",
+		       modname, param);
+	}
+	return 0;
+}
+
 /* Allocate and load the module: note that size of section 0 is always
    zero, and we rely on this for optional sections. */
 static int load_module(struct load_info *info, const char __user *uargs,
@@ -3292,7 +3303,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
 
 	/* Module is ready to execute: parsing args may do that. */
 	err = parse_args(mod->name, mod->args, mod->kp, mod->num_kp,
-			 -32768, 32767, &ddebug_dyndbg_module_param_cb);
+			 -32768, 32767, unknown_module_param_cb);
 	if (err < 0)
 		goto bug_cleanup;
 

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