Re: [PATCH 3/5] module: Restore the moduleparam prefix length check

[Petr Pavlu <petr.pavlu@suse.com>]

On 7/17/25 9:23 PM, Daniel Gomez wrote:
> On 30/06/2025 16.32, Petr Pavlu wrote:
>> The moduleparam code allows modules to provide their own definition of
>> MODULE_PARAM_PREFIX, instead of using the default KBUILD_MODNAME ".".
>>
>> Commit 730b69d22525 ("module: check kernel param length at compile time,
>> not runtime") added a check to ensure the prefix doesn't exceed
>> MODULE_NAME_LEN, as this is what param_sysfs_builtin() expects.
>>
>> Later, commit 58f86cc89c33 ("VERIFY_OCTAL_PERMISSIONS: stricter checking
>> for sysfs perms.") removed this check, but there is no indication this was
>> intentional.
>>
>> Since the check is still useful for param_sysfs_builtin() to function
>> properly, reintroduce it in __module_param_call(), but in a modernized form
>> using static_assert().
>>
>> While here, clean up the __module_param_call() comments. In particular,
>> remove the comment "Default value instead of permissions?", which comes
>> from commit 9774a1f54f17 ("[PATCH] Compile-time check re world-writeable
>> module params"). This comment was related to the test variable
>> __param_perm_check_##name, which was removed in the previously mentioned
>> commit 58f86cc89c33.
>>
>> Fixes: 58f86cc89c33 ("VERIFY_OCTAL_PERMISSIONS: stricter checking for sysfs perms.")
>> Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
>> ---
>>  include/linux/moduleparam.h | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/moduleparam.h b/include/linux/moduleparam.h
>> index bfb85fd13e1f..110e9d09de24 100644
>> --- a/include/linux/moduleparam.h
>> +++ b/include/linux/moduleparam.h
>> @@ -282,10 +282,9 @@ struct kparam_array
>>  #define __moduleparam_const const
>>  #endif
>>  
>> -/* This is the fundamental function for registering boot/module
>> -   parameters. */
>> +/* This is the fundamental function for registering boot/module parameters. */
>>  #define __module_param_call(prefix, name, ops, arg, perm, level, flags)	\
>> -	/* Default value instead of permissions? */			\
>> +	static_assert(sizeof(""prefix) - 1 <= MAX_PARAM_PREFIX_LEN);	\
> 
> Can you clarify if -1 to remove the dot from prefix?
> 
> Final code 
> 	static_assert(sizeof(""prefix) - 1 <= __MODULE_NAME_LEN);	\
> 
> with __MODULE_NAME_LEN being:
> 
> #define __MODULE_NAME_LEN (64 - sizeof(unsigned long))

Correct, -1 is to account for the dot at the end of the prefix.

I actually also wanted to assert that the prefix ends with a dot, but
unfortunately prefix[sizeof(prefix)-2] (with prefix being a string
literal) is not a constant expression in C.

-- 
Thanks,
Petr