linux-kernel - Re: [PATCH v2] hpet: Fix missing '=' character in the __setup() code of hpet_mmap

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Date:   Fri, 21 Dec 2018 17:48:38 +0530
From:   Buland Singh <bsingh@...hat.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     clemens@...isch.de, arnd@...db.de, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] hpet: Fix missing '=' character in the __setup() code
 of hpet_mmap_enable

On 12/20/18 7:39 PM, Greg KH wrote:
> On Thu, Dec 20, 2018 at 07:12:55PM +0530, Buland Singh wrote:
>> On 12/20/18 5:59 PM, Greg KH wrote:
>>> On Thu, Dec 20, 2018 at 05:35:24PM +0530, Buland Singh wrote:
>>>> Commit '3d035f580699 ("drivers/char/hpet.c: allow user controlled mmap for
>>>> user processes")' introduced a new kernel command line parameter hpet_mmap,
>>>> that is required to expose the memory map of the HPET registers to
>>>> user-space. Unfortunately the kernel command line parameter 'hpet_mmap' is
>>>> broken and never takes effect due to missing '=' character in the __setup()
>>>> code of hpet_mmap_enable.
>>>>
>>>> Before this patch:
>>>>
>>>> dmesg output with the kernel command line parameter hpet_mmap=1
>>>>
>>>> [    0.204152] HPET mmap disabled
>>>>
>>>> dmesg output with the kernel command line parameter hpet_mmap=0
>>>>
>>>> [    0.204192] HPET mmap disabled
>>>>
>>>> After this patch:
>>>>
>>>> dmesg output with the kernel command line parameter hpet_mmap=1
>>>>
>>>> [    0.203945] HPET mmap enabled
>>>>
>>>> dmesg output with the kernel command line parameter hpet_mmap=0
>>>>
>>>> [    0.204652] HPET mmap disabled
>>>>
>>>> Fixes: 3d035f580699 ("drivers/char/hpet.c: allow user controlled mmap for user processes")
>>>> Signed-off-by: Buland Singh <bsingh@...hat.com>
>>>> ---
>>>>    drivers/char/hpet.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
>>>> index 4a22b4b41aef..9bffcd37cc7b 100644
>>>> --- a/drivers/char/hpet.c
>>>> +++ b/drivers/char/hpet.c
>>>> @@ -377,7 +377,7 @@ static __init int hpet_mmap_enable(char *str)
>>>>    	pr_info("HPET mmap %s\n", hpet_mmap_enabled ? "enabled" : "disabled");
>>>>    	return 1;
>>>>    }
>>>> -__setup("hpet_mmap", hpet_mmap_enable);
>>>> +__setup("hpet_mmap=", hpet_mmap_enable);
>>
>> Hello Greag,
>>
>>> This has _never_ worked?  Since 3.13?
>>
>> Yes, that's true :)
>>
>>> Why not just remove the thing as it is obvious no one actually has ever used it.  > That would make the code even simpler :)
>>
>> Data Plane Development Kit (DPDK)[1] provides API that requires the CONFIG_HPET_MMAP
>> kernel configuration option to be enabled[2]. Some end users might want to use the
>> HPET MMAP functionality within the application.
> 
> But, obviously, they really don't need to do that from the kernel
> command line as no one has ever noticed this didn't work :)
> 
> Also, that page:
> 
>> [2] https://doc.dpdk.org/guides-18.08/linux_gsg/enable_func.html
> 
> Does not say to use this command line option either.  So if no one has
> ever used it, please, let us just delete it.
> 
> thanks,
> 
> greg k-h
> 

Hello Greg,
                                                                            
It is better to allow a user to 'enable/disable' the HPET mmap from the
kernel command line as per the requirement rather than recompiling the
kernel to 'enable/disable' this functionality.
                                                                            
Also as per the description in the initial patch (commit 3d035f58), the
'CONFIG_HPET_MMAP' Kconfig option has a security risk involved. Hence,
keeping the CONFIG_HPET_MMAP_DEFAULT (disabled) and allowing a user to
alter the default behavior using the kernel command line parameter
hpet_mmap is a better solution.

Regards,
Buland Singh