Re: [PATCH v2] hpet: Fix missing '=' character in the __setup() code of hpet_mmap_enable

[Buland Singh]

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@xxxxxxxxxx>
> > > > ---
> > > >    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