linux-kernel - Re: [PATCH 1/5] lib: Add a generic version of devmem_is

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Message-ID: <a037dac961c989d027eab293a0280643@mailhost.ics.forth.gr>
Date:   Fri, 10 Jul 2020 08:48:17 +0300
From:   Nick Kossifidis <mick@....forth.gr>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>, mark.rutland@....com,
        steve@....org, gregory.0xf0@...il.com, catalin.marinas@....com,
        linus.walleij@...aro.org,
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        zaslonko@...ux.ibm.com, glider@...gle.com, krzk@...nel.org,
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        Palmer Dabbelt <palmer@...belt.com>, mhiramat@...nel.org,
        akpm@...ux-foundation.org, davem@...emloft.net
Subject: Re: [PATCH 1/5] lib: Add a generic version of devmem_is_allowed()

Στις 2020-07-10 08:38, Christoph Hellwig έγραψε:
> On Thu, Jul 09, 2020 at 11:49:21PM +0300, Mike Rapoport wrote:
>> > +#ifndef CONFIG_GENERIC_DEVMEM_IS_ALLOWED
>> > +extern int devmem_is_allowed(unsigned long pfn);
>> > +#endif
> 
> Nit: no need for the extern here.
> 
>> > +config GENERIC_LIB_DEVMEM_IS_ALLOWED
>> > +	bool
>> > +	select ARCH_HAS_DEVMEM_IS_ALLOWED
>> 
>> This seems to work the other way around from the usual Kconfig chains.
>> In the most cases ARCH_HAS_SOMETHING selects GENERIC_SOMETHING.
>> 
>> I believe nicer way would be to make
>> 
>> config STRICT_DEVMEM
>> 	bool "Filter access to /dev/mem"
>> 	depends on MMU && DEVMEM
>> 	depends on ARCH_HAS_DEVMEM_IS_ALLOWED || 
>> GENERIC_LIB_DEVMEM_IS_ALLOWED
>> 
>> config GENERIC_LIB_DEVMEM_IS_ALLOWED
>> 	bool
>> 
>> and then s/select ARCH_HAS_DEVMEM_IS_ALLOWED/select 
>> GENERIC_LIB_DEVMEM_IS_ALLOWED/
>> in the arch Kconfigs and drop ARCH_HAS_DEVMEM_IS_ALLOWED in the end.
> 
> To take a step back:  Is there any reason to not just always
> STRICT_DEVMEM? Maybe for a few architectures that don't currently
> support a strict /dev/mem the generic version isn't quite correct, but
> someone selecting the option and finding the issue is the best way to
> figure that out..
> 

During prototyping / testing having full access to all physical memory 
through /dev/mem is very useful. We should have it enabled by default 
but leave the config option there so that users / developers can disable 
it if needed IMHO.