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

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Message-ID: <20200710055124.GA30265@infradead.org>
Date:   Fri, 10 Jul 2020 06:51:24 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Nick Kossifidis <mick@....forth.gr>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Mike Rapoport <rppt@...ux.ibm.com>, mark.rutland@....com,
        steve@....org, gregory.0xf0@...il.com, catalin.marinas@....com,
        linus.walleij@...aro.org,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        zaslonko@...ux.ibm.com, glider@...gle.com, krzk@...nel.org,
        zong.li@...ive.com, mchehab+samsung@...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()

On Fri, Jul 10, 2020 at 08:48:17AM +0300, Nick Kossifidis wrote:
> ???????? 2020-07-10 08:38, Christoph Hellwig ????????????:
> > 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.

I did not suggest to take the config option away.  Just to
unconditionally allow enabling the option on all architectures.