linux-kernel - Re: [PATCH v3 1/2] kaslr: shift linear region randomization ahead of memory

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Date:   Sun, 12 May 2019 23:41:30 +0800
From:   pierre kuo <vichy.kuo@...il.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Price <steven.price@....com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 1/2] kaslr: shift linear region randomization ahead of memory_limit

hi Ard:
> > The following is schematic diagram of the program before and after the
> > modification.
> >
> > Before:
> > if (memstart_addr + linear_region_size < memblock_end_of_DRAM()) {} --(a)
> > if (memory_limit != PHYS_ADDR_MAX) {}                               --(b)
> > if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && phys_initrd_size) {}       --(c)
> > if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {}                           --(d)*
> >
> > After:
> > if (memstart_addr + linear_region_size < memblock_end_of_DRAM()) {} --(a)
> > if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {}                           --(d)*
> > if (memory_limit != PHYS_ADDR_MAX) {}                               --(b)
> > if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && phys_initrd_size) {}       --(c)
> >
> > After grouping modification of memstart_address by moving linear region
> > randomization ahead of memory_init, driver can safely using macro,
> > __phys_to_virt, in (b) or (c), if necessary.
> >
>
> Why is this an advantage?

1st, by putting (d) right behind (a),  driver can safely uses macro,
__phys_to_virt that depends on memstart_address, in (b) and (c) if necessary.
That means:
(a)
(d)
----- finish modification of memstart_address --------
(b) --> can safely use __phys_to_virt
(c) --> can safely use __phys_to_virt

2nd, it can make current driver more concise.
Letting (d) right behind (a), as below v3 patch shows,
https://lkml.org/lkml/2019/4/8/683
it can put initrd_start/initrd_end to be calculated only when linear
mapping check
pass and concise the driver by eliminating
"if (IS_ENABLED(CONFIG_BLK_DEV_INITRD) && phys_initrd_size) " as patch shows.

Thanks for your message.