linux-kernel - Re: [PATCH v2] MIPS: kernel: Reserve exception base early to prevent corruption

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Date:   Sun, 7 Mar 2021 22:20:01 +0100
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Serge Semin <fancer.lancer@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, linux-mips@...r.kernel.org,
        linux-kernel@...r.kernel.org, Kamal Dasu <kdasu.kdev@...il.com>
Subject: Re: [PATCH v2] MIPS: kernel: Reserve exception base early to prevent
 corruption

On Sun, Mar 07, 2021 at 11:06:12PM +0300, Serge Semin wrote:
> > +
> > +	if (cpu_has_mips_r2_r6)
> > +		reserve_exception_space(0, 0x400);
> 
> Are you sure it shouldn't be (!cpu_has_mips_r2_r6)?. What I see here
> contradicts to what is said in Changelog v2.

d'oh, of course it has to be !cpu_has_mips_r2_r6.

> Anyway regarding the problem in general. AFAICS the next code uses the
> lowest memory to place some specific exception handlers:
> board_cache_error_setup pointer:
>   arch/mips/mm/c-r4k.c: r4k_cache_error_setup() - SiByte CPUs: CPU_SB1, CPU_SB1A (up to 0x180)
>   arch/mips/mm/c-octeon.c: octeon_cache_error_setup() - Cavium CPU: CPU_CAVIUM_OCTEON (up to 0x180)
> board_nmi_handler_setup pointer:
>   arch/mips/kernel/smp-bmips.c: bmips_nmi_handler_setup() - Broadcom CPU: CPU_BMIPS (up to 0x400)
>   arch/mips/loongson2ef/common/init.c: mips_nmi_setup() - Loongson 2E CPU: MACH_LOONGSON2EF (up to 0x400)
>   arch/mips/loongson64/init.c: mips_nmi_setup() - Loongson 64 CPU: MACH_LOONGSON64 (up to 0x400, VEIC:0xB00)
>   arch/mips/mti-malta/malta-init.c: mips_nmi_setup() - Malta CPU: MIPS_MALTA (up to 0x400, VEIC: 0xB00)
>   arch/mips/pistachio/init.c: mips_nmi_setup() - Pistachio CPU: MACH_PISTACHIO (up to 0x400, VEIC: 0xB00)
> board_ejtag_handler_setup:
>   arch/mips/mti-malta/malta-init.c: mips_ejtag_setup() - Malta CPU: MIPS_MALTA (up to 0x380, VEIC: 0xa80)
>   arch/mips/pistachio/init.c: mips_ejtag_setup() - Pistachio CPU: MACH_PISTACHIO (up to 0x380, VEIC: 0xa80)
> bmips_ebase_setup:
>   arch/mips/kernel/smp-bmips.c: bmips_ebase_setup() - Broadcom CPU: CPU_BMIPS (up to 0x400 - NMI/reset, and 0x1000 - normal)
> plat_mem_setup:
>   arch/mips/bmips/setup.c: bcm63xx_fixup_cpu1() - Broadcom CPU: CPU_BMIPS (up to 0x220)
>   
> 
> Are you sure all of them have "cpu_has_mips_r2_r6" macro returning
> true (false) in order to safely use the lowest region in accordance
> with the conditional statement you've added?

some of them are not R2 (SB1), others are. So best bet would be to
simply reserve the first 0x1000 bytes for every CPU and special handling
for the BMIPS case. Does this cover all cases ?

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]