linux-kernel - Re: [PATCH v3 3/6] riscv: Move DTB_EARLY_BASE

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Date:   Fri, 27 Jan 2023 15:27:40 +0000
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Alexandre Ghiti <alexghiti@...osinc.com>
CC:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Conor Dooley <conor@...nel.org>,
        <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        <kasan-dev@...glegroups.com>, <linux-efi@...r.kernel.org>
Subject: Re: [PATCH v3 3/6] riscv: Move DTB_EARLY_BASE_VA to the kernel
 address space

Hey Alex,

On Wed, Jan 25, 2023 at 09:23:30AM +0100, Alexandre Ghiti wrote:
> The early virtual address should lie in the kernel address space for
> inline kasan instrumentation to succeed, otherwise kasan tries to
> dereference an address that does not exist in the address space (since
> kasan only maps *kernel* address space, not the userspace).
> 
> Simply use the very first address of the kernel address space for the
> early fdt mapping.
> 
> It allowed an Ubuntu kernel to boot successfully with inline
> instrumentation.
> 
> Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>

Been poking around in this area the last few days trying to hunt down
some bugs... Things look functionally the same w/ this patch and we do
get rid of the odd looking pointer which is nice.
Reviewed-by: Conor Dooley <conor.dooley@...rochip.com>

Probably would've made the cause of 50e63dd8ed92 ("riscv: fix reserved
memory setup") more difficult to find so glad I got that out of the way
well before this patch!

Thanks,
Conor.

> ---
>  arch/riscv/mm/init.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 478d6763a01a..87f6a5d475a6 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -57,7 +57,7 @@ unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]
>  EXPORT_SYMBOL(empty_zero_page);
>  
>  extern char _start[];
> -#define DTB_EARLY_BASE_VA      PGDIR_SIZE
> +#define DTB_EARLY_BASE_VA      (ADDRESS_SPACE_END - (PTRS_PER_PGD / 2 * PGDIR_SIZE) + 1)
>  void *_dtb_early_va __initdata;
>  uintptr_t _dtb_early_pa __initdata;
>  
> -- 
> 2.37.2
> 
> 

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