linux-kernel - Re: [PATCH 3/3] riscv: Make sure the kernel mapping does not overlap with IS_ERR

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Message-ID: <1778a14a-6bb0-7ff8-1b11-32be51c015da@ghiti.fr>
Date:   Mon, 26 Jul 2021 07:47:11 +0200
From:   Alex Ghiti <alex@...ti.fr>
To:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] riscv: Make sure the kernel mapping does not overlap
 with IS_ERR_VALUE

Hi Palmer,

Le 29/06/2021 à 11:13, Alexandre Ghiti a écrit :
> The check that is done in setup_bootmem currently only works for 32-bit
> kernel since the kernel mapping has been moved outside of the linear
> mapping for 64-bit kernel. So make sure that for 64-bit kernel, the kernel
> mapping does not overlap with the last 4K of the addressable memory.
> 
> Signed-off-by: Alexandre Ghiti <alex@...ti.fr>
> ---
>   arch/riscv/mm/init.c | 18 ++++++++++++++++--
>   1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index a1a0c4afa80f..a90c41bc9485 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -156,7 +156,7 @@ static void __init setup_bootmem(void)
>   {
>   	phys_addr_t vmlinux_end = __pa_symbol(&_end);
>   	phys_addr_t vmlinux_start = __pa_symbol(&_start);
> -	phys_addr_t max_mapped_addr = __pa(~(ulong)0);
> +	phys_addr_t __maybe_unused max_mapped_addr;
>   	phys_addr_t dram_end;
>   
>   #ifdef CONFIG_XIP_KERNEL
> @@ -179,14 +179,20 @@ static void __init setup_bootmem(void)
>   	memblock_reserve(vmlinux_start, vmlinux_end - vmlinux_start);
>   
>   	dram_end = memblock_end_of_DRAM();
> +#ifndef CONFIG_64BIT
>   	/*
>   	 * memblock allocator is not aware of the fact that last 4K bytes of
>   	 * the addressable memory can not be mapped because of IS_ERR_VALUE
>   	 * macro. Make sure that last 4k bytes are not usable by memblock
> -	 * if end of dram is equal to maximum addressable memory.
> +	 * if end of dram is equal to maximum addressable memory. For 64-bit
> +	 * kernel, this problem can't happen here as the end of the virtual
> +	 * address space is occupied by the kernel mapping then this check must
> +	 * be done in create_kernel_page_table.

This comment is wrong, I have just sent a patch fixing this:

"riscv: Fix comment regarding kernel mapping overlapping with IS_ERR_VALUE"

>   	 */
> +	max_mapped_addr = __pa(~(ulong)0);
>   	if (max_mapped_addr == (dram_end - 1))
>   		memblock_set_current_limit(max_mapped_addr - 4096);
> +#endif
>   
>   	min_low_pfn = PFN_UP(memblock_start_of_DRAM());
>   	max_low_pfn = max_pfn = PFN_DOWN(dram_end);
> @@ -556,6 +562,7 @@ static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size,
>   	uintptr_t va, end_va;
>   
>   	end_va = kernel_virt_addr + load_sz;
> +

I saw that you removed this useless new line, thanks.

>   	for (va = kernel_virt_addr; va < end_va; va += map_size)
>   		create_pgd_mapping(pgdir, va,
>   				   load_pa + (va - kernel_virt_addr),
> @@ -602,6 +609,13 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>   	/* Sanity check alignment and size */
>   	BUG_ON((PAGE_OFFSET % PGDIR_SIZE) != 0);
>   	BUG_ON((load_pa % map_size) != 0);
> +#ifdef CONFIG_64BIT
> +	/*
> +	 * The last 4K bytes of the addressable memory can not be mapped because
> +	 * of IS_ERR_VALUE macro.
> +	 */
> +	BUG_ON((kernel_virt_addr + load_sz) > ADDRESS_SPACE_END - SZ_4K);
> +#endif
>   
>   	pt_ops.alloc_pte = alloc_pte_early;
>   	pt_ops.get_pte_virt = get_pte_virt_early;
> 

The merge you have done is correct to me. Sorry about the above 
mistakes, should have caught those before.

Thanks,

Alex