linux-kernel - Re: [PATCH v14 1/5] x86/boot: Introduce get_acpi_rsdp() to parse RSDP in cmdline from KEXEC

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Date:   Tue, 18 Dec 2018 09:27:04 +0800
From:   Chao Fan <fanc.fnst@...fujitsu.com>
To:     Ingo Molnar <mingo@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <x86@...nel.org>, <bp@...en8.de>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>,
        <keescook@...omium.org>, <bhe@...hat.com>, <msys.mizuma@...il.com>,
        <indou.takao@...fujitsu.com>, <caoj.fnst@...fujitsu.com>
Subject: Re: [PATCH v14 1/5] x86/boot: Introduce get_acpi_rsdp() to parse
 RSDP in cmdline from KEXEC

On Mon, Dec 17, 2018 at 06:25:10PM +0100, Ingo Molnar wrote:
>
>* Chao Fan <fanc.fnst@...fujitsu.com> wrote:
>
>> Memory information in SRAT is necessary to fix the conflict between
>> KASLR and memory-hotremove.
>> 
>> ACPI SRAT (System/Static Resource Affinity Table) shows the details
>> about memory ranges, including ranges of memory provided by hot-added
>> memory devices. SRAT is introduced by Root System Description
>> Pointer(RSDP), so RSDP should be found firstly.
>> 
>> When booting form KEXEC/EFI/BIOS, the methods to find RSDP
>> are different. When booting from KEXEC, 'acpi_rsdp' may have been
>> added to cmdline, so parse cmdline to find RSDP.
>
>What is the nature of the basic conflict? Should the kernel only allow 
>hot-removing memory ranges that are explicitly marked as such in the 
>SRAT? I presume that's already so, correct? Or should the kernel avoiding 
>KASLR-randomizing into hot-removable memory areas?

The basic conflict is the kernel avoiding KASLR-randomizing into
hot-removable memory areas.

>
>Some minor nits:
>
>> Signed-off-by: Chao Fan <fanc.fnst@...fujitsu.com>
>> ---
>>  arch/x86/boot/compressed/acpi.c | 30 ++++++++++++++++++++++++++++++
>>  1 file changed, 30 insertions(+)
>>  create mode 100644 arch/x86/boot/compressed/acpi.c
>> 
>> diff --git a/arch/x86/boot/compressed/acpi.c b/arch/x86/boot/compressed/acpi.c
>> new file mode 100644
>> index 000000000000..44f19546c169
>> --- /dev/null
>> +++ b/arch/x86/boot/compressed/acpi.c
>> @@ -0,0 +1,30 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +#define BOOT_CTYPE_H
>> +#include "misc.h"
>> +#include "error.h"
>> +
>> +#include <linux/efi.h>
>> +#include <linux/numa.h>
>> +#include <linux/acpi.h>
>> +#include <asm/efi.h>
>> +
>> +/* Max length of 64-bit hex address string is 18, prefix "0x" + 16 hex digits */
>> +#define MAX_ADDRESS_LENGTH 18
>
>I'd suggest adding the +1 for the string termination \0 as well, and use 
>that below.
>
>Also, 'max address length' is way too generic (and hence pretty 
>meaningless) - how about MAX_HEX_ADDRESS_STRING_LEN or so?

OK, I will change the name and add the \0.

Thanks,
Chao Fan

>
>> +
>> +static acpi_physical_address get_acpi_rsdp(void)
>> +{
>> +#ifdef CONFIG_KEXEC
>> +	char val[MAX_ADDRESS_LENGTH+1];
>> +	unsigned long long res;
>> +	int len;
>> +
>> +	len = cmdline_find_option("acpi_rsdp", val, MAX_ADDRESS_LENGTH+1);
>> +	if (len > 0) {
>> +		char *e;
>> +
>> +		val[len] = '\0';
>> +		return (acpi_physical_address)simple_strtoull(val, &e, 16);
>
>'return' is not a function - no need for the parenthesis.
>
>Thanks,
>
>	Ingo
>
>