linux-kernel - Re: [PATCH v2 2/2] reboot: fix parsing of reboot cpu number

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Date:   Fri, 30 Oct 2020 15:30:49 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Matteo Croce <mcroce@...ux.microsoft.com>
Cc:     linux-kernel@...r.kernel.org, Guenter Roeck <linux@...ck-us.net>,
        Arnd Bergmann <arnd@...db.de>, Mike Rapoport <rppt@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Robin Holt <robinmholt@...il.com>,
        Fabian Frederick <fabf@...net.be>, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/2] reboot: fix parsing of reboot cpu number

On Tue 2020-10-27 14:35:45, Matteo Croce wrote:
> From: Matteo Croce <mcroce@...rosoft.com>
> 
> The kernel cmdline reboot= argument allows to specify the CPU used
> for rebooting, with the syntax `s####` among the other flags, e.g.
> 
>   reboot=soft,s4
>   reboot=warm,s31,force
> 
> In the early days the parsing was done with simple_strtoul(), later
> deprecated in favor of the safer kstrtoint() which handles overflow.
> 
> But kstrtoint() returns -EINVAL if there are non-digit characters
> in a string, so if this flag is not the last given, it's silently
> ignored as well as the subsequent ones.
> 
> To fix it, revert the usage of simple_strtoul(), which is no longer
> deprecated, and restore the old behaviour.
> 
> While at it, merge two identical code blocks into one.

> --- a/kernel/reboot.c
> +++ b/kernel/reboot.c
> @@ -552,25 +552,19 @@ static int __init reboot_setup(char *str)
>  
>  		case 's':
>  		{
> -			int rc;
> -
> -			if (isdigit(*(str+1))) {
> -				rc = kstrtoint(str+1, 0, &reboot_cpu);
> -				if (rc)
> -					return rc;
> -				if (reboot_cpu >= num_possible_cpus()) {
> -					reboot_cpu = 0;
> -					return -ERANGE;
> -				}
> -			} else if (str[1] == 'm' && str[2] == 'p' &&
> -				   isdigit(*(str+3))) {
> -				rc = kstrtoint(str+3, 0, &reboot_cpu);
> -				if (rc)
> -					return rc;
> -				if (reboot_cpu >= num_possible_cpus()) {
> -					reboot_cpu = 0;

                                                     ^^^^^^

> +			int cpu;
> +
> +			/*
> +			 * reboot_cpu is s[mp]#### with #### being the processor
> +			 * to be used for rebooting. Skip 's' or 'smp' prefix.
> +			 */
> +			str += str[1] == 'm' && str[2] == 'p' ? 3 : 1;
> +
> +			if (isdigit(str[0])) {
> +				cpu = simple_strtoul(str, NULL, 0);
> +				if (cpu >= num_possible_cpus())
>  					return -ERANGE;
> -				}
> +				reboot_cpu = cpu;

The original value stays when the new one is out of range. It is
small functional change that should get mentioned in the commit
message or better fixed separately.

Hmm, I suggest to split this into 3 patches and switch the order:

  + 1st patch should simply revert the commit 616feab75397
   ("kernel/reboot.c: convert simple_strtoul to kstrtoint").

  + 2nd patch should merge the two branches without any
    functional change.

  + 3rd patch should add the check for num_possible_cpus()
    and update the value only when it is valid.

I am sorry that I did not suggested this when reviewed v1.
I have missed this functional change at that time.

Best Regards,
Petr