linux-kernel - Re: [PATCH] mm/mmap: Tighten up cmdline_parse_stack_guard

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Date:   Wed, 30 Aug 2023 08:47:12 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mmap: Tighten up cmdline_parse_stack_guard_gap()



On 8/29/23 18:21, Matthew Wilcox wrote:
> On Mon, Aug 28, 2023 at 10:52:12AM +0530, Anshuman Khandual wrote:
>> -static int __init cmdline_parse_stack_guard_gap(char *p)
>> +static int __init cmdline_parse_stack_guard_gap(char *str)
>>  {
>>  	unsigned long val;
>> -	char *endptr;
>>  
>> -	val = simple_strtoul(p, &endptr, 10);
>> -	if (!*endptr)
>> -		stack_guard_gap = val << PAGE_SHIFT;
>> +	if (!str)
>> +		return 0;
> 
> Please explain how this function can be called with a NULL pointer.

This is an additional check just in case. We have similar constructs
in the following __setup() functions as well.

__setup("hashdist=", set_hashdist)
__setup("numa_balancing=", setup_numabalancing)
__setup("transparent_hugepage=", setup_transparent_hugepage)

Also it might be a better to warn, when returning unhandled with 0
like in those scenarios.

> 
>> -	return 1;
>> +	val = simple_strtoul(str, &str, 10);
>> +	if (!*str && val) {
>> +		stack_guard_gap = val << PAGE_SHIFT;
>> +		return 1;
>> +	}
>> +	return 0;
>>  }
> 
> Now you've removed the abillity for someone to say stack_guard_gap=0,
> which seems potentially useful.

In that case, should the following two scenarios be differentiated ?

* stack_guard_gap=	- Retains DEFAULT_STACK_GUARD_GAP
* stack_guard_gap=0	- Changes to 0 pages