Re: [PATCH] mm/mremap: Fail map duplication attempts for private mappings

[Mike Kravetz]

On 07/13/2017 12:11 PM, Vlastimil Babka wrote:
> [+CC linux-api]
> 
> On 07/13/2017 05:58 PM, Mike Kravetz wrote:
>> mremap will create a 'duplicate' mapping if old_size == 0 is
>> specified.  Such duplicate mappings make no sense for private
>> mappings.  If duplication is attempted for a private mapping,
>> mremap creates a separate private mapping unrelated to the
>> original mapping and makes no modifications to the original.
>> This is contrary to the purpose of mremap which should return
>> a mapping which is in some way related to the original.
>>
>> Therefore, return EINVAL in the case where if an attempt is
>> made to duplicate a private mapping.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

After considering Michal's concerns with follow on patch, it appears
this patch provides the most desired behavior.  Any other concerns
or issues with this patch?

If this moves forward, I will create man page updates to describe the
mremap(old_size == 0) behavior.

-- 
Mike Kravetz

> 
>> ---
>>  mm/mremap.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/mm/mremap.c b/mm/mremap.c
>> index cd8a1b1..076f506 100644
>> --- a/mm/mremap.c
>> +++ b/mm/mremap.c
>> @@ -383,6 +383,13 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
>>  	if (!vma || vma->vm_start > addr)
>>  		return ERR_PTR(-EFAULT);
>>  
>> +	/*
>> +	 * !old_len  is a special case where a mapping is 'duplicated'.
>> +	 * Do not allow this for private mappings.
>> +	 */
>> +	if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE)))
>> +		return ERR_PTR(-EINVAL);
>> +
>>  	if (is_vm_hugetlb_page(vma))
>>  		return ERR_PTR(-EINVAL);
>>  
>>
>