Re: [PATCH v2] mm/mremap: Fail map duplication attempts for private mappings

[Mike Kravetz <mike.kravetz@oracle.com>]

On 07/21/2017 07:36 AM, Michal Hocko wrote:
> On Thu 20-07-17 13:37:59, Mike Kravetz wrote:
>> mremap will create a 'duplicate' mapping if old_size == 0 is
>> specified.  Such duplicate mappings make no sense for private
>> mappings.
> 
> sorry for the nit picking but this is not true strictly speaking.
> It makes some sense, arguably (e.g. take an atomic snapshot of the
> mapping). It doesn't make any sense with the _current_ implementation.
> 
>> 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.  Also, print a warning
>> message (once) if such an attempt is made.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> 
> I do not insist on the comment update suggested
> http://lkml.kernel.org/r/20170720082058.GF9058@dhcp22.suse.cz
> but I would appreciate it...
> 
> Other than that looks reasonably to me
> 
> Acked-by: Michal Hocko <mhocko@suse.com>

My apologies.  I overlooked your comment about the comment when
creating the patch.  Below is the patch with commit message and
comment updated.

>From 5c4a1602bd6a942544ed011dc0a72fd258e874b2 Mon Sep 17 00:00:00 2001
From: Mike Kravetz <mike.kravetz@oracle.com>
Date: Wed, 12 Jul 2017 13:52:47 -0700
Subject: [PATCH] mm/mremap: Fail map duplication attempts for private mappings

mremap will attempt to create a 'duplicate' mapping if old_size
== 0 is specified.  In the case of private mappings, mremap
will actually create a fresh separate private mapping unrelated
to the original.  This does not fit with the design semantics of
mremap as the intention is to create a new mapping based on the
original.

Therefore, return EINVAL in the case where an attempt is made
to duplicate a private mapping.  Also, print a warning message
(once) if such an attempt is made.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 mm/mremap.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c
index cd8a1b1..75b167d 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -383,6 +383,19 @@ 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 an attempt is made to 'duplicate'
+	 * a mapping.  This makes no sense for private mappings as it will
+	 * instead create a fresh/new mapping unrelated to the original.  This
+	 * is contrary to the basic idea of mremap which creates new mappings
+	 * based on the original.  There are no known use cases for this
+	 * behavior.  As a result, fail such attempts.
+	 */
+	if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) {
+		pr_warn_once("%s (%d): attempted to duplicate a private mapping with mremap.  This is not supported.\n", current->comm, current->pid);
+		return ERR_PTR(-EINVAL);
+	}
+
 	if (is_vm_hugetlb_page(vma))
 		return ERR_PTR(-EINVAL);
 
-- 
2.7.5

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