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Fri, 04 Dec 2020 07:10:29 -0800 (PST) Received: from ziepe.ca ([206.223.160.26]) by smtp.gmail.com with ESMTPSA id o9sm5206035qte.35.2020.12.04.07.10.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 04 Dec 2020 07:10:29 -0800 (PST) Received: from jgg by mlx with local (Exim 4.94) (envelope-from ) id 1klCj6-005uXv-6R; Fri, 04 Dec 2020 11:10:28 -0400 Date: Fri, 4 Dec 2020 11:10:28 -0400 From: Jason Gunthorpe To: David Hildenbrand Cc: Liu Zixian , akpm@linux-foundation.org, linmiaohe@huawei.com, louhongxiang@huawei.com, linux-mm@kvack.org, hushiyuan@huawei.com, stable@vger.kernel.org Subject: Re: [PATCH v2] fix mmap return value when vma is merged after call_mmap() Message-ID: <20201204151028.GZ5487@ziepe.ca> References: <20201203085350.22624-1-liuzixian4@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Dec 04, 2020 at 03:11:54PM +0100, David Hildenbrand wrote: > On 03.12.20 09:53, Liu Zixian wrote: > > On success, mmap should return the begin address of newly mapped area, > > but patch "mm: mmap: merge vma after call_mmap() if possible" > > set vm_start of newly merged vma to return value addr. > > Users of mmap will get wrong address if vma is merged after call_mmap(). > > We fix this by moving the assignment to addr before merging vma. > > > > Fixes: d70cec898324 ("mm: mmap: merge vma after call_mmap() if possible") > > Signed-off-by: Liu Zixian > > v2: > > We want to do "addr = vma->vm_start;" unconditionally, > > so move assignment to addr before if(unlikely) block. > > mm/mmap.c | 26 ++++++++++++-------------- > > 1 file changed, 12 insertions(+), 14 deletions(-) > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > index d91ecb00d38c..5c8b4485860d 100644 > > +++ b/mm/mmap.c > > @@ -1808,6 +1808,17 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > > if (error) > > goto unmap_and_free_vma; > > > > + /* Can addr have changed?? > > + * > > + * Answer: Yes, several device drivers can do it in their > > + * f_op->mmap method. -DaveM > > + * Bug: If addr is changed, prev, rb_link, rb_parent should > > + * be updated for vma_link() > > + */ > > > Why do we tolerate device drivers doing such stuff at all? > WARN_ON_ONCE() is just as good as BUG_ON() in many environments. If we > support it, then no warning. If we don't support it, then why tolerate it? The commit that introduced this seemed pretty clear it is to catch possibly wrong drivers. I suppose the idea was to give a migration time where things would "work" and drivers could be fixed. Since it has now been 8 years it should be either dropped or turned into: /* Drivers are not permitted to change vm_start */ if (WARN_ON(addr != vma->vm_start)) { err = EINVAL goto unmap_and_free_vma } commit 2897b4d29d9fca82a57b09b8a216a5d604966e4b Author: Joonsoo Kim Date: Wed Dec 12 13:51:53 2012 -0800 mm: WARN_ON_ONCE if f_op->mmap() change vma's start address During reviewing the source code, I found a comment which mention that after f_op->mmap(), vma's start address can be changed. I didn't verify that it is really possible, because there are so many f_op->mmap() implementation. But if there are some mmap() which change vma's start address, it is possible error situation, because we already prepare prev vma, rb_link and rb_parent and these are related to original address. So add WARN_ON_ONCE for finding that this situtation really happens. Jason