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Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Arnd Bergmann , Greg Kroah-Hartman , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Lance Yang , syzbot+c181d3198e98f8aef8b9@syzkaller.appspotmail.com Subject: Re: [PATCH] drivers/char/mem: mmap readonly MAP_SHARED-/dev/zero correctly Message-Id: <20260924195826.70a87d6fb78d754bc73892cd@linux-foundation.org> In-Reply-To: <9efc7bc1-8d9d-49ac-b856-003e81558411@kernel.org> References: <20260924-fix-dev-zero-readonly-shared-v1-1-153c2111e323@kernel.org> <9efc7bc1-8d9d-49ac-b856-003e81558411@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 4F7FF100003 X-Stat-Signature: gbpbya95j5nw35hb3uxkun5ynqtzow1o X-HE-Tag: 1790305108-915448 X-HE-Meta: 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 XcdjP+Bp 9CDTzNUoYH+hvkCloJh5hgcj8eoly3f//4ckVU0XgdBKxh4Dr79w99btMzv31YPed+WpZPu6mRVzD4TEGDmUJNCDgu2g3G+mJntcmyegKfPb8fCv3UrNNUGpxhHE8Ne6DphL+nvTuRBBaw0LkaLu+nALXePj+yTueJfzVcTKGBbhtkRm6enTrGdvQHFVDMCl2NGtHF/nj67NzK9BQddC1Q5wUN5LWqKnHjjybuyOs9bn46eQmj/aGkSeJgQSKEyjGgQvJx2gtKjF+9wYeUs0I3ae71c5bTGjArOL/5Fwm9Dul/Yh0Xf4t6yZGS+2MpQKWRgGIC4uSUskd72en6j6E+2DDdk5eW2tqtrPa7QI0GoorYhinySX1InIHzAtkPjPFvftQFhonjtV2jd7u8Vm81kw3HKldjpsZhUtc96dcbdf0toHrfeyI9WGvJJDK1ywfORjuMcBsG1oWFks= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, 24 Sep 2026 17:37:29 +0200 "David Hildenbrand (Arm)" wrote: > On 9/24/26 16:48, Lorenzo Stoakes (ARM) wrote: > > Rather surprisingly, opening /dev/zero read-only then mmap()'ing it > > MAP_SHARED gets you true anonymous memory (albeit in a VMA with > > non-NULL vma->vm_file). > > > > ... > > > --- a/drivers/char/mem.c > > +++ b/drivers/char/mem.c > > @@ -503,7 +503,7 @@ static int mmap_zero_prepare(struct vm_area_desc *desc) > > #ifndef CONFIG_MMU > > return -ENOSYS; > > #endif > > - if (vma_desc_test(desc, VMA_SHARED_BIT)) > > + if (vma_desc_test(desc, VMA_MAYSHARE_BIT)) > > return shmem_zero_setup_desc(desc); > > > > So instead of shared zeropages we'd now get zero-filled shmem pages. "zeropage". Singular. Used to be! The accounting differences, possible changes in reclaim, memcg charging, maybe swap behavior. Switching to a different fault handler. It's hard to foresee all the effects of this. > The alternative would be to just convert it to a proper read-only COW mapping in > mmap code: > * Not clearing VM_MAYWRITE, but keeping VM_WRITE clear > * Clearing VMA_SHARED and VMA_MAYSHARE > > Sure, someone could then mprotect(PROT_WRITE that thing) or > FOLL_FORCE|FOLL_WRITE to get anonymous memory. Just raising that as an alternative. I dunno, the whole thing feels imprudent. To alter such longstanding core(ish) behavior. And why? Because a shiny new assertion said "hey, that isn't quite right". Wouldn't it be better to squish the warning somehow and to set about this change in a very careful way?