From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965493AbbCPTDz (ORCPT ); Mon, 16 Mar 2015 15:03:55 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55317 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932978AbbCPTDx (ORCPT ); Mon, 16 Mar 2015 15:03:53 -0400 Date: Mon, 16 Mar 2015 20:01:54 +0100 From: Oleg Nesterov To: Andy Lutomirski , Hugh Dickins , Linus Torvalds Cc: Jan Kratochvil , Sergio Durigan Junior , GDB Patches , Pedro Alves , "linux-kernel@vger.kernel.org" Subject: install_special_mapping && vm_pgoff (Was: vvar, gup && coredump) Message-ID: <20150316190154.GA18472@redhat.com> References: <878ufc9kau.fsf@redhat.com> <20150305154827.GA9441@host1.jankratochvil.net> <87zj7r5fpz.fsf@redhat.com> <20150305205744.GA13165@host1.jankratochvil.net> <20150311200052.GA22654@redhat.com> <20150312143438.GA4338@redhat.com> <20150312165423.GA10073@redhat.com> <20150312174653.GA13086@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150312174653.GA13086@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/12, Oleg Nesterov wrote: > > OTOH. We can probably add ->access() into special_mapping_vmops, this > way __access_remote_vm() could work even if gup() fails ? So I tried to think how special_mapping_vmops->access() can work, it needs to rely on ->vm_pgoff. But afaics this logic is just broken. Lets even forget about vvar vma which uses remap_file_pages(). Lets look at "[vdso]" which uses the "normal" pages. The comment in special_mapping_fault() says * special mappings have no vm_file, and in that case, the mm * uses vm_pgoff internally. Yes. But afaics mm/ doesn't do this correctly. So * do not copy this code into drivers! looks like a good recommendation ;) I think that this logic is wrong even if ARRAY_SIZE(pages) == 1, but I am not sure. But since vdso use 2 pages, it is trivial to show that this logic is wrong. To verify, I changed show_map_vma() to expose pgoff even if !file, but this test-case can show the problem too: #include #include #include #include #include #include void *find_vdso_vaddr(void) { FILE *perl; char buf[32] = {}; perl = popen("perl -e 'open STDIN,qq|/proc/@{[getppid]}/maps|;" "/^(.*?)-.*vdso/ && print hex $1 while <>'", "r"); fread(buf, sizeof(buf), 1, perl); fclose(perl); return (void *)atol(buf); } #define PAGE_SIZE 4096 int main(void) { void *vdso = find_vdso_vaddr(); assert(vdso); // of course they should differ, and they do so far printf("vdso pages differ: %d\n", !!memcmp(vdso, vdso + PAGE_SIZE, PAGE_SIZE)); // split into 2 vma's assert(mprotect(vdso, PAGE_SIZE, PROT_READ) == 0); // force another fault on the next check assert(madvise(vdso, 2 * PAGE_SIZE, MADV_DONTNEED) == 0); // now they no longer differ, the 2nd vm_pgoff is wrong printf("vdso pages differ: %d\n", !!memcmp(vdso, vdso + PAGE_SIZE, PAGE_SIZE)); return 0; } output: vdso pages differ: 1 vdso pages differ: 0 And not only "split_vma" is wrong, I think that "move_vma" is not right too. Note this check in copy_vma(), /* * If anonymous vma has not yet been faulted, update new pgoff * to match new location, to increase its chance of merging. */ if (unlikely(!vma->vm_file && !vma->anon_vma)) { pgoff = addr >> PAGE_SHIFT; faulted_in_anon_vma = false; } I can easily misread this code. But it doesn't look right too. If vdso was cow'ed (breakpoint installed by gdb) and sys_nremap()'ed, then the new pgoff will be wrong too after, say, MADV_DONTNEED. Or I am totally confused? Oleg.