From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BA252C433EF for ; Thu, 21 Oct 2021 18:01:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9D31E61994 for ; Thu, 21 Oct 2021 18:01:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232359AbhJUSDW (ORCPT ); Thu, 21 Oct 2021 14:03:22 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:41632 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229968AbhJUSDV (ORCPT ); Thu, 21 Oct 2021 14:03:21 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1634839265; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=sf9FY6qH/YzdVN2hxg25wEYTT/Ij/RZUft7pA2XFpnw=; b=YHF4gv4+dKVvbvz6vN61TBjkQKl+eaveGxyZurf0ZxpGDKGGakxVqt6bgqnUp4es+QcU2p w6y18HHxyKbNFQ2p/VCTugZcf+B57gAUzSezk8knLEJC3iEJszFTZsSCi+fusRB43sccog 8uDcPLkjmLuSrUcQtMdI+ayaEwvhgKM= Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-374-YWf4rXTbMiC_1u7QF9ylyw-1; Thu, 21 Oct 2021 14:01:04 -0400 X-MC-Unique: YWf4rXTbMiC_1u7QF9ylyw-1 Received: by mail-wm1-f71.google.com with SMTP id n189-20020a1c27c6000000b00322f2e380f2so181194wmn.6 for ; Thu, 21 Oct 2021 11:01:03 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=sf9FY6qH/YzdVN2hxg25wEYTT/Ij/RZUft7pA2XFpnw=; b=L9s0Rqn/SfZDLVyW34oKCp5h+taVEm5ICMBJ85DnoH+imZEWRzbSvtgVNNOSG8UZBB JYVE8vqkOWBDP2B+ujtPQZpuLkdSz62r5qqlx7kfgV3J1zMTHk1+NZuNRu76yrjViUKP snNeI4gV9LuDkWM1Fuok7L8PK56/FdZ39T+kz7WCJ9jvle0cFvmAvaI4+9t5cVOtamKV eu1bCcLGTpF+Y5qlGG7The29zPHmccJb60ffkW5o6yBnz72CKoKGYnfiuNeLDGAztrLC 2NVO8btfL5C8dsuvwv3l8OIBxfNvdxVKdgKvQB8HXOOVMRU6NKPiUiRY3aeTqcVjxqDL TSdw== X-Gm-Message-State: AOAM530rpg8iYOyflwGIgc1XxHS2DOrIJ8/CLPBnp23+VE9lO658MSow yN6480mxcsaA5I0Fh1zlAYshDXSlWAxJdTOhGVkh5yhl6dNayLa3DEIjA9CPpVARQvFJzUaWcSS 6JRNx+GUFZPkALMao1M0Lt8GcCzo/efelpS2yotsG1Q== X-Received: by 2002:a1c:191:: with SMTP id 139mr8334173wmb.186.1634839262828; Thu, 21 Oct 2021 11:01:02 -0700 (PDT) X-Google-Smtp-Source: ABdhPJyp8jl7JjGdAuqkt28+Op/uIaVHGEOr5lv9I7JBHJzSrtRts/AxKHOPTiWybiNdJOqKTnZSHdECUlACCMo2WSE= X-Received: by 2002:a1c:191:: with SMTP id 139mr8334114wmb.186.1634839262466; Thu, 21 Oct 2021 11:01:02 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Andreas Gruenbacher Date: Thu, 21 Oct 2021 20:00:50 +0200 Message-ID: Subject: Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl() To: Catalin Marinas Cc: Linus Torvalds , Al Viro , Christoph Hellwig , "Darrick J. Wong" , Jan Kara , Matthew Wilcox , cluster-devel , linux-fsdevel , Linux Kernel Mailing List , "ocfs2-devel@oss.oracle.com" , Josef Bacik , Will Deacon Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Oct 21, 2021 at 7:09 PM Catalin Marinas wrote: > On Thu, Oct 21, 2021 at 04:42:33PM +0200, Andreas Gruenbacher wrote: > > On Thu, Oct 21, 2021 at 12:06 PM Catalin Marinas > > wrote: > > > On Thu, Oct 21, 2021 at 02:46:10AM +0200, Andreas Gruenbacher wrote: > > > > When a page fault would occur, we > > > > get back an error instead, and then we try to fault in the offending > > > > pages. If a page is resident and we still get a fault trying to access > > > > it, trying to fault in the same page again isn't going to help and we > > > > have a true error. > > > > > > You can't be sure the second fault is a true error. The unlocked > > > fault_in_*() may race with some LRU scheme making the pte not accessible > > > or a write-back making it clean/read-only. copy_to_user() with > > > pagefault_disabled() fails again but that's a benign fault. The > > > filesystem should re-attempt the fault-in (gup would correct the pte), > > > disable page faults and copy_to_user(), potentially in an infinite loop. > > > If you bail out on the second/third uaccess following a fault_in_*() > > > call, you may get some unexpected errors (though very rare). Maybe the > > > filesystems avoid this problem somehow but I couldn't figure it out. > > > > Good point, we can indeed only bail out if both the user copy and the > > fault-in fail. > > > > But probing the entire memory range in fault domain granularity in the > > page fault-in functions still doesn't actually make sense. Those > > functions really only need to guarantee that we'll be able to make > > progress eventually. From that point of view, it should be enough to > > probe the first byte of the requested memory range, so when one of > > those functions reports that the next N bytes should be accessible, > > this really means that the first byte surely isn't permanently > > inaccessible and that the rest is likely accessible. Functions > > fault_in_readable and fault_in_writeable already work that way, so > > this only leaves function fault_in_safe_writeable to worry about. > > I agree, that's why generic_perform_write() works. It does a get_user() > from the first byte in that range and the subsequent copy_from_user() > will make progress of at least one byte if it was readable. Eventually > it will hit the byte that faults. The gup-based fault_in_*() are a bit > more problematic. > > Your series introduces fault_in_safe_writeable() and I think for MTE > doing a _single_ get_user(uaddr) (in addition to the gup checks for > write) would be sufficient as long as generic_file_read_iter() advances > by at least one byte (eventually). > > This discussion started with the btrfs search_ioctl() where, even if > some bytes were written in copy_to_sk(), it always restarts from an > earlier position, reattempting to write the same bytes. Since > copy_to_sk() doesn't guarantee forward progress even if some bytes are > writable, Linus' suggestion was for fault_in_writable() to probe the > whole range. I consider this overkill since btrfs is the only one that > needs probing every 16 bytes. The other cases like the new > fault_in_safe_writeable() can be fixed by probing the first byte only > followed by gup. Hmm. Direct I/O request sizes are multiples of the underlying device block size, so we'll also get stuck there if fault-in won't give us a full block. This is getting pretty ugly. So scratch that idea; let's stick with probing the whole range. Thanks, Andreas > I think we need to better define the semantics of the fault_in + uaccess > sequences. For uaccess, we document "a hard requirement that not storing > anything at all (i.e. returning size) should happen only when nothing > could be copied" (from linux/uaccess.h). I think we can add a > requirement for the new size_t-based fault_in_* variants without > mandating that the whole range is probed, something like: "returning > leftover < size guarantees that a subsequent user access at uaddr copies > at least one byte eventually". I said "eventually" but maybe we can come > up with some clearer wording for a liveness property. > > Such requirement would ensure that infinite loops of fault_in_* + > uaccess make progress as long as they don't reset the probed range. Code > like btrfs search_ioctl() would need to be adjusted to avoid such range > reset and guarantee forward progress. > > -- > Catalin >