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([2001:a61:2482:101:3351:6160:8173:cc31]) by smtp.gmail.com with ESMTPSA id b67sm1346031wmh.29.2020.03.31.23.38.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 31 Mar 2020 23:38:32 -0700 (PDT) Cc: mtk.manpages@gmail.com, Al Viro , Christian Brauner , Aleksa Sarai , linux-man@vger.kernel.org, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH man-pages v2 2/2] openat2.2: document new openat2(2) syscall To: Aleksa Sarai References: <20200202151907.23587-1-cyphar@cyphar.com> <20200202151907.23587-3-cyphar@cyphar.com> <1567baea-5476-6d21-4f03-142def0f62e3@gmail.com> <20200331143911.lokfoq3lqfri2mgy@yavin.dot.cyphar.com> From: "Michael Kerrisk (man-pages)" Message-ID: Date: Wed, 1 Apr 2020 08:38:29 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200331143911.lokfoq3lqfri2mgy@yavin.dot.cyphar.com> Content-Type: text/plain; charset=UTF-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Aleksa, On 3/31/20 4:39 PM, Aleksa Sarai wrote: > On 2020-03-30, Michael Kerrisk (man-pages) wrote: >> Hello Aleksa, >> >> On 2/2/20 4:19 PM, Aleksa Sarai wrote: >>> Rather than trying to merge the new syscall documentation into open.2 >>> (which would probably result in the man-page being incomprehensible), >>> instead the new syscall gets its own dedicated page with links between >>> open(2) and openat2(2) to avoid duplicating information such as the list >>> of O_* flags or common errors. >>> >>> In addition to describing all of the key flags, information about the >>> extensibility design is provided so that users can better understand why >>> they need to pass sizeof(struct open_how) and how their programs will >>> work across kernels. After some discussions with David Laight, I also >>> included explicit instructions to zero the structure to avoid issues >>> when recompiling with new headers. >>> >>> Signed-off-by: Aleksa Sarai >> >> Thanks. I've applied this patch, but also done quite a lot of >> editing of the page. The current draft is below (and also pushed >> to Git). Could I ask you to review the page, to see if I injected >> any error during my edits. > > Looks good to me. > >> In addition, I've added a number of FIXMEs in comments >> in the page source. Can you please check these, and let me >> know your thoughts. > > Will do, see below. > >> .\" FIXME I find the "previously-functional systems" in the previous >> .\" sentence a little odd (since openat2() ia new sysycall), so I would >> .\" like to clarify a little... >> .\" Are you referring to the scenario where someone might take an >> .\" existing application that uses openat() and replaces the uses >> .\" of openat() with openat2()? In which case, is it correct to >> .\" understand that you mean that one should not just indiscriminately >> .\" add the RESOLVE_NO_XDEV flag to all of the openat2() calls? >> .\" If I'm not on the right track, could you point me in the right >> .\" direction please. > > This is mostly meant as a warning to hopefully avoid applications > because the developer didn't realise that system paths may contain > symlinks or bind-mounts. For an application which has switched to > openat2() and then uses RESOLVE_NO_SYMLINKS for a non-security reason, > it's possible that on some distributions (or future versions of a > distribution) that their application will stop working because a system > path suddenly contains a symlink or is a bind-mount. > > This was a concern which was brought up on LWN some time ago. If you can > think of a phrasing that makes this more clear, I'd appreciate it. Thanks. I've made the text: Applications that employ the RESOLVE_NO_XDEV flag are encouraged to make its use configurable (unless it is used for a specific security purpose), as bind mounts are widely used by end-users. Setting this flag indiscriminately—i.e., for purposes not specif‐ ically related to security—for all uses of openat2() may result in spurious errors on previously-func‐ tional systems. This may occur if, for example, a system pathname that is used by an application is modified (e.g., in a new distribution release) so that a pathname component (now) contains a bind mount. Okay? >> .\" FIXME: what specific details in symlink(7) are being referred >> .\" by the following sentence? It's not clear. > > The section on magic-links, but you're right that the sentence ordering > is a bit odd. It should probably go after the first sentence. I must admit that I'm still confused. There's only the briefest of mentions of magic links in symlink(7). Perhaps that needs to be fixed? And, while I think of it, the text just preceding that FIXME says: Due to the potential danger of unknowingly opening these magic links, it may be preferable for users to disable their resolution entirely. This sentence reads a little strangely. Could you please give me some concrete examples, and I will try rewording that sentence a bit. >> .\" FIXME I found the following hard to understand (in particular, the >> .\" meaning of "scoped" is unclear) , and reworded as below. Is it okay? >> .\" Absolute symbolic links and ".." path components will be scoped to >> .\" .IR dirfd . > > Scoped does broadly mean "interpreted relative to", though the > difference is mainly that when I said scoped it's meant to be more of an > assertive claim ("the kernel promises to always treat this path inside > dirfd"). But "interpreted relative to" is a clearer way of phrasing the > semantics, so I'm okay with this change. Okay. >> .\" FIXME The next piece is unclear (to me). What kind of ".." escape >> .\" attempts does chroot() not detect that RESOLVE_IN_ROOT does? > > If the root is moved, you can escape from a chroot(2). But this sentence > might not really belong in a man-page since it's describing (important) > aspects of the implementation and not the semantics. So, should I just remove the sentence? Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/