From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.3 required=5.0 tests=MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 32188 invoked from network); 15 Mar 2021 21:51:38 -0000 Received: from mother.openwall.net (195.42.179.200) by inbox.vuxu.org with ESMTPUTF8; 15 Mar 2021 21:51:38 -0000 Received: (qmail 1323 invoked by uid 550); 15 Mar 2021 21:51:36 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 1300 invoked from network); 15 Mar 2021 21:51:36 -0000 Date: Mon, 15 Mar 2021 17:51:23 -0400 From: Rich Felker To: Dominic Chen Cc: musl@lists.openwall.com Message-ID: <20210315215120.GF32655@brightrain.aerifal.cx> References: <50e56e3e-d448-c568-b3d8-fbab98939ff8@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50e56e3e-d448-c568-b3d8-fbab98939ff8@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) Subject: Re: [musl] Issue with fread() and unaligned readv() On Mon, Mar 15, 2021 at 05:39:43PM -0400, Dominic Chen wrote: > Not sure this counts as a problem in musl or the application, but > I've been debugging a return error of EINVAL from `fread(&buf, 8, > 16, f)`, where `f = fopen("/proc/self/pagemap", "r")`. Internally, > musl converts this into a call to `readv(f->fd, iov, 2)`, where `iov > = {{iov_base = buf, iov_len = 127}, {iov_base = f->buf, iov_len = > 1024}}`. However, it turns out that the kernel VFS read > implementation inside `pagemap_read` checks that both the file > position and count are divisible by PM_ENTRY_BYTES (8 on x86_64), > otherwise it rejects the read with EINVAL. In comparison, glibc's > `_IO_file_xsgetn` does appear to try to maintain read alignment, > although I haven't looked at it in detail. You can't use stdio to read or write special files/devices that depend on the reads or writes happening in particular units, because the relationship between stdio operations and the underlying buffer-fill/flush operations on the underlying fd is unspecified. It's really unfortunate that the kernel lies that procfs files are regular files but doesn't give them regular-file semantics, but you really need to use direct operations on the fd in the units the interface requires, rather than stdio, to work with these files. Rich