Re: [Libguestfs] [PATCH nbdkit 4/7] cache, cow: Use full pread/pwrite operations

On Mon, Jul 26, 2021 at 06:28:57PM +0100, Richard W.M. Jones wrote: >Although it probably cannot happen on Linux, POSIX allows pread/pwrite >to return or write fewer bytes than requested. The cache and cow >filters didn't handle this situation. Replace the raw >pread(2)/pwrite(2) syscalls with alternate versions which can handle >this. >--- >common/utils/Makefile.am | 1 + >common/utils/utils.h | 2 + >common/utils/full-rw.c | 81 ++++++++++++++++++++++++++++++++++++++++ >filters/cache/blk.c | 10 ++--- >filters/cow/blk.c | 6 +-- >5 files changed, 92 insertions(+), 8 deletions(-) > >diff --git a/common/utils/Makefile.am b/common/utils/Makefile.am >index 1708a4c8..14e9dfc4 100644 >--- a/common/utils/Makefile.am >+++ b/common/utils/Makefile.am >@@ -40,6 +40,7 @@ libutils_la_SOURCES = \ > cleanup-nbdkit.c \ > cleanup.h \ > environ.c \ >+ full-rw.c \ > quote.c \ > utils.c \ > utils.h \ >diff --git a/common/utils/utils.h b/common/utils/utils.h >index f8f70212..83397ae1 100644 >--- a/common/utils/utils.h >+++ b/common/utils/utils.h >@@ -40,5 +40,7 @@ extern int set_cloexec (int fd); >extern int set_nonblock (int fd); >extern char **copy_environ (char **env, ...) __attribute__((__sentinel__)); >extern char *make_temporary_directory (void); >+extern ssize_t full_pread (int fd, void *buf, size_t count, off_t offset); >+extern ssize_t full_pwrite (int fd, const void *buf, size_t count, off_t offset); > >#endif /* NBDKIT_UTILS_H */ >diff --git a/common/utils/full-rw.c b/common/utils/full-rw.c >new file mode 100644 >index 00000000..55b32cdd >--- /dev/null >+++ b/common/utils/full-rw.c >@@ -0,0 +1,81 @@ >+/* nbdkit >+ * Copyright (C) 2021 Red Hat Inc. >+ * >+ * Redistribution and use in source and binary forms, with or without >+ * modification, are permitted provided that the following conditions are >+ * met: >+ * >+ * * Redistributions of source code must retain the above copyright >+ * notice, this list of conditions and the following disclaimer. >+ * >+ * * Redistributions in binary form must reproduce the above copyright >+ * notice, this list of conditions and the following disclaimer in the >+ * documentation and/or other materials provided with the distribution. >+ * >+ * * Neither the name of Red Hat nor the names of its contributors may be >+ * used to endorse or promote products derived from this software without >+ * specific prior written permission. >+ * >+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND >+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, >+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A >+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR >+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, >+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT >+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF >+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND >+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, >+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT >+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF >+ * SUCH DAMAGE. >+ */ >+ >+/* These functions are like pread(2)/pwrite(2) but they always read or >+ * write the full amount, or fail. >+ */ >+ >+#include <config.h> >+ >+#include <stdio.h> >+#include <stdlib.h> >+#include <unistd.h> >+#include <errno.h> >+ >+ssize_t >+full_pread (int fd, void *buf, size_t count, off_t offset) >+{ >+ ssize_t ret = 0, r; >+ >+ while (count > 0) { >+ r = pread (fd, buf, count, offset); >+ if (r == -1) return -1; >+ if (r == 0) { >+ /* Presumably the caller wasn't expecting end-of-file here, so >+ * return an error. >+ */ >+ errno = EIO; >+ return -1; >+ } >+ ret += r; >+ offset += r; >+ count -= r; >+ } >+ >+ return ret; >+} >+ >+ssize_t >+full_pwrite (int fd, const void *buf, size_t count, off_t offset) >+{ >+ ssize_t ret = 0, r; >+ >+ while (count > 0) { >+ r = pwrite (fd, buf, count, offset); >+ if (r == -1) return -1; >+ ret += r; >+ offset += r; >+ count -= r; >+ } >+ Shouldn't these continue on EINTR?