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

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; + } +