From 5bd332a683811586039f99f31c01d4f2f7181334 Mon Sep 17 00:00:00 2001

From: "Richard W.M. Jones" <rjones@redhat.com>

Date: Mon, 26 Jul 2021 15:21:18 +0100

Subject: [PATCH] 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.

(cherry picked from commit ce0db9d7736dd28dd0f10951ce65853e50b35e41)

---

 common/utils/Makefile.am |  1 +

 common/utils/full-rw.c   | 81 ++++++++++++++++++++++++++++++++++++++++

 common/utils/utils.h     |  2 +

 filters/cache/blk.c      | 10 ++---

 filters/cow/blk.c        |  6 +--

 5 files changed, 92 insertions(+), 8 deletions(-)

 create mode 100644 common/utils/full-rw.c

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/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;

+  }

+

+  return ret;

+}

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/filters/cache/blk.c b/filters/cache/blk.c

index f85ada35..42bd3779 100644

--- a/filters/cache/blk.c

+++ b/filters/cache/blk.c

@@ -250,7 +250,7 @@ _blk_read_multiple (nbdkit_next *next,

                       " (offset %" PRIu64 ")",

                       blknum, (uint64_t) offset);

-      if (pwrite (fd, block, blksize * runblocks, offset) == -1) {

+      if (full_pwrite (fd, block, blksize * runblocks, offset) == -1) {

         *err = errno;

         nbdkit_error ("pwrite: %m");

         return -1;

@@ -262,7 +262,7 @@ _blk_read_multiple (nbdkit_next *next,

     }

   }

   else {                        /* Read cache. */

-    if (pread (fd, block, blksize * runblocks, offset) == -1) {

+    if (full_pread (fd, block, blksize * runblocks, offset) == -1) {

       *err = errno;

       nbdkit_error ("pread: %m");

       return -1;

@@ -339,7 +339,7 @@ blk_cache (nbdkit_next *next,

       nbdkit_debug ("cache: cache block %" PRIu64 " (offset %" PRIu64 ")",

                     blknum, (uint64_t) offset);

-    if (pwrite (fd, block, blksize, offset) == -1) {

+    if (full_pwrite (fd, block, blksize, offset) == -1) {

       *err = errno;

       nbdkit_error ("pwrite: %m");

       return -1;

@@ -380,7 +380,7 @@ blk_writethrough (nbdkit_next *next,

     nbdkit_debug ("cache: writethrough block %" PRIu64 " (offset %" PRIu64 ")",

                   blknum, (uint64_t) offset);

-  if (pwrite (fd, block, blksize, offset) == -1) {

+  if (full_pwrite (fd, block, blksize, offset) == -1) {

     *err = errno;

     nbdkit_error ("pwrite: %m");

     return -1;

@@ -414,7 +414,7 @@ blk_write (nbdkit_next *next,

     nbdkit_debug ("cache: writeback block %" PRIu64 " (offset %" PRIu64 ")",

                   blknum, (uint64_t) offset);

-  if (pwrite (fd, block, blksize, offset) == -1) {

+  if (full_pwrite (fd, block, blksize, offset) == -1) {

     *err = errno;

     nbdkit_error ("pwrite: %m");

     return -1;

diff --git a/filters/cow/blk.c b/filters/cow/blk.c

index 4ec8d1b8..121b0dd4 100644

--- a/filters/cow/blk.c

+++ b/filters/cow/blk.c

@@ -278,7 +278,7 @@ blk_read_multiple (nbdkit_next *next,

     memset (block + n, 0, tail);

   }

   else if (state == BLOCK_ALLOCATED) { /* Read overlay. */

-    if (pread (fd, block, BLKSIZE * runblocks, offset) == -1) {

+    if (full_pread (fd, block, BLKSIZE * runblocks, offset) == -1) {

       *err = errno;

       nbdkit_error ("pread: %m");

       return -1;

@@ -353,7 +353,7 @@ blk_cache (nbdkit_next *next,

   memset (block + n, 0, tail);

   if (mode == BLK_CACHE_COW) {

-    if (pwrite (fd, block, BLKSIZE, offset) == -1) {

+    if (full_pwrite (fd, block, BLKSIZE, offset) == -1) {

       *err = errno;

       nbdkit_error ("pwrite: %m");

       return -1;

@@ -372,7 +372,7 @@ blk_write (uint64_t blknum, const uint8_t *block, int *err)

     nbdkit_debug ("cow: blk_write block %" PRIu64 " (offset %" PRIu64 ")",

                   blknum, (uint64_t) offset);

-  if (pwrite (fd, block, BLKSIZE, offset) == -1) {

+  if (full_pwrite (fd, block, BLKSIZE, offset) == -1) {

     *err = errno;

     nbdkit_error ("pwrite: %m");

     return -1;

-- 

2.31.1