[Libguestfs] [nbdkit PATCH 4/4] blocksize-policy: Add blocksize-write-disconnect=N parameter

Eric Blake
Wed, 26 Oct 2022 22:18:02 -0000

Allow this filter to emulate qemu's behavior of a hard disconnect on
write attempts larger than 32M.
---
 .../nbdkit-blocksize-policy-filter.pod        |  21 ++++
 tests/Makefile.am                             |  12 +-
 filters/blocksize-policy/policy.c             |  27 ++++-
 tests/test-blocksize-write-disconnect.sh      | 107 ++++++++++++++++++
 4 files changed, 164 insertions(+), 3 deletions(-)
 create mode 100755 tests/test-blocksize-write-disconnect.sh

diff --git a/filters/blocksize-policy/nbdkit-blocksize-policy-filter.pod
b/filters/blocksize-policy/nbdkit-blocksize-policy-filter.pod
index 691ad289..a377829f 100644
--- a/filters/blocksize-policy/nbdkit-blocksize-policy-filter.pod
+++ b/filters/blocksize-policy/nbdkit-blocksize-policy-filter.pod
@@ -10,6 +10,7 @@ maximum block size, and apply error policy
         [blocksize-minimum=N]
         [blocksize-preferred=N]
         [blocksize-maximum=N]
+        [blocksize-write-disconnect=N]

 =head1 DESCRIPTION

@@ -49,6 +50,13 @@ read-modify-write for an unaligned write).  With this filter
you can
 use C<blocksize-error-policy=error> to reject these requests in the
 filter with an EINVAL error.  The plugin will not see them.

+Normally, nbdkit will accept write requests up to 64M in length, and
+reply with a gracful error message rather than a hard disconnect for a
+buffer up to twice that large.  But many other servers (for example,
+qemu-nbd) will give a hard disconnect for a write request larger than
+32M.  With this filter you can use C<blocksize-write-disconnect=32M>
+to emulate the behavior of other servers.
+
 =head2 Combining with L<nbdkit-blocksize-filter(1)>

 A related filter is L<nbdkit-blocksize-filter(1)>.  That filter can
@@ -87,6 +95,19 @@ means pass the request through to the plugin.
 Use C<error> to return an EINVAL error back to the client.  The plugin
 will not see the badly formed request in this case.

+=item B<blocksize-write-disconnect=>N
+
+(nbdkit E<ge> 1.34)
+
+If a client sends a write request which is larger than the specified
+I<size> (using the usual size modifiers like C<32M>), abruptly
close
+the connection.  This can be used to emulate qemu's behavior of
+disconnecting for write requests larger than 32M, rather than nbdkit's
+default of keeping the connection alive for write requests up to 128M
+(although nbdkit does not let the plugin see requests larger than
+64M).  The write disconnect size is independent of any advertised
+maximum block size or its accompanying error policy.
+
 =item B<blocksize-minimum=>N

 =item B<blocksize-preferred=>N
diff --git a/tests/Makefile.am b/tests/Makefile.am
index e951381d..d59b797c 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1464,8 +1464,16 @@ EXTRA_DIST += \
 	$(NULL)

 # blocksize-policy filter test.
-TESTS += test-blocksize-policy.sh test-blocksize-error-policy.sh
-EXTRA_DIST += test-blocksize-policy.sh test-blocksize-error-policy.sh
+TESTS += \
+	test-blocksize-policy.sh \
+	test-blocksize-error-policy.sh \
+	test-blocksize-write-disconnect.sh \
+	$(NULL)
+EXTRA_DIST += \
+	test-blocksize-policy.sh \
+	test-blocksize-error-policy.sh \
+	test-blocksize-write-disconnect.sh \
+	$(NULL)

 # cache filter test.
 TESTS += \
diff --git a/filters/blocksize-policy/policy.c
b/filters/blocksize-policy/policy.c
index 4ec07d36..f7cff2f1 100644
--- a/filters/blocksize-policy/policy.c
+++ b/filters/blocksize-policy/policy.c
@@ -42,11 +42,13 @@
 #include <nbdkit-filter.h>

 #include "ispowerof2.h"
+#include "rounding.h"

 /* Block size constraints configured on the command line (0 = unset). */
 static uint32_t config_minimum;
 static uint32_t config_preferred;
 static uint32_t config_maximum;
+static uint32_t config_disconnect;

 /* Error policy. */
 static enum { EP_ALLOW, EP_ERROR } error_policy = EP_ALLOW;
@@ -90,6 +92,12 @@ policy_config (nbdkit_next_config *next, nbdkit_backend
*nxdata,
     config_maximum = r;
     return 0;
   }
+  else if (strcmp (key, "blocksize-write-disconnect") == 0) {
+    r = nbdkit_parse_size (value);
+    if (r == -1 || r > UINT32_MAX) goto parse_error;
+    config_disconnect = r;
+    return 0;
+  }

   return next (nxdata, key, value);
 }
@@ -147,6 +155,14 @@ policy_config_complete (nbdkit_next_config_complete *next,
     }
   }

+  if (config_minimum && config_disconnect) {
+    if (config_disconnect <= config_minimum) {
+      nbdkit_error ("blocksize-write-disonnect must be larger than "
+                    "blocksize-minimum");
+      return -1;
+    }
+  }
+
   return next (nxdata);
 }

@@ -192,6 +208,8 @@ policy_block_size (nbdkit_next *next, void *handle,

     if (config_maximum)
       *maximum = config_maximum;
+    else if (config_disconnect)
+      *maximum = ROUND_DOWN (config_disconnect, *minimum);
     else
       *maximum = 0xffffffff;
   }
@@ -220,7 +238,7 @@ policy_block_size (nbdkit_next *next, void *handle,
  * below.
  *
  * The 'data' flag is true for pread and pwrite (where we check the
- * maximum bound).  We don't check maximum for non-data- carrying
+ * maximum bound).  We don't check maximum for non-data-carrying
  * calls like zero.
  *
  * The NBD specification mandates EINVAL for block size constraint
@@ -303,6 +321,13 @@ policy_pwrite (nbdkit_next *next,
                void *handle, const void *buf, uint32_t count, uint64_t offset,
                uint32_t flags, int *err)
 {
+  if (config_disconnect && count > config_disconnect) {
+    nbdkit_error ("disconnecting client due to oversize write
request");
+    nbdkit_disconnect (true);
+    *err = ESHUTDOWN;
+    return -1;
+  }
+
   if (check_policy (next, handle, "pwrite", true, count, offset, err)
== -1)
     return -1;

diff --git a/tests/test-blocksize-write-disconnect.sh
b/tests/test-blocksize-write-disconnect.sh
new file mode 100755
index 00000000..14a35100
--- /dev/null
+++ b/tests/test-blocksize-write-disconnect.sh
@@ -0,0 +1,107 @@
+#!/usr/bin/env bash
+# nbdkit
+# Copyright (C) 2019-2022 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.
+
+source ./functions.sh
+set -e
+set -x
+
+requires_plugin eval
+requires nbdsh -c 'print(h.get_block_size)'
+requires nbdsh -c 'print(h.get_strict_mode)'
+requires_nbdsh_uri
+requires dd iflag=count_bytes </dev/null
+
+# Libnbd does not let us test pwrite larger than 64M, so we can't
+# test nbdkit's graceful behavior of writes up to 128M.
+# In this test, odd size writes fail with EINVAL from the filter (size 1 too
+# small, all others unaligned); evens 2 to 8M pass, 8M+2 to 16M fail with
+# ENOMEM from the plugin, 16M+2 to 32M fail with EINVAL from the filter,
+# 32M+1 to 64M kill the connection (ENOTCONN visible to client), and
+# 64M+1 and above fails with ERANGE in libnbd.
+
+nbdkit -v -U - eval \
+       block_size="echo 2 4096 16M" \
+       get_size="echo 64M" \
+       pread=' dd if=/dev/zero count=$3 iflag=count_bytes ' \
+       pwrite=' if test $3 -gt $((8*1024*1024)); then
+           echo ENOMEM >&2; exit 1
+         else
+           dd of=/dev/null
+         fi' \
+       --filter=blocksize-policy \
+       blocksize-error-policy=error blocksize-write-disconnect=32M \
+       --run '
+nbdsh -u "$uri" -c "
+import errno
+
+def check(h, size, expect_value, expect_traffic=True):
+  assert h.aio_is_ready() is True
+  buf = b\"0\" * size
+  if hasattr(h, \"stats_bytes_sent\"):
+    start = h.stats_bytes_sent()
+  try:
+    h.pwrite(buf, 0)
+    assert expect_value == 0
+  except nbd.Error as ex:
+    assert expect_value == ex.errnum
+  if hasattr(h, \"stats_bytes_sent\"):
+    if expect_traffic:
+      assert h.stats_bytes_sent() > start
+    else:
+      assert h.stats_bytes_sent() == start
+
+h.set_strict_mode(0)  # Bypass client-side safety checks
+# Beyond 64M
+check(h, 64*1024*1024 + 1, errno.ERANGE, False)
+check(h, 64*1024*1024 + 2, errno.ERANGE, False)
+# Small reads
+check(h, 1, errno.EINVAL)
+check(h, 2, 0)
+# Near 8M boundary
+check(h, 8*1024*1024 - 2, 0)
+check(h, 8*1024*1024 - 1, errno.EINVAL)
+check(h, 8*1024*1024, 0)
+check(h, 8*1024*1024 + 1, errno.EINVAL)
+check(h, 8*1024*1024 + 2, errno.ENOMEM)
+# Near 16M boundary
+check(h, 16*1024*1024 - 2, errno.ENOMEM)
+check(h, 16*1024*1024 - 1, errno.EINVAL)
+check(h, 16*1024*1024, errno.ENOMEM)
+check(h, 16*1024*1024 + 1, errno.EINVAL)
+check(h, 16*1024*1024 + 2, errno.EINVAL)
+# Near 32M boundary
+check(h, 32*1024*1024 - 2, errno.EINVAL)
+check(h, 32*1024*1024 - 1, errno.EINVAL)
+check(h, 32*1024*1024, errno.EINVAL)
+check(h, 32*1024*1024 + 1, errno.ENOTCONN)
+assert h.aio_is_ready() is False
+"'
--
2.37.3