Libguestfs - Apr 2022 - [INCOMPLETE PATCH nbdkit] ssh: Allow the remote file to be created

This patch is incomplete (missing tests) so for discussion only, but
it implements nbdkit-ssh-plugin create=true option which creates the
remote file.

Example:

$ nbdkit ssh host=foo /var/tmp/newdisk create=true create-size=100M
$ nbdsh -u nbd://localhost

The disk is only created on first connection (which works the same way
as nbdkit-vddk-plugin).  So if you just want to create the remote disk
and not do anything else, you should do something like this:

nbdkit -U - ssh host=foo /var/tmp/newdisk create=true create-size=100M \
       --run 'nbdinfo --can connect $uri'

If the remote disk already exists, it is truncated but not recreated
(in this case you'll notice the remote ctime is not touched).

For some reason I don't understand, create-mode=0777 is ineffective.
sftp-server has a -u option to set the umask, or else it uses the
remote user's umask.  I verified that openssh does *not* use the -u
option and that the umask of sftp-server is 0002, yet the permissions
on the file are still 0700.

Rich.

This adds new parameters, create=(true|false), create-size=SIZE and
create-mode=MODE to create and truncate the remote file.
---
 plugins/ssh/nbdkit-ssh-plugin.pod | 37 +++++++++++++++++--
 plugins/ssh/ssh.c                 | 60 +++++++++++++++++++++++++++++--
 2 files changed, 93 insertions(+), 4 deletions(-)

diff --git a/plugins/ssh/nbdkit-ssh-plugin.pod
b/plugins/ssh/nbdkit-ssh-plugin.pod
index 3f401c15..648fa94d 100644
--- a/plugins/ssh/nbdkit-ssh-plugin.pod
+++ b/plugins/ssh/nbdkit-ssh-plugin.pod
@@ -5,8 +5,10 @@ nbdkit-ssh-plugin - access disk images over the SSH protocol
 =head1 SYNOPSIS
 
  nbdkit ssh host=HOST [path=]PATH
-            [compression=true] [config=CONFIG_FILE] [identity=FILENAME]
-            [known-hosts=FILENAME] [password=PASSWORD|-|+FILENAME]
+            [compression=true] [config=CONFIG_FILE]
+            [create=true] [create-mode=MODE] [create-size=SIZE]
+            [identity=FILENAME] [known-hosts=FILENAME]
+            [password=PASSWORD|-|+FILENAME]
             [port=PORT] [timeout=SECS] [user=USER]
             [verify-remote-host=false]
 
@@ -62,6 +64,37 @@ The C<config> parameter is optional.  If it is
I<not> specified at all
 then F<~/.ssh/config> and F</etc/ssh/ssh_config> are both read.
 Missing or unreadable files are ignored.
 
+=item B<create=true>
+
+(nbdkit E<ge> 1.32)
+
+If set, the remote file will be created if it does not exist already.
+Note the remote file is created on first NBD connection to nbdkit.
+
+If using this option, you must use C<create-size>.  C<create-mode>
can
+be used to control the permissions of the new file.
+
+=item B<create-mode=>MODE
+
+(nbdkit E<ge> 1.32)
+
+If using C<create=true> specify the default permissions of the new
+remote file.  You can use octal modes like C<create-mode=0777>.  The
+default is C<0600>, ie. only readable and writable by the remote user.
+
+The SFTP server may apply a L<umask(2)> which is specific to the
+remote system and cannot be queried from the client, so the final
+permissions may be more restrictive than requested.  For OpenSSH I
+found that files are always created with mode & 0700 whatever mode is
+specified here, but other servers might be different.
+
+=item B<create-size=>SIZE
+
+(nbdkit E<ge> 1.32)
+
+If using C<create=true>, specify the virtual size of the new disk.
+C<SIZE> can use modifiers like C<100M> etc.
+
 =item B<host=>HOST
 
 Specify the name or IP address of the remote host.
diff --git a/plugins/ssh/ssh.c b/plugins/ssh/ssh.c
index 3b2b21c8..66998813 100644
--- a/plugins/ssh/ssh.c
+++ b/plugins/ssh/ssh.c
@@ -63,6 +63,9 @@ static const char *known_hosts = NULL;
 static const_string_vector identities = empty_vector;
 static uint32_t timeout = 0;
 static bool compression = false;
+static bool create = false;
+static int64_t create_size = -1;
+static unsigned create_mode = S_IRUSR | S_IWUSR /* 0600 */;
 
 /* config can be:
  * NULL => parse options from default file
@@ -167,6 +170,27 @@ ssh_config (const char *key, const char *value)
       return -1;
     compression = r;
   }
+  else if (strcmp (key, "create") == 0) {
+    r = nbdkit_parse_bool (value);
+    if (r == -1)
+      return -1;
+    create = r;
+  }
+  else if (strcmp (key, "create-size") == 0) {
+    create_size = nbdkit_parse_size (value);
+    if (create_size == -1)
+      return -1;
+  }
+  else if (strcmp (key, "create-mode") == 0) {
+    r = nbdkit_parse_unsigned (key, value, &create_mode);
+    if (r == -1)
+      return -1;
+    /* OpenSSH checks this too. */
+    if (create_mode > 0777) {
+      nbdkit_error ("create-mode must be <= 0777");
+      return -1;
+    }
+  }
 
   else {
     nbdkit_error ("unknown parameter '%s'", key);
@@ -186,6 +210,13 @@ ssh_config_complete (void)
     return -1;
   }
 
+  /* If create=true, create-size must be supplied. */
+  if (create && create_size == -1) {
+    nbdkit_error ("if using create=true, you must specify the size "
+                  "of the new remote file using create-size=SIZE");
+    return -1;
+  }
+
   return 0;
 }
 
@@ -200,7 +231,10 @@ ssh_config_complete (void)
   "identity=<FILENAME>        Prepend private key (identity)
file.\n" \
   "timeout=SECS               Set SSH connection timeout.\n" \
   "verify-remote-host=false   Ignore known_hosts.\n" \
-  "compression=true           Enable compression."
+  "compression=true           Enable compression.\n" \
+  "create=true                Create the remote file.\n" \
+  "create-mode=MODE           Set the permissions of the remote
file.\n" \
+  "create-size=SIZE           Set the size of the remote file."
 
 /* The per-connection handle. */
 struct ssh_handle {
@@ -480,7 +514,8 @@ ssh_open (int readonly)
     goto err;
   }
   access_type = readonly ? O_RDONLY : O_RDWR;
-  h->file = sftp_open (h->sftp, path, access_type, S_IRWXU);
+  if (create) access_type |= O_CREAT;
+  h->file = sftp_open (h->sftp, path, access_type, create_mode);
   if (!h->file) {
     nbdkit_error ("cannot open file for %s: %s",
                   readonly ? "reading" : "writing",
@@ -488,6 +523,27 @@ ssh_open (int readonly)
     goto err;
   }
 
+  if (create) {
+    /* There's no sftp_truncate call.  However OpenSSH lets you call
+     * SSH_FXP_SETSTAT + SSH_FILEXFER_ATTR_SIZE which invokes
+     * truncate(2) on the server.  Libssh doesn't provide a binding
+     * for SSH_FXP_FSETSTAT so we have to pass the session + path.
+     */
+    struct sftp_attributes_struct attrs = {
+      .flags = SSH_FILEXFER_ATTR_SIZE,
+      .size = create_size,
+    };
+
+    r = sftp_setstat (h->sftp, path, &attrs);
+    if (r != SSH_OK) {
+      nbdkit_error ("truncate failed: %s", ssh_get_error
(h->session));
+      goto err;
+    }
+  }
+
+  /* On the next open, don't create or truncate the file. */
+  create = false;
+
   nbdkit_debug ("opened libssh handle");
 
   return h;
-- 
2.35.1