[Libguestfs] [nbdkit PATCH] offset: Better handling of parameters

[Eric Blake]

The man page claims both offset and range are optional (matching the
code), but the --help text claims offset is mandatory, and the comment
to the no-op offset_config_complete claims we require both parameters.
We did not check for an offset larger than the underlying size when
there was no range, and even when there is a range, we were not
careful about integer overflow (offset=5E range=5E happily claims to
export a 5E image; but all bets are off if you later try to access
beyond the real underlying size).  And in several cases, such as if
the plugin's get_size fails but range was not provided, we are not
returning -1 for failure.

Fixes: 3db69f56
Signed-off-by: Eric Blake <eblake at redhat.com>
---

The first post-1.14 bugfix.  Too bad I didn't spot it earlier today.

 filters/offset/offset.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/filters/offset/offset.c b/filters/offset/offset.c
index 7df1ed13..1039a577 100644
--- a/filters/offset/offset.c
+++ b/filters/offset/offset.c
@@ -64,16 +64,9 @@ offset_config (nbdkit_next_config *next, void *nxdata,
     return next (nxdata, key, value);
 }

-/* Check the user did pass both parameters. */
-static int
-offset_config_complete (nbdkit_next_config_complete *next, void *nxdata)
-{
-  return next (nxdata);
-}
-
 #define offset_config_help \
-  "offset=<OFFSET>     (required) The start offset to serve.\n" \
-  "range=<LENGTH>                 The total size to serve."
+  "offset=<OFFSET>            The start offset to serve (default 0).\n" \
+  "range=<LENGTH>             The total size to serve (default rest of file)."

 /* Get the file size. */
 static int64_t
@@ -82,16 +75,23 @@ offset_get_size (struct nbdkit_next_ops *next_ops, void *nxdata,
 {
   int64_t real_size = next_ops->get_size (nxdata);

+  if (real_size == -1)
+    return -1;
+
   if (range >= 0) {
-    if (offset + range > real_size) {
+    if (offset > real_size - range) {
       nbdkit_error ("offset+range is larger than the real size "
                     "of the underlying file or device");
       return -1;
     }
     return range;
   }
-  else
-    return real_size - offset;
+  else if (offset > real_size) {
+    nbdkit_error ("offset is larger than the real size "
+                  "of the underlying file or device");
+    return -1;
+  }
+  return real_size - offset;
 }

 /* Read data. */
@@ -176,7 +176,6 @@ static struct nbdkit_filter filter = {
   .longname          = "nbdkit offset filter",
   .version           = PACKAGE_VERSION,
   .config            = offset_config,
-  .config_complete   = offset_config_complete,
   .config_help       = offset_config_help,
   .get_size          = offset_get_size,
   .pread             = offset_pread,
-- 
2.21.0