block/file-*: *_parse_filename() and colons

The file drivers' *_parse_filename() implementations just strip the
optional protocol prefix off the filename. However, for e.g.
"file:foo:bar", this would lead to "foo:bar" being stored as the BDS's
filename which looks like it should be managed using the "foo" protocol.
This is especially troublesome if you then try to resolve a backing
filename based on "foo:bar".

This issue can only occur if the stripped part is a relative filename
("file:/foo:bar" will be shortened to "/foo:bar" and having a slash
before the first colon means that "/foo" is not recognized as a protocol
part). Therefore, we can easily fix it by prepending "./" to such
filenames.

Before this patch:
$ ./qemu-img create -f qcow2 backing.qcow2 64M
Formatting 'backing.qcow2', fmt=qcow2 size=67108864 encryption=off
    cluster_size=65536 lazy_refcounts=off refcount_bits=16
$ ./qemu-img create -f qcow2 -b backing.qcow2 file:top:image.qcow2
Formatting 'file:top:image.qcow2', fmt=qcow2 size=67108864
    backing_file=backing.qcow2 encryption=off cluster_size=65536
    lazy_refcounts=off refcount_bits=16
$ ./qemu-io file:top:image.qcow2
can't open device file:top:image.qcow2: Could not open backing file:
    Unknown protocol 'top'

After this patch:
$ ./qemu-io file:top:image.qcow2
[no error]

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170522195217.12991-3-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 35 insertions, 0 deletions

diff --git a/block.c b/block.c
index b72b872..fa1d06d 100644
--- a/block.c
+++ b/block.c
@@ -197,6 +197,41 @@ void path_combine(char *dest, int dest_size,
     }
 }
 
+/*
+ * Helper function for bdrv_parse_filename() implementations to remove optional
+ * protocol prefixes (especially "file:") from a filename and for putting the
+ * stripped filename into the options QDict if there is such a prefix.
+ */
+void bdrv_parse_filename_strip_prefix(const char *filename, const char *prefix,
+                                      QDict *options)
+{
+    if (strstart(filename, prefix, &filename)) {
+        /* Stripping the explicit protocol prefix may result in a protocol
+         * prefix being (wrongly) detected (if the filename contains a colon) */
+        if (path_has_protocol(filename)) {
+            QString *fat_filename;
+
+            /* This means there is some colon before the first slash; therefore,
+             * this cannot be an absolute path */
+            assert(!path_is_absolute(filename));
+
+            /* And we can thus fix the protocol detection issue by prefixing it
+             * by "./" */
+            fat_filename = qstring_from_str("./");
+            qstring_append(fat_filename, filename);
+
+            assert(!path_has_protocol(qstring_get_str(fat_filename)));
+
+            qdict_put(options, "filename", fat_filename);
+        } else {
+            /* If no protocol prefix was detected, we can use the shortened
+             * filename as-is */
+            qdict_put_str(options, "filename", filename);
+        }
+    }
+}
+
+
 /* Returns whether the image file is opened as read-only. Note that this can
  * return false and writing to the image file is still not possible because the
  * image is inactivated. */