[PATCH 2/9] migration: Fix file migration with fdset

[Fabiano Rosas]

When the migration using the "file:" URI was implemented, I don't
think any of us noticed that if you pass in a file name with the
format "/dev/fdset/N", this allows a file descriptor to be passed in
to QEMU and that behaves just like the "fd:" URI. So the "file:"
support has been added without regard for the fdset part and we got
some things wrong.

The first issue is that we should not truncate the migration file if
we're allowing an fd + offset. We need to leave the file contents
untouched.

The second issue is that there's an expectation that QEMU removes the
fd after the migration has finished. That's what the "fd:" code
does. Otherwise a second migration on the same VM could attempt to
provide an fdset with the same name and QEMU would reject it.

We can fix the first issue by detecting when we're using the fdset
vs. the plain file name. This requires storing the fdset_id
somewhere. We can then use this stored fdset_id to do cleanup at the
end and also fix the second issue.

Fixes: 385f510df5 ("migration: file URI offset")
Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 migration/file.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 46 insertions(+), 2 deletions(-)

diff --git a/migration/file.c b/migration/file.c
index ab18ba505a..8f30999400 100644
--- a/migration/file.c
+++ b/migration/file.c
@@ -10,6 +10,7 @@
 #include "qemu/cutils.h"
 #include "qemu/error-report.h"
 #include "qapi/error.h"
+#include "qapi/qapi-commands-misc.h"
 #include "channel.h"
 #include "file.h"
 #include "migration.h"
@@ -23,6 +24,7 @@
 
 static struct FileOutgoingArgs {
     char *fname;
+    int64_t fdset_id;
 } outgoing_args;
 
 /* Remove the offset option from @filespec and return it in @offsetp. */
@@ -44,10 +46,39 @@ int file_parse_offset(char *filespec, uint64_t *offsetp, 
Error **errp)
     return 0;
 }
 
+static void file_remove_fdset(void)
+{
+    if (outgoing_args.fdset_id != -1) {
+        qmp_remove_fd(outgoing_args.fdset_id, false, -1, NULL);
+        outgoing_args.fdset_id = -1;
+    }
+}
+
+static bool file_parse_fdset(const char *filename, int64_t *fdset_id,
+                             Error **errp)
+{
+    const char *fdset_id_str;
+
+    *fdset_id = -1;
+
+    if (!strstart(filename, "/dev/fdset/", &fdset_id_str)) {
+        return true;
+    }
+
+    *fdset_id = qemu_parse_fd(fdset_id_str);
+    if (*fdset_id == -1) {
+        error_setg_errno(errp, EINVAL, "Could not parse fdset %s", 
fdset_id_str);
+        return false;
+    }
+
+    return true;
+}
+
 void file_cleanup_outgoing_migration(void)
 {
     g_free(outgoing_args.fname);
     outgoing_args.fname = NULL;
+    file_remove_fdset();
 }
 
 bool file_send_channel_create(gpointer opaque, Error **errp)
@@ -81,11 +112,24 @@ void file_start_outgoing_migration(MigrationState *s,
     g_autofree char *filename = g_strdup(file_args->filename);
     uint64_t offset = file_args->offset;
     QIOChannel *ioc;
+    int flags = O_CREAT | O_WRONLY;
 
     trace_migration_file_outgoing(filename);
 
-    fioc = qio_channel_file_new_path(filename, O_CREAT | O_WRONLY | O_TRUNC,
-                                     0600, errp);
+    if (!file_parse_fdset(filename, &outgoing_args.fdset_id, errp)) {
+        return;
+    }
+
+    /*
+     * Only truncate if it's QEMU opening the file. If an fd has been
+     * passed in the file will already contain data written by the
+     * management layer.
+     */
+    if (outgoing_args.fdset_id == -1) {
+        flags |= O_TRUNC;
+    }
+
+    fioc = qio_channel_file_new_path(filename, flags, 0600, errp);
     if (!fioc) {
         return;
     }
-- 
2.35.3