[Xen-devel] [PATCH] libxl: create backend/ xenstore dir for driver domains

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

Cleaning up backend xenstore entries is a responsibility of the backend.
When backend lives outside of dom0, the domain needs proper permissions
to do it. Normally it is given permission to remove the device dir
itself, but not the dir containing it (named after frontend ID). After a
whole those empty leftover directories accumulate to the point xenstore
returning E2BIG on listing them.

Fix this by giving backend domain write access also to backend/
directory itself when c_info->driver_domain option is set. The code
removing relevant dir is already there (just lacked permissions to do so).

Note this also allows the backend domain to create new entries,
pretending to host backend devices it don't have. But since libxl uses
/libxl/ xenstore dir for this information (still outside of backend
domain control), this shouldn't be an issue.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
---
 tools/libxl/libxl_create.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index a6d40b753e..38ca9b85a4 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -763,6 +763,13 @@ retry_transaction:
          */
         libxl__xs_mknod(gc, t, GCSPRINTF("%s/device-model", dom_path), rwperm,
                         ARRAY_SIZE(rwperm));
+
+        /*
+         * Create a local "backend" directory for each guest, writable by that
+         * guest, to allow it properly cleanup removed devices
+         */
+        libxl__xs_mknod(gc, t, GCSPRINTF("%s/backend", dom_path), rwperm,
+                        ARRAY_SIZE(rwperm));
     }
 
     vm_list = libxl_list_vm(ctx, &nb_vm);
-- 
2.21.0


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