From 6c4c780eda082ef395b70af89b8b8942b9ad4a9d Mon Sep 17 00:00:00 2001
Message-Id: <6c4c780eda082ef395b70af89b8b8942b9ad4a9d@dist-git>
From: Michal Privoznik <mprivozn@redhat.com>
Date: Mon, 8 Jan 2018 10:16:19 +0100
Subject: [PATCH] qemuDomainAttachDeviceMknodHelper: Remove symlink before
 creating it

https://bugzilla.redhat.com/show_bug.cgi?id=1528502

So imagine you have /dev/blah symlink which points to /dev/sda.
You attach /dev/blah as disk to your domain. Libvirt correctly
creates the /dev/blah -> /dev/sda symlink in the qemu namespace.
However, then you detach the disk, change the symlink so that it
points to /dev/sdb and tries to attach the disk again. This time,
however, the attach fails (well, qemu attaches wrong disk)
because the code assumes that symlinks don't change. Well they
do.

This is inspired by test fix written by Eduardo Habkost.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
(cherry picked from commit db98e7f67ea0d7699410f514f01947cef5128a6c)
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
---
 src/qemu/qemu_domain.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index 8967f2774d..b0c78cc7c4 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -9559,13 +9559,23 @@ qemuDomainAttachDeviceMknodHelper(pid_t pid ATTRIBUTE_UNUSED,
 
     if (isLink) {
         VIR_DEBUG("Creating symlink %s -> %s", data->file, data->target);
+
+        /* First, unlink the symlink target. Symlinks change and
+         * therefore we have no guarantees that pre-existing
+         * symlink is still valid. */
+        if (unlink(data->file) < 0 &&
+            errno != ENOENT) {
+            virReportSystemError(errno,
+                                 _("Unable to remove symlink %s"),
+                                 data->file);
+            goto cleanup;
+        }
+
         if (symlink(data->target, data->file) < 0) {
-            if (errno != EEXIST) {
-                virReportSystemError(errno,
-                                     _("Unable to create symlink %s"),
-                                     data->target);
-                goto cleanup;
-            }
+            virReportSystemError(errno,
+                                 _("Unable to create symlink %s (pointing to %s)"),
+                                 data->file, data->target);
+            goto cleanup;
         } else {
             delDevice = true;
         }
-- 
2.15.1