[PATCH 1/2] virdevmapper: Don't error on kernels without DM support

[Michal Privoznik <mprivozn@xxxxxxxxxx>]

In one of my latest patch (v6.6.0~30) I was trying to remove
libdevmapper use in favor of our own implementation. However, the
code did not take into account that device mapper can be not
compiled into the kernel (e.g. be a separate module that's not
loaded) in which case /proc/devices won't have the device-mapper
major number and thus virDevMapperGetTargets() and/or
virIsDevMapperDevice() fails.

Fixes: 22494556542c676d1b9e7f1c1f2ea13ac17e1e3e
Reported-by: Andrea Bolognani <abologna@xxxxxxxxxx>
Signed-off-by: Michal Privoznik <mprivozn@xxxxxxxxxx>
---
 src/util/virdevmapper.c | 41 +++++++++++++++++++++++++++++++----------
 1 file changed, 31 insertions(+), 10 deletions(-)

diff --git a/src/util/virdevmapper.c b/src/util/virdevmapper.c
index fe7f611496..cc33d8211e 100644
--- a/src/util/virdevmapper.c
+++ b/src/util/virdevmapper.c
@@ -47,10 +47,11 @@
 G_STATIC_ASSERT(BUF_SIZE > sizeof(struct dm_ioctl));
 
 static unsigned int virDMMajor;
+static virMutex virDevMapperInitMutex = VIR_MUTEX_INITIALIZER;
 
 
 static int
-virDevMapperOnceInit(void)
+virDevMapperGetMajor(unsigned int *dmmaj)
 {
     g_autofree char *buf = NULL;
     VIR_AUTOSTRINGLIST lines = NULL;
@@ -69,23 +70,34 @@ virDevMapperOnceInit(void)
 
         if (sscanf(lines[i], "%u %ms\n", &maj, &dev) == 2 &&
             STREQ(dev, DM_NAME)) {
-            virDMMajor = maj;
+            *dmmaj = maj;
             break;
         }
     }
 
     if (!lines[i]) {
-        virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("Unable to find major for %s"),
-                       DM_NAME);
-        return -1;
+        /* Don't error here. Maybe the kernel was built without
+         * devmapper. Let the caller decide. */
+        return -2;
     }
 
     return 0;
 }
 
 
-VIR_ONCE_GLOBAL_INIT(virDevMapper);
+static int
+virDevMapperInit(void)
+{
+    int rc = 0;
+
+    virMutexLock(&virDevMapperInitMutex);
+
+    if (virDMMajor == 0)
+        rc = virDevMapperGetMajor(&virDMMajor);
+
+    virMutexUnlock(&virDevMapperInitMutex);
+    return rc;
+}
 
 
 static void *
@@ -220,7 +232,6 @@ virDevMapperGetTargetsImpl(int controlFD,
     size_t i;
 
     memset(&dm, 0, sizeof(dm));
-    *devPaths_ret = NULL;
 
     if (ttl == 0) {
         errno = ELOOP;
@@ -298,14 +309,24 @@ virDevMapperGetTargets(const char *path,
                        char ***devPaths)
 {
     VIR_AUTOCLOSE controlFD = -1;
+    int rc;
     const unsigned int ttl = 32;
 
     /* Arbitrary limit on recursion level. A devmapper target can
      * consist of devices or yet another targets. If that's the
      * case, we have to stop recursion somewhere. */
 
-    if (virDevMapperInitialize() < 0)
+    *devPaths = NULL;
+
+    if ((rc = virDevMapperInit()) < 0) {
+        if (rc == -2) {
+            /* Devmapper is not available. Yet. Maybe the module
+             * will be loaded later, but do not error out for now. */
+            return 0;
+        }
+
         return -1;
+    }
 
     if ((controlFD = virDMOpen()) < 0)
         return -1;
@@ -319,7 +340,7 @@ virIsDevMapperDevice(const char *dev_name)
 {
     struct stat buf;
 
-    if (virDevMapperInitialize() < 0)
+    if (virDevMapperInit() < 0)
         return false;
 
     if (!stat(dev_name, &buf) &&
-- 
2.26.2