When trying to migrate a huge page enabled guest, I've noticed
the following crash. Apparently, if no specific hugepages are
requested:

  <memoryBacking>
    <hugepages/>
  </memoryBacking>

and there are no hugepages configured on the destination, we try
to dereference a NULL pointer.

Program received signal SIGSEGV, Segmentation fault.
0x00007fcc907fb20e in qemuGetHugepagePath (hugepage=0x0) at 
qemu/qemu_conf.c:1447
1447        if (virAsprintf(&ret, "%s/libvirt/qemu", hugepage->mnt_dir) < 0)
(gdb) bt
#0  0x00007fcc907fb20e in qemuGetHugepagePath (hugepage=0x0) at 
qemu/qemu_conf.c:1447
#1  0x00007fcc907fb2f5 in qemuGetDefaultHugepath (hugetlbfs=0x0, nhugetlbfs=0) 
at qemu/qemu_conf.c:1466
#2  0x00007fcc907b4afa in qemuBuildMemoryBackendStr (size=4194304, pagesize=0, 
guestNode=0, userNodeset=0x0, autoNodeset=0x0, def=0x7fcc70019070, 
qemuCaps=0x7fcc70004000, cfg=0x7fcc5c011800, backendType=0x7fcc95087228, 
backendProps=0x7fcc95087218,
    force=false) at qemu/qemu_command.c:3297
#3  0x00007fcc907b4f91 in qemuBuildMemoryCellBackendStr (def=0x7fcc70019070, 
qemuCaps=0x7fcc70004000, cfg=0x7fcc5c011800, cell=0, auto_nodeset=0x0, 
backendStr=0x7fcc70020360) at qemu/qemu_command.c:3413
#4  0x00007fcc907c0406 in qemuBuildNumaArgStr (cfg=0x7fcc5c011800, 
def=0x7fcc70019070, cmd=0x7fcc700040c0, qemuCaps=0x7fcc70004000, 
auto_nodeset=0x0) at qemu/qemu_command.c:7470
#5  0x00007fcc907c5fdf in qemuBuildCommandLine (driver=0x7fcc5c07b8a0, 
logManager=0x7fcc70003c00, def=0x7fcc70019070, monitor_chr=0x7fcc70004bb0, 
monitor_json=true, qemuCaps=0x7fcc70004000, migrateURI=0x7fcc700199c0 "defer", 
snapshot=0x0,
    vmop=VIR_NETDEV_VPORT_PROFILE_OP_MIGRATE_IN_START, standalone=false, 
enableFips=false, nodeset=0x0, nnicindexes=0x7fcc95087498, 
nicindexes=0x7fcc950874a0, domainLibDir=0x7fcc700047c0 
"/var/lib/libvirt/qemu/domain-1-fedora") at qemu/qemu_command.c:9547
#6  0x00007fcc90807d37 in qemuProcessLaunch (conn=0x7fcc54000b10, 
driver=0x7fcc5c07b8a0, vm=0x7fcc70005320, asyncJob=QEMU_ASYNC_JOB_MIGRATION_IN, 
incoming=0x7fcc70015d70, snapshot=0x0, 
vmop=VIR_NETDEV_VPORT_PROFILE_OP_MIGRATE_IN_START, flags=4)
    at qemu/qemu_process.c:5238
#7  0x00007fcc90815233 in qemuMigrationPrepareAny (driver=0x7fcc5c07b8a0, 
dconn=0x7fcc54000b10,
    cookiein=0x7fcc70001ec0 "<qemu-migration>\n  <name>fedora</name>\n  
<uuid>63840878-0deb-4095-97e6-fc444d9bc9fa</uuid>\n  
<hostname>lisa</hostname>\n  
<hostuuid>da1c2801-52f3-11cb-a9db-e905fe22010c</hostuuid>\n  <feature 
name='lock"..., cookieinlen=328,
    cookieout=0x7fcc95087aa0, cookieoutlen=0x7fcc95087a90, def=0x7fcc950878f0, 
origname=0x0, st=0x0, protocol=0x7fcc908ab42f "tcp", port=49152, autoPort=true, 
listenAddress=0x0, nmigrate_disks=0, migrate_disks=0x0, nbdPort=0, 
compression=0x7fcc70000a30,
    flags=64) at qemu/qemu_migration.c:3752
#8  0x00007fcc908161ae in qemuMigrationPrepareDirect (driver=0x7fcc5c07b8a0, 
dconn=0x7fcc54000b10,
    cookiein=0x7fcc70001ec0 "<qemu-migration>\n  <name>fedora</name>\n  
<uuid>63840878-0deb-4095-97e6-fc444d9bc9fa</uuid>\n  
<hostname>lisa</hostname>\n  
<hostuuid>da1c2801-52f3-11cb-a9db-e905fe22010c</hostuuid>\n  <feature 
name='lock"..., cookieinlen=328,
    cookieout=0x7fcc95087aa0, cookieoutlen=0x7fcc95087a90, uri_in=0x0, 
uri_out=0x7fcc700008c0, def=0x7fcc950878f0, origname=0x0, listenAddress=0x0, 
nmigrate_disks=0, migrate_disks=0x0, nbdPort=0, compression=0x7fcc70000a30, 
flags=64)
    at qemu/qemu_migration.c:4100
#9  0x00007fcc908673f3 in qemuDomainMigratePrepare3Params 
(dconn=0x7fcc54000b10, params=0x7fcc70000930, nparams=1,
    cookiein=0x7fcc70001ec0 "<qemu-migration>\n  <name>fedora</name>\n  
<uuid>63840878-0deb-4095-97e6-fc444d9bc9fa</uuid>\n  
<hostname>lisa</hostname>\n  
<hostuuid>da1c2801-52f3-11cb-a9db-e905fe22010c</hostuuid>\n  <feature 
name='lock"..., cookieinlen=328,
    cookieout=0x7fcc95087aa0, cookieoutlen=0x7fcc95087a90, 
uri_out=0x7fcc700008c0, flags=64) at qemu/qemu_driver.c:12204
#10 0x00007fcc9d5499da in virDomainMigratePrepare3Params (dconn=0x7fcc54000b10, 
params=0x7fcc70000930, nparams=1,
    cookiein=0x7fcc70001ec0 "<qemu-migration>\n  <name>fedora</name>\n  
<uuid>63840878-0deb-4095-97e6-fc444d9bc9fa</uuid>\n  
<hostname>lisa</hostname>\n  
<hostuuid>da1c2801-52f3-11cb-a9db-e905fe22010c</hostuuid>\n  <feature 
name='lock"..., cookieinlen=328,
    cookieout=0x7fcc95087aa0, cookieoutlen=0x7fcc95087a90, 
uri_out=0x7fcc700008c0, flags=64) at libvirt-domain.c:5141
#11 0x000055c71ce1c9f5 in remoteDispatchDomainMigratePrepare3Params 
(server=0x55c71f0a0cc0, client=0x55c71f0bef20, msg=0x55c71f0ae460, 
rerr=0x7fcc95087b80, args=0x7fcc70000900, ret=0x7fcc70000af0) at remote.c:5117
#12 0x000055c71cdfbd9e in remoteDispatchDomainMigratePrepare3ParamsHelper 
(server=0x55c71f0a0cc0, client=0x55c71f0bef20, msg=0x55c71f0ae460, 
rerr=0x7fcc95087b80, args=0x7fcc70000900, ret=0x7fcc70000af0) at 
remote_dispatch.h:7820
#13 0x00007fcc9d5cd810 in virNetServerProgramDispatchCall (prog=0x55c71f0ad070, 
server=0x55c71f0a0cc0, client=0x55c71f0bef20, msg=0x55c71f0ae460) at 
rpc/virnetserverprogram.c:437
#14 0x00007fcc9d5cd371 in virNetServerProgramDispatch (prog=0x55c71f0ad070, 
server=0x55c71f0a0cc0, client=0x55c71f0bef20, msg=0x55c71f0ae460) at 
rpc/virnetserverprogram.c:307
#15 0x000055c71ce35903 in virNetServerProcessMsg (srv=0x55c71f0a0cc0, 
client=0x55c71f0bef20, prog=0x55c71f0ad070, msg=0x55c71f0ae460) at 
rpc/virnetserver.c:148
#16 0x000055c71ce359c3 in virNetServerHandleJob (jobOpaque=0x55c71f0aef30, 
opaque=0x55c71f0a0cc0) at rpc/virnetserver.c:169
#17 0x00007fcc9d47a6f3 in virThreadPoolWorker (opaque=0x55c71f0ae540) at 
util/virthreadpool.c:167
#18 0x00007fcc9d479c95 in virThreadHelper (data=0x55c71f0ae560) at 
util/virthread.c:206
#19 0x00007fcc9aabd494 in start_thread () from /lib64/libpthread.so.0
#20 0x00007fcc9a8025dd in clone () from /lib64/libc.so.6

Signed-off-by: Michal Privoznik <mpriv...@redhat.com>
---
 src/qemu/qemu_command.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index 038c38f..0bafc3f 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -3294,6 +3294,12 @@ qemuBuildMemoryBackendStr(unsigned long long size,
             if (!(mem_path = qemuGetHugepagePath(&cfg->hugetlbfs[i])))
                 goto cleanup;
         } else {
+            if (!cfg->nhugetlbfs) {
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               "%s", _("hugetlbfs filesystem is not mounted "
+                                       "or disabled by administrator config"));
+                goto cleanup;
+            }
             if (!(mem_path = qemuGetDefaultHugepath(cfg->hugetlbfs,
                                                     cfg->nhugetlbfs)))
                 goto cleanup;
-- 
2.8.4

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