When setting up mount namespace for a qemu domain the following
steps are executed:

1) get list of mountpoints under /dev/
2) move them to /var/run/libvirt/qemu/$domName.ext
3) start constructing new device tree under /var/run/libvirt/qemu/$domName.dev
4) move the mountpoint of the new device tree to /dev
5) restore original mountpoints from step 2)

Not the problem with this approach is that if some device in step
3) requires access to a mountpoint from step 2) it will fail as
the mountpoint is not there anymore. For instance consider the
following domain disk configuration:

    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/dev/shm/vhostmd0'/>
      <target dev='vdb' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
    </disk>

In this case operation fails as we are unable to create vhostmd0
in the new device tree because after step 2) there is no /dev/shm
anymore. Leave aside fact that we shouldn't try to create devices
living in other mountpoints. That's a separate bug that will be
addressed later.

Currently, the order described above is rearranged to:

1) get list of mountpoints under /dev/
2) start constructing new device tree under /var/run/libvirt/qemu/$domName.dev
3) move them to /var/run/libvirt/qemu/$domName.ext
4) move the mountpoint of the new device tree to /dev
5) restore original mountpoints from step 3)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 src/qemu/qemu_domain.c | 54 +++++++++++++++++++++++++-------------------------
 1 file changed, 27 insertions(+), 27 deletions(-)

diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index 00b0b4a..be02d54 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -7950,33 +7950,6 @@ qemuDomainBuildNamespace(virQEMUDriverConfigPtr cfg,
     if (qemuDomainSetupDev(cfg, mgr, vm, devPath) < 0)
         goto cleanup;
 
-    /* Save some mount points because we want to share them with the host */
-    for (i = 0; i < ndevMountsPath; i++) {
-        struct stat sb;
-
-        if (devMountsSavePath[i] == devPath)
-            continue;
-
-        if (stat(devMountsPath[i], &sb) < 0) {
-            virReportSystemError(errno,
-                                 _("Unable to stat: %s"),
-                                 devMountsPath[i]);
-            goto cleanup;
-        }
-
-        /* At this point, devMountsPath is either a regular file or a directory. */
-        if ((S_ISDIR(sb.st_mode) && virFileMakePath(devMountsSavePath[i]) < 0) ||
-            (S_ISREG(sb.st_mode) && virFileTouch(devMountsSavePath[i], sb.st_mode) < 0)) {
-            virReportSystemError(errno,
-                                 _("Failed to create %s"),
-                                 devMountsSavePath[i]);
-            goto cleanup;
-        }
-
-        if (virFileMoveMount(devMountsPath[i], devMountsSavePath[i]) < 0)
-            goto cleanup;
-    }
-
     if (qemuDomainSetupAllDisks(cfg, vm, devPath) < 0)
         goto cleanup;
 
@@ -8001,6 +7974,33 @@ qemuDomainBuildNamespace(virQEMUDriverConfigPtr cfg,
     if (qemuDomainSetupAllRNGs(cfg, vm, devPath) < 0)
         goto cleanup;
 
+    /* Save some mount points because we want to share them with the host */
+    for (i = 0; i < ndevMountsPath; i++) {
+        struct stat sb;
+
+        if (devMountsSavePath[i] == devPath)
+            continue;
+
+        if (stat(devMountsPath[i], &sb) < 0) {
+            virReportSystemError(errno,
+                                 _("Unable to stat: %s"),
+                                 devMountsPath[i]);
+            goto cleanup;
+        }
+
+        /* At this point, devMountsPath is either a regular file or a directory. */
+        if ((S_ISDIR(sb.st_mode) && virFileMakePath(devMountsSavePath[i]) < 0) ||
+            (S_ISREG(sb.st_mode) && virFileTouch(devMountsSavePath[i], sb.st_mode) < 0)) {
+            virReportSystemError(errno,
+                                 _("Failed to create %s"),
+                                 devMountsSavePath[i]);
+            goto cleanup;
+        }
+
+        if (virFileMoveMount(devMountsPath[i], devMountsSavePath[i]) < 0)
+            goto cleanup;
+    }
+
     if (virFileMoveMount(devPath, "/dev") < 0)
         goto cleanup;
 
-- 
2.10.2

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On Fri, 2017-04-28 at 13:22 +0200, Michal Privoznik wrote:
> When setting up mount namespace for a qemu domain the following
> steps are executed:
> 
> 1) get list of mountpoints under /dev/
> 2) move them to /var/run/libvirt/qemu/$domName.ext
> 3) start constructing new device tree under /var/run/libvirt/qemu/$domName.dev
> 4) move the mountpoint of the new device tree to /dev
> 5) restore original mountpoints from step 2)
> 
> Not the problem with this approach is that if some device in step

You may have wanted to write "Note" rather than "Not".
Otherwise ACK.

--
Cedric

> 3) requires access to a mountpoint from step 2) it will fail as
> the mountpoint is not there anymore. For instance consider the
> following domain disk configuration:
> 
>     <disk type='file' device='disk'>
>       <driver name='qemu' type='raw'/>
>       <source file='/dev/shm/vhostmd0'/>
>       <target dev='vdb' bus='virtio'/>
>       <address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
>     </disk>
> 
> In this case operation fails as we are unable to create vhostmd0
> in the new device tree because after step 2) there is no /dev/shm
> anymore. Leave aside fact that we shouldn't try to create devices
> living in other mountpoints. That's a separate bug that will be
> addressed later.
> 
> Currently, the order described above is rearranged to:
> 
> 1) get list of mountpoints under /dev/
> 2) start constructing new device tree under /var/run/libvirt/qemu/$domName.dev
> 3) move them to /var/run/libvirt/qemu/$domName.ext
> 4) move the mountpoint of the new device tree to /dev
> 5) restore original mountpoints from step 3)
> 
> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> ---
>  src/qemu/qemu_domain.c | 54 +++++++++++++++++++++++++-------------------------
>  1 file changed, 27 insertions(+), 27 deletions(-)
> 
> diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
> index 00b0b4a..be02d54 100644
> --- a/src/qemu/qemu_domain.c
> +++ b/src/qemu/qemu_domain.c
> @@ -7950,33 +7950,6 @@ qemuDomainBuildNamespace(virQEMUDriverConfigPtr cfg,
>      if (qemuDomainSetupDev(cfg, mgr, vm, devPath) < 0)
>          goto cleanup;
>  
> -    /* Save some mount points because we want to share them with the host */
> -    for (i = 0; i < ndevMountsPath; i++) {
> -        struct stat sb;
> -
> -        if (devMountsSavePath[i] == devPath)
> -            continue;
> -
> -        if (stat(devMountsPath[i], &sb) < 0) {
> -            virReportSystemError(errno,
> -                                 _("Unable to stat: %s"),
> -                                 devMountsPath[i]);
> -            goto cleanup;
> -        }
> -
> -        /* At this point, devMountsPath is either a regular file or a directory. */
> -        if ((S_ISDIR(sb.st_mode) && virFileMakePath(devMountsSavePath[i]) < 0) ||
> -            (S_ISREG(sb.st_mode) && virFileTouch(devMountsSavePath[i], sb.st_mode) < 0)) {
> -            virReportSystemError(errno,
> -                                 _("Failed to create %s"),
> -                                 devMountsSavePath[i]);
> -            goto cleanup;
> -        }
> -
> -        if (virFileMoveMount(devMountsPath[i], devMountsSavePath[i]) < 0)
> -            goto cleanup;
> -    }
> -
>      if (qemuDomainSetupAllDisks(cfg, vm, devPath) < 0)
>          goto cleanup;
>  
> @@ -8001,6 +7974,33 @@ qemuDomainBuildNamespace(virQEMUDriverConfigPtr cfg,
>      if (qemuDomainSetupAllRNGs(cfg, vm, devPath) < 0)
>          goto cleanup;
>  
> +    /* Save some mount points because we want to share them with the host */
> +    for (i = 0; i < ndevMountsPath; i++) {
> +        struct stat sb;
> +
> +        if (devMountsSavePath[i] == devPath)
> +            continue;
> +
> +        if (stat(devMountsPath[i], &sb) < 0) {
> +            virReportSystemError(errno,
> +                                 _("Unable to stat: %s"),
> +                                 devMountsPath[i]);
> +            goto cleanup;
> +        }
> +
> +        /* At this point, devMountsPath is either a regular file or a directory. */
> +        if ((S_ISDIR(sb.st_mode) && virFileMakePath(devMountsSavePath[i]) < 0) ||
> +            (S_ISREG(sb.st_mode) && virFileTouch(devMountsSavePath[i], sb.st_mode) < 0)) {
> +            virReportSystemError(errno,
> +                                 _("Failed to create %s"),
> +                                 devMountsSavePath[i]);
> +            goto cleanup;
> +        }
> +
> +        if (virFileMoveMount(devMountsPath[i], devMountsSavePath[i]) < 0)
> +            goto cleanup;
> +    }
> +
>      if (virFileMoveMount(devPath, "/dev") < 0)
>          goto cleanup;
>  

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