When virDomainGetFSInfo() is called over a QEMU/KVM domain it
results into calling of 'guest-get-fsinfo' guest agent command to
which it replies with info on guest (mounted) filesystems. When
filling return structure we also try to do basic lookup and
translate guest agent provided disk address into disk target (as
seen in domain XML). This can of course fail - guest can have
variety of disks not recorded in domain XML (iSCSI, scsi_debug,
NFS to name a few). If that's the case, a debug message is logged
and no disk target is added into the return structure.

However, due to the way our code is written the caller is led to
believe that the target was added into the structure. This may
lead to a situation where the array of disk targets (strings)
contains NULL. But our RPC structure says the array contains only
non-NULL strings. This results in somewhat 'cryptic' (at least to
users) error message:

  error: Unable to get filesystem information
  error: Unable to encode message payload

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1919783
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
---
 src/qemu/qemu_driver.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index d30cf75b73..0da9264b49 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -18932,9 +18932,8 @@ qemuAgentFSInfoToPublic(qemuAgentFSInfoPtr agent,
 
     if (agent->disks)
         ret->devAlias = g_new0(char *, agent->ndisks);
-    ret->ndevAlias = agent->ndisks;
 
-    for (i = 0; i < ret->ndevAlias; i++) {
+    for (i = 0; i < agent->ndisks; i++) {
         qemuAgentDiskAddressPtr agentdisk = agent->disks[i];
         virDomainDiskDefPtr diskDef;
 
@@ -18945,7 +18944,7 @@ qemuAgentFSInfoToPublic(qemuAgentFSInfoPtr agent,
                                          agentdisk->target,
                                          agentdisk->unit);
         if (diskDef != NULL)
-            ret->devAlias[i] = g_strdup(diskDef->dst);
+            ret->devAlias[ret->ndevAlias++] = g_strdup(diskDef->dst);
         else
             VIR_DEBUG("Missing target name for '%s'.", ret->mountpoint);
     }
-- 
2.26.2

On a Monday in 2021, Michal Privoznik wrote:
>When virDomainGetFSInfo() is called over a QEMU/KVM domain it
>results into calling of 'guest-get-fsinfo' guest agent command to
>which it replies with info on guest (mounted) filesystems. When
>filling return structure we also try to do basic lookup and
>translate guest agent provided disk address into disk target (as
>seen in domain XML). This can of course fail - guest can have
>variety of disks not recorded in domain XML (iSCSI, scsi_debug,
>NFS to name a few). If that's the case, a debug message is logged
>and no disk target is added into the return structure.
>
>However, due to the way our code is written the caller is led to
>believe that the target was added into the structure. This may
>lead to a situation where the array of disk targets (strings)
>contains NULL. But our RPC structure says the array contains only
>non-NULL strings. This results in somewhat 'cryptic' (at least to
>users) error message:
>
>  error: Unable to get filesystem information
>  error: Unable to encode message payload
>
>Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1919783
>Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
>---
> src/qemu/qemu_driver.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>

Reviewed-by: Ján Tomko <jtomko@redhat.com>

Jano