[PATCH 2/2] qemu_process: Start QEMU for caps probing more robustly

[Michal Privoznik <mprivozn@xxxxxxxxxx>]

When probing QEMU capabilities, we look at whatever <emulator/>
was specified in the domain XML and execute it with couple of
arguments (-daemonize being one of them) Then, we use
virCommandSetErrorBuffer() to read stderr of the child process
hoping to read possible error message just before the process
daemonized itself. Well, this works as long as the emulator
binary behaves.

If the binary is evil and basically does the following:

  #!/bin/bash
  sleep 1h

then virCommandRun() called from qemuProcessQMPLaunch() doesn't
return for whole hour (because it's stuck in reading stderr of
the child process). This behavior of ours is very suboptimal.

The solution is to not rely on the binary behaving correctly on
-daemonize argument but to daemonize the process ourselves (via
virCommandDaemonize()) and then wait for the monitor to show up
with a timeout. This in turn means, that we can no longer use
virCommandSetErrorBuffer() but we can do the equivalent with
virCommandSetErrorFD() and a bit of code.

Sure, this doesn't shield us from malicious binaries 100% but
helps preventing depletion of worker threads.

Signed-off-by: Michal Privoznik <mprivozn@xxxxxxxxxx>
---
 src/qemu/qemu_process.c | 58 +++++++++++++++++++++++++++++------------
 src/qemu/qemu_process.h |  1 -
 2 files changed, 42 insertions(+), 17 deletions(-)

diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index 2e149699b0..d038f7e2ae 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -9166,7 +9166,6 @@ qemuProcessQMPFree(qemuProcessQMP *proc)
     g_free(proc->monpath);
     g_free(proc->monarg);
     g_free(proc->pidfile);
-    g_free(proc->stdErr);
     g_free(proc);
 }
 
@@ -9285,7 +9284,9 @@ static int
 qemuProcessQMPLaunch(qemuProcessQMP *proc)
 {
     const char *machine;
-    int status = 0;
+    VIR_AUTOCLOSE errfd = -1;
+    virTimeBackOffVar timebackoff;
+    const unsigned long long timeout = 30 * 1000; /* ms */
     int rc;
 
     if (proc->forceTCG)
@@ -9310,9 +9311,7 @@ qemuProcessQMPLaunch(qemuProcessQMP *proc)
                                      "-nographic",
                                      "-machine", machine,
                                      "-qmp", proc->monarg,
-                                     "-pidfile", proc->pidfile,
-                                     "-daemonize",
-                                    NULL);
+                                     NULL);
     virCommandAddEnvPassCommon(proc->cmd);
     virCommandClearCaps(proc->cmd);
 
@@ -9326,26 +9325,53 @@ qemuProcessQMPLaunch(qemuProcessQMP *proc)
     virCommandSetGID(proc->cmd, proc->runGid);
     virCommandSetUID(proc->cmd, proc->runUid);
 
-    virCommandSetErrorBuffer(proc->cmd, &(proc->stdErr));
+    virCommandSetPidFile(proc->cmd, proc->pidfile);
+    virCommandSetErrorFD(proc->cmd, &errfd);
+    virCommandDaemonize(proc->cmd);
 
-    if (virCommandRun(proc->cmd, &status) < 0)
+    if (virCommandRun(proc->cmd, NULL) < 0)
         return -1;
 
-    if (status != 0) {
-        VIR_DEBUG("QEMU %s exited with status %d", proc->binary, status);
-        virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("Failed to start QEMU binary %s for probing: %s"),
-                       proc->binary,
-                       proc->stdErr ? proc->stdErr : _("unknown error"));
-        return -1;
-    }
-
     if ((rc = virPidFileReadPath(proc->pidfile, &proc->pid)) < 0) {
         virReportSystemError(-rc, _("Failed to read pidfile %s"), proc->pidfile);
         return -1;
     }
 
+    if (virTimeBackOffStart(&timebackoff, 1, timeout) < 0)
+        goto error;
+    while (virTimeBackOffWait(&timebackoff)) {
+        char errbuf[1024] = { 0 };
+
+        if (virFileExists(proc->monpath))
+            break;
+
+        if (virProcessKill(proc->pid, 0) == 0)
+            continue;
+
+        ignore_value(saferead(errfd, errbuf, sizeof(errbuf) - 1));
+        virReportError(VIR_ERR_INTERNAL_ERROR,
+                       _("Failed to start QEMU binary %s for probing: %s"),
+                       proc->binary,
+                       errbuf[0] ? errbuf : _("unknown error"));
+        goto error;
+    }
+
+    if (!virFileExists(proc->monpath)) {
+        virReportError(VIR_ERR_OPERATION_TIMEOUT, "%s",
+                       _("QEMU monitor did not show up"));
+        goto error;
+    }
+
     return 0;
+
+ error:
+    virCommandAbort(proc->cmd);
+    if (proc->pid >= 0)
+        virProcessKillPainfully(proc->pid, true);
+    if (proc->pidfile)
+        unlink(proc->pidfile);
+
+    return -1;
 }
 
 
diff --git a/src/qemu/qemu_process.h b/src/qemu/qemu_process.h
index 289cd74eb7..f73722846b 100644
--- a/src/qemu/qemu_process.h
+++ b/src/qemu/qemu_process.h
@@ -221,7 +221,6 @@ struct _qemuProcessQMP {
     char *libDir;
     uid_t runUid;
     gid_t runGid;
-    char *stdErr;
     char *monarg;
     char *monpath;
     char *pidfile;
-- 
2.34.1