[PATCH 2/3] chardev: preserve the logfile fd across cpr-exec #VSTOR-148304

["Denis V. Lunev" <den@openvz.org>]

From: Denis V. Lunev <den@openvz.org>

A chardev with logfile= opens the log in qemu_char_open() on every
start, including the start of the new QEMU after cpr-exec. The backend
fds are taken from the cpr fd registry there, the log fd is not, so the
new process reopens the path itself.

libvirt normally opens the log through virtlogd and passes a pipe in an
fdset. The qemu-update command line cannot carry that fdset, so it
names the log file path. The exec'd QEMU, running as the qemu user,
then fails to create a log it has no write access to, e.g. a nova
console.log on node-local storage:

  qemu-kvm: -chardev pty,id=charserial0,logfile=.../console.log,
  logappend=on: Could not create '.../console.log': Permission denied

and the VM is destroyed. Where the open succeeds, the new QEMU writes
the file directly, behind virtlogd's back.

Save the log fd in the cpr registry under the chardev label and take it
from there on cpr-exec incoming, as the backends do. The logfile path
is not opened at all then. If the source did not save it, run without
a log rather than reopen the path. A failed chardev-change re-claims
the log through the base class cpr_claim_fds(), since the replacement's
log has displaced the entry.

Signed-off-by: Denis V. Lunev <den@openvz.org>
---
 chardev/char.c          | 30 +++++++++++++++++++++++++-----
 include/migration/cpr.h |  1 +
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/chardev/char.c b/chardev/char.c
index b64767f574e..c037d2e7730 100644
--- a/chardev/char.c
+++ b/chardev/char.c
@@ -40,6 +40,7 @@
 #include "qemu/id.h"
 #include "qemu/coroutine.h"
 #include "qemu/yank.h"
+#include "migration/cpr.h"
 
 #include "chardev-internal.h"
 
@@ -261,9 +262,21 @@ static void qemu_char_open(Chardev *chr, ChardevBackend *backend,
         } else {
             flags |= O_TRUNC;
         }
-        chr->logfd = qemu_create(common->logfile, flags, 0666, errp);
-        if (chr->logfd < 0) {
-            return;
+        if (cpr_exec_is_incoming()) {
+            chr->logfd = cpr_find_fd(chr->label, CPR_CHARDEV_LOG_ID);
+            if (chr->logfd < 0) {
+                warn_report("chardev '%s': log file was not preserved, "
+                            "continuing without it", chr->label);
+            }
+        } else {
+            chr->logfd = qemu_create(common->logfile, flags, 0666, errp);
+            if (chr->logfd < 0) {
+                return;
+            }
+            if (!cpr_is_incoming()) {
+                cpr_delete_fd(chr->label, CPR_CHARDEV_LOG_ID);
+                cpr_save_fd(chr->label, CPR_CHARDEV_LOG_ID, chr->logfd);
+            }
         }
     }
 
@@ -297,6 +310,10 @@ static int null_chr_write(Chardev *chr, const uint8_t *buf, int len)
 
 static void char_cpr_claim_fds(Chardev *chr)
 {
+    if (chr->logfd >= 0) {
+        cpr_delete_fd(chr->label, CPR_CHARDEV_LOG_ID);
+        cpr_save_fd(chr->label, CPR_CHARDEV_LOG_ID, chr->logfd);
+    }
 }
 
 static void char_class_init(ObjectClass *oc, const void *data)
@@ -315,11 +332,14 @@ static void char_finalize(Object *obj)
     if (chr->be) {
         chr->be->chr = NULL;
     }
-    g_free(chr->filename);
-    g_free(chr->label);
     if (chr->logfd != -1) {
+        if (cpr_find_fd(chr->label, CPR_CHARDEV_LOG_ID) == chr->logfd) {
+            cpr_delete_fd(chr->label, CPR_CHARDEV_LOG_ID);
+        }
         close(chr->logfd);
     }
+    g_free(chr->filename);
+    g_free(chr->label);
     qemu_mutex_destroy(&chr->chr_write_lock);
 }
 
diff --git a/include/migration/cpr.h b/include/migration/cpr.h
index d74ff1ea602..8fb79eb0a28 100644
--- a/include/migration/cpr.h
+++ b/include/migration/cpr.h
@@ -15,6 +15,7 @@
 
 #define CPR_LISTENSOCK_ID       0
 #define CPR_CONNSOCK_ID         1
+#define CPR_CHARDEV_LOG_ID      2
 
 #define QEMU_CPR_FILE_MAGIC     0x51435052
 #define QEMU_CPR_FILE_VERSION   0x00000001
-- 
2.53.0