trace: forbid use of %m in trace event format strings

The '%m' format instructs glibc's printf()/syslog() implementation to
insert the contents of strerror(errno). Since this is a glibc extension
it should generally be avoided in QEMU due to need for portability to a
variety of platforms.

Even though vfio is Linux-only code that could otherwise use "%m", it
must still be avoided in trace-events files because several of the
backends do not use the format string and so this error information is
invisible to them.

The errno string value should be given as an explicit trace argument
instead, making it accessible to all backends. This also allows it to
work correctly with future patches that use the format string with
systemtap's simple printf code.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20190123120016.4538-4-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/vfio/pci.c

@@ -2581,7 +2581,7 @@ static void vfio_populate_device(VFIOPCIDevice *vdev, Error **errp)
     ret = ioctl(vdev->vbasedev.fd, VFIO_DEVICE_GET_IRQ_INFO, &irq_info);
     if (ret) {
         /* This can fail for an old kernel or legacy PCI dev */
-        trace_vfio_populate_device_get_irq_info_failure();
+        trace_vfio_populate_device_get_irq_info_failure(strerror(errno));
     } else if (irq_info.count == 1) {
         vdev->pci_aer = true;
     } else {

hw/vfio/trace-events

@@ -37,7 +37,7 @@ vfio_pci_hot_reset_has_dep_devices(const char *name) "%s: hot reset dependent de
 vfio_pci_hot_reset_dep_devices(int domain, int bus, int slot, int function, int group_id) "\t%04x:%02x:%02x.%x group %d"
 vfio_pci_hot_reset_result(const char *name, const char *result) "%s hot reset: %s"
 vfio_populate_device_config(const char *name, unsigned long size, unsigned long offset, unsigned long flags) "Device %s config:\n  size: 0x%lx, offset: 0x%lx, flags: 0x%lx"
-vfio_populate_device_get_irq_info_failure(void) "VFIO_DEVICE_GET_IRQ_INFO failure: %m"
+vfio_populate_device_get_irq_info_failure(const char *errstr) "VFIO_DEVICE_GET_IRQ_INFO failure: %s"
 vfio_realize(const char *name, int group_id) " (%s) group %d"
 vfio_mdev(const char *name, bool is_mdev) " (%s) is_mdev %d"
 vfio_add_ext_cap_dropped(const char *name, uint16_t cap, uint16_t offset) "%s 0x%x@0x%x"

scripts/tracetool/__init__.py

@@ -274,6 +274,10 @@ class Event(object):
         props = groups["props"].split()
         fmt = groups["fmt"]
         fmt_trans = groups["fmt_trans"]
+        if fmt.find("%m") != -1 or fmt_trans.find("%m") != -1:
+            raise ValueError("Event format '%m' is forbidden, pass the error "
+                             "as an explicit trace argument")
+
         if len(fmt_trans) > 0:
             fmt = [fmt_trans, fmt]
         args = Arguments.build(groups["args"])