[Qemu-devel] [PATCH v4 10/17] trace: don't abort qemu if ftrace can't be initialized

[Daniel P. Berrange]

If the ftrace backend is compiled into QEMU, any attempt
to start QEMU while non-root will fail due to the
inability to open /sys/kernel/debug/tracing/trace_on.

Add a fallback into the code so that it connects up the
trace_marker_fd variable to /dev/null when setting
EACCESS on the 'trace_on' file. This allows QEMU to
run, with ftrace turned into a no-op.

Signed-off-by: Daniel P. Berrange <address@hidden>
---
 trace/ftrace.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/trace/ftrace.c b/trace/ftrace.c
index e953922..3588bb0 100644
--- a/trace/ftrace.c
+++ b/trace/ftrace.c
@@ -51,6 +51,12 @@ bool ftrace_init(void)
         snprintf(path, PATH_MAX, "%s/tracing/tracing_on", debugfs);
         trace_fd = open(path, O_WRONLY);
         if (trace_fd < 0) {
+            if (errno == EACCES) {
+                trace_marker_fd = open("/dev/null", O_WRONLY);
+                if (trace_marker_fd != -1) {
+                    return true;
+                }
+            }
             perror("Could not open ftrace 'tracing_on' file");
             return false;
         } else {
-- 
2.7.4