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trace: don't abort qemu if ftrace can't be initialized

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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/tracing_on.

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

[Fixed s/setting/getting/ and s/EACCESS/EACCES/ errors pointed out by
Eric Blake <eblake@redhat.com>.
--Stefan]

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1475588159-30598-13-git-send-email-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Daniel P. Berrange 2016-10-04 14:35:51 +01:00 committed by Stefan Hajnoczi
parent 7f1b588f20
commit 8ed5372874
1 changed files with 6 additions and 0 deletions
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6
trace/ftrace.c
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@ -51,6 +51,12 @@ bool ftrace_init(void)
snprintf(path, PATH_MAX, "%s/tracing/tracing_on", debugfs); snprintf(path, PATH_MAX, "%s/tracing/tracing_on", debugfs);
trace_fd = open(path, O_WRONLY); trace_fd = open(path, O_WRONLY);
if (trace_fd < 0) { 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"); perror("Could not open ftrace 'tracing_on' file");
return false; return false;
} else { } else {