Re: [PATCH] perf: Fix probing for PERF_FLAG_FD_CLOEXEC flag

From: Adrian Hunter
Date: Thu Feb 19 2015 - 11:18:18 EST

On 19/02/2015 4:55 p.m., David Ahern wrote:
On 2/19/15 12:06 AM, Adrian Hunter wrote:
/* not supported, confirm error related to PERF_FLAG_FD_CLOEXEC */
- fd = sys_perf_event_open(&attr, pid, cpu, -1, 0);
+ fd = sys_perf_event_open(&attr, 0, cpu, -1, 0);

I would prefer to avoid pid = 0 unless necessary and so just do the same
thing again i.e.

while (1) {
fd = sys_perf_event_open(&attr, pid, cpu, -1, 0);
if (fd < 0 && pid == -1 && errno == EACCES) {
pid = 0;

The probing is getting of hand. In this case the intent is a probe for a flag
and flags are the first thing checked kernel side. Given that the parameters
> passed to sys_perf_event_open should be as simple and known safe as possible.
> pid = -1 has known limitations. Why can't pid just be getpid() in both cases?

Simplifies this function a lot and removes the need for sched_getcpu(). So
pid = getpid();

fd = sys_perf_event_open(&attr, pid, -1, -1, PERF_FLAG_FD_CLOEXEC);

and if that fails

fd = sys_perf_event_open(&attr, pid, -1, -1, 0);

Why is anything more complicated needed?

Yes, I am sorry it is a pain. I don't know why I didn't add a comment
to the code :-(. Using -1 for the pid is a workaround to avoid gratuitous
jump label changes. If pid=0 is used and then a system-wide trace is done
with Intel PT, there will be a jump label change shortly after the tracing
starts. That means the running code gets changed, but Intel PT decoding
has to walk the code to reconstruct the trace - so errors result. There
will always be occasional jump label changes, but this avoids one that
would otherwise always happen.
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