linux-kernel - Re: [PATCH] perf: Fix probing for PERF_FLAG_FD

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Date:	Thu, 19 Feb 2015 09:06:12 +0200
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	David Ahern <david.ahern@...cle.com>, acme@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: Fix probing for PERF_FLAG_FD_CLOEXEC flag

On 19/02/15 02:01, David Ahern wrote:
> Commit f6edb53c4993ffe92ce521fb449d1c146cea6ec2 converted the probe to
> a CPU wide event first (pid == -1). For kernels that do not support
> the PERF_FLAG_FD_CLOEXEC flag the probe fails with EINVAL. Since this
> errno is not handled pid is not reset to 0 and the subsequent use of
> pid = -1 as an argument brings in an additional failure path if
> perf_event_paranoid > 0:
> 
> $ perf record -- sleep 1
> perf_event_open(..., 0) failed unexpectedly with error 13 (Permission denied)
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.007 MB /tmp/perf.data (11 samples) ]
> 
> Since this function only needs to get past this check in kernel/events/core.c:
> 
>         /* for future expandability... */
>         if (flags & ~PERF_FLAG_ALL)
>                 return -EINVAL;
> 
> pid = 0 is sufficient to confirm if the flag is supported or not.
> 
> Also, ensure the fd of the confirmation check is closed.
> 
> Needs to go to 3.18 stable tree as well.
> 
> Signed-off-by: David Ahern <david.ahern@...cle.com>
> Cc: Adrian Hunter <adrian.hunter@...el.com>
> ---
>  tools/perf/util/cloexec.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/perf/util/cloexec.c b/tools/perf/util/cloexec.c
> index 47b78b3f0325..3cc34edf2403 100644
> --- a/tools/perf/util/cloexec.c
> +++ b/tools/perf/util/cloexec.c
> @@ -47,16 +47,17 @@ static int perf_flag_probe(void)
>  		  err, strerror_r(err, sbuf, sizeof(sbuf)));
>  
>  	/* 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;
			continue;
		}
		break;
	}

>  	err = errno;
>  
> +	if (fd >= 0)
> +		close(fd);
> +
>  	if (WARN_ONCE(fd < 0 && err != EBUSY,
>  		      "perf_event_open(..., 0) failed unexpectedly with error %d (%s)\n",
>  		      err, strerror_r(err, sbuf, sizeof(sbuf))))
>  		return -1;
>  
> -	close(fd);
> -
>  	return 0;
>  }
>  
> 

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