linux-kernel - Re: [PATCH 1/2] libtracefs: Add random number to keep synthetic variables unique

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Date:   Thu, 12 Aug 2021 11:34:57 +0300
From:   Yordan Karadzhov <y.karadz@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        linux-trace-devel@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Tom Zanussi <zanussi@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH 1/2] libtracefs: Add random number to keep synthetic
 variables unique



On 12.08.21 г. 3:55, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> The 'hist' triggers expect that all variables are unique. If two synthetic
> events are created, it is possible that they will use the same variable
> names, and this can break the logic for synthetic events. Add a random
> number to the argument names that will help prevent that from happening.
> There's no guarantee that there wont be collisions, but the chances of
> that happening is 1 in 65535. If this is a problem, we could possibly look
> for variables that are already in use.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>   src/tracefs-hist.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/src/tracefs-hist.c b/src/tracefs-hist.c
> index 7f9cf3820611..8783ef4364bd 100644
> --- a/src/tracefs-hist.c
> +++ b/src/tracefs-hist.c
> @@ -13,6 +13,8 @@
>   #include <errno.h>
>   #include <fcntl.h>
>   #include <limits.h>
> +#include <time.h>
> +#include <sys/types.h>
>   
>   #include "tracefs.h"
>   #include "tracefs-local.h"
> @@ -558,6 +560,7 @@ struct tracefs_synth {
>   	unsigned int		end_parens;
>   	unsigned int		end_state;
>   	int			*start_type;
> +	char			arg_name[16];
>   	int			arg_cnt;
>   };
>   
> @@ -957,7 +960,15 @@ static char *new_arg(struct tracefs_synth *synth)
>   	char *arg;
>   	int ret;
>   
> -	ret = asprintf(&arg, "__arg__%d", cnt);
> +	/* Create a unique argument name */
> +	if (!synth->arg_name[0]) {
> +		srand(time(NULL));

Nit: Have in mind that time(NULL) has 1 second resolution. Fast consecutive calls (within a second) of this function can 
generate identical random numbers.
This can be mitigated if we do something like this:

		struct timeval now;

		gettimeofday(&now, NULL);
		srand(now.tv_usec);


-- Yordan

> +		ret = rand() + gettid();
> +		/* Truncate so that ret is at most 65535 (total 13 bytes) */
> +		ret &= (1 << 16) - 1;
> +		sprintf(synth->arg_name, "__arg_%d_", ret);
> +	}
> +	ret = asprintf(&arg, "%s%d", synth->arg_name, cnt);
>   	if (ret < 0)
>   		return NULL;
>   
>