[DTrace-devel] [PATCH] bpf: use correct loop bound for conf->cpus traversal in cpuinfo map creation

[Kris Van Hees]

On Fri, Mar 01, 2024 at 08:45:35PM +0000, Nick Alcock via DTrace-devel wrote:
> We were using the wrong bound, causing a buffer overrun on machines with
> fewer online CPUs than possible CPUs.

... on machines with online CPUs that do not have sequential CPU ids.

(You could even add an exaxmple, like: 1, 2, 3, 6.)

> (Add an assertion to verify that there are never more online CPUs
> than possible CPUs.)
> 
> Orabug: 36356681
> Signed-off-by: Nick Alcock <nick.alcock at oracle.com>

Reviewed-by: Kris Van Hees <kris.van.hees at oracle.com>

... with the commit msg change listed above

> ---
>  libdtrace/dt_bpf.c  | 5 ++++-
>  libdtrace/dt_conf.c | 2 ++
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/libdtrace/dt_bpf.c b/libdtrace/dt_bpf.c
> index 69e41ba7194e9..2ceb9f7ac61cd 100644
> --- a/libdtrace/dt_bpf.c
> +++ b/libdtrace/dt_bpf.c
> @@ -613,10 +613,13 @@ gmap_create_cpuinfo(dtrace_hdl_t *dtp)
>  	int			i, rc;
>  	uint32_t		key = 0;
>  	dtrace_conf_t		*conf = &dtp->dt_conf;
> -	size_t			ncpus = conf->max_cpuid + 1;
> +	size_t			ncpus = conf->num_online_cpus;
>  	dt_bpf_cpuinfo_t	*data;
>  	cpuinfo_t		*ci;
>  
> +	/*
> +	 * num_possible_cpus <= num_online_cpus: see dt_conf_init.
> +	 */
>  	data = dt_calloc(dtp, dtp->dt_conf.num_possible_cpus,
>  			 sizeof(dt_bpf_cpuinfo_t));
>  	if (data == NULL)
> diff --git a/libdtrace/dt_conf.c b/libdtrace/dt_conf.c
> index b1fb46a4ca33d..dff75d77fe35b 100644
> --- a/libdtrace/dt_conf.c
> +++ b/libdtrace/dt_conf.c
> @@ -98,6 +98,8 @@ dt_conf_init(dtrace_hdl_t *dtp)
>  	if (conf->num_online_cpus == 0 || conf->cpus == NULL)
>  		return;
>  
> +	assert(conf->num_possible_cpus >= conf->num_online_cpus);
> +
>  	conf->max_cpuid = conf->cpus[conf->num_online_cpus - 1].cpu_id;
>  
>  	/* Retrieve the chip ID (physical_package_id) for each CPU. */
> -- 
> 2.43.0.272.gce700b77fd
> 
> 
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