Re: [PATCH bpf] libbpf: count present CPUs, not theoretically possible

[Daniel Borkmann <daniel@iogearbox.net>]

On 9/28/19 8:30 AM, Andrii Nakryiko wrote:
> This patch switches libbpf_num_possible_cpus() from using possible CPU
> set to present CPU set. This fixes issues with incorrect auto-sizing of
> PERF_EVENT_ARRAY map on HOTPLUG-enabled systems.

Those issues should be described in more detail here in the changelog,
otherwise noone knows what is meant exactly when glancing at the git log.

> On HOTPLUG enabled systems, /sys/devices/system/cpu/possible is going to
> be a set of any representable (i.e., potentially possible) CPU, which is
> normally way higher than real amount of CPUs (e.g., 0-127 on VM I've
> tested on, while there were just two CPU cores actually present).
> /sys/devices/system/cpu/present, on the other hand, will only contain
> CPUs that are physically present in the system (even if not online yet),
> which is what we really want, especially when creating per-CPU maps or
> perf events.
> 
> On systems with HOTPLUG disabled, present and possible are identical, so
> there is no change of behavior there.
> 
> Signed-off-by: Andrii Nakryiko <andriin@fb.com>
> ---
>   tools/lib/bpf/libbpf.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index e0276520171b..45351c074e45 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -5899,7 +5899,7 @@ void bpf_program__bpil_offs_to_addr(struct bpf_prog_info_linear *info_linear)
>   
>   int libbpf_num_possible_cpus(void)
>   {
> -	static const char *fcpu = "/sys/devices/system/cpu/possible";
> +	static const char *fcpu = "/sys/devices/system/cpu/present";

Problem is that this is going to break things *badly* for per-cpu maps as
BPF_DECLARE_PERCPU() relies on possible CPUs, not present ones. And given
present<=possible you'll end up corrupting user space when you do a lookup
on the map since kernel side operates on possible as well.

>   	int len = 0, n = 0, il = 0, ir = 0;
>   	unsigned int start = 0, end = 0;
>   	int tmp_cpus = 0;
>