Re: [Potential Spoof] [PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function thread safe

[Andrey Ignatov <rdna@fb.com>]

Takshak Chahande <ctakshak@fb.com> [Wed, 2019-07-31 15:11 -0700]:
> Having static variable `cpus` in libbpf_num_possible_cpus function
> without guarding it with mutex makes this function thread-unsafe.
> 
> If multiple threads accessing this function, in the current form; it
> leads to incrementing the static variable value `cpus` in the multiple
> of total available CPUs.
> 
> Used local stack variable to calculate the number of possible CPUs and
> then updated the static variable using WRITE_ONCE().
> 
> Changes since v1:
>  * added stack variable to calculate cpus
>  * serialized static variable update using WRITE_ONCE()
>  * fixed Fixes tag

This "Changes" section should be after "---" line not to be included in
the final commit message.

Not sure if resubmit is needed because of it, but other than this looks
good to me.

Acked-by: Andrey Ignatov <rdna@fb.com>


> Fixes: 6446b3155521 ("bpf: add a new API libbpf_num_possible_cpus()")
> Signed-off-by: Takshak Chahande <ctakshak@fb.com>
> ---
>  tools/lib/bpf/libbpf.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 6718d0b90130..2e84fa5b8479 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4995,13 +4995,15 @@ int libbpf_num_possible_cpus(void)
>  	static const char *fcpu = "/sys/devices/system/cpu/possible";
>  	int len = 0, n = 0, il = 0, ir = 0;
>  	unsigned int start = 0, end = 0;
> +	int tmp_cpus = 0;
>  	static int cpus;
>  	char buf[128];
>  	int error = 0;
>  	int fd = -1;
>  
> -	if (cpus > 0)
> -		return cpus;
> +	tmp_cpus = READ_ONCE(cpus);
> +	if (tmp_cpus > 0)
> +		return tmp_cpus;
>  
>  	fd = open(fcpu, O_RDONLY);
>  	if (fd < 0) {
> @@ -5024,7 +5026,7 @@ int libbpf_num_possible_cpus(void)
>  	}
>  	buf[len] = '\0';
>  
> -	for (ir = 0, cpus = 0; ir <= len; ir++) {
> +	for (ir = 0, tmp_cpus = 0; ir <= len; ir++) {
>  		/* Each sub string separated by ',' has format \d+-\d+ or \d+ */
>  		if (buf[ir] == ',' || buf[ir] == '\0') {
>  			buf[ir] = '\0';
> @@ -5036,13 +5038,15 @@ int libbpf_num_possible_cpus(void)
>  			} else if (n == 1) {
>  				end = start;
>  			}
> -			cpus += end - start + 1;
> +			tmp_cpus += end - start + 1;
>  			il = ir + 1;
>  		}
>  	}
> -	if (cpus <= 0) {
> -		pr_warning("Invalid #CPUs %d from %s\n", cpus, fcpu);
> +	if (tmp_cpus <= 0) {
> +		pr_warning("Invalid #CPUs %d from %s\n", tmp_cpus, fcpu);
>  		return -EINVAL;
>  	}
> -	return cpus;
> +
> +	WRITE_ONCE(cpus, tmp_cpus);
> +	return tmp_cpus;
>  }
> -- 
> 2.17.1
> 

-- 
Andrey Ignatov