Re: [RFC PATCH bpf-next v2 3/9] tools: bpftool: add probes for kernel configuration options

[Stanislav Fomichev <sdf@fomichev.me>]

On 12/20, Quentin Monnet wrote:
> Add probes to dump a number of options set (or not set) for compiling
> the kernel image. These parameters provide information about what BPF
> components should be available on the system. A number of them are not
> directly related to eBPF, but are in fact used in the kernel as
> conditions on which to compile, or not to compile, some of the eBPF
> helper functions.
> 
> Sample output:
> 
>     # bpftool feature probe kernel
>     Scanning system configuration...
>     ...
>     CONFIG_BPF is set to y
>     CONFIG_BPF_SYSCALL is set to y
>     CONFIG_HAVE_EBPF_JIT is set to y
>     ...
> 
>     # bpftool --pretty --json feature probe kernel
>     {
>         "system_config": {
>             ...
>             "CONFIG_BPF": "y",
>             "CONFIG_BPF_SYSCALL": "y",
>             "CONFIG_HAVE_EBPF_JIT": "y",
>             ...
>         }
>     }
> 
> v2:
> - Remove C-style macros output from this patch.
> - NOT addressed: grouping of those config options into subsections
>   (I don't see an easy way of grouping them at the moment, please see
>   also the discussion on v1 thread).
> 
> Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
> Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
> ---
>  tools/bpf/bpftool/feature.c | 137 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 137 insertions(+)
> 
> diff --git a/tools/bpf/bpftool/feature.c b/tools/bpf/bpftool/feature.c
> index 410d35857cf4..238d7b80f426 100644
> --- a/tools/bpf/bpftool/feature.c
> +++ b/tools/bpf/bpftool/feature.c
> @@ -48,6 +48,30 @@ print_bool_feature(const char *feat_name, const char *plain_name, bool res)
>  		printf("%s is %savailable\n", plain_name, res ? "" : "NOT ");
>  }
>  
> +static void print_kernel_option(const char *name, const char *value)
> +{
> +	char *endptr;
> +	int res;
> +
> +	if (json_output) {
> +		if (!value) {
> +			jsonw_null_field(json_wtr, name);
> +			return;
> +		}
> +		errno = 0;
> +		res = strtol(value, &endptr, 0);
> +		if (!errno && *endptr == '\n')
> +			jsonw_int_field(json_wtr, name, res);
> +		else
> +			jsonw_string_field(json_wtr, name, value);
> +	} else {
> +		if (value)
> +			printf("%s is set to %s\n", name, value);
> +		else
> +			printf("%s is not set\n", name);
> +	}
> +}
> +
>  static void
>  print_start_section(const char *json_title, const char *plain_title)
>  {
> @@ -190,6 +214,118 @@ static void probe_jit_kallsyms(void)
>  	}
>  }
>  
> +static char *get_kernel_config_option(FILE *fd, const char *option)
> +{
> +	size_t line_n = 0, optlen = strlen(option);
> +	char *res, *strval, *line = NULL;
> +	ssize_t n;
> +
> +	rewind(fd);
> +	while ((n = getline(&line, &line_n, fd)) > 0) {
> +		if (strncmp(line, option, optlen))
> +			continue;
> +		/* Check we have at least '=', value, and '\n' */
> +		if (strlen(line) < optlen + 3)
> +			continue;
> +		if (*(line + optlen) != '=')
> +			continue;
> +
> +		/* Trim ending '\n' */
> +		line[strlen(line) - 1] = '\0';
> +
> +		/* Copy and return config option value */
> +		strval = line + optlen + 1;
> +		res = strdup(strval);
> +		free(line);
> +		return res;
> +	}
> +	free(line);
> +
> +	return NULL;
> +}
> +
> +static void probe_kernel_image_config(void)
> +{
> +	const char * const options[] = {
> +		"CONFIG_BPF",
> +		"CONFIG_BPF_SYSCALL",
> +		"CONFIG_HAVE_EBPF_JIT",
> +		"CONFIG_BPF_JIT",
> +		"CONFIG_BPF_JIT_ALWAYS_ON",
> +		"CONFIG_NET",
> +		"CONFIG_XDP_SOCKETS",
> +		"CONFIG_CGROUPS",
> +		"CONFIG_CGROUP_BPF",
> +		"CONFIG_CGROUP_NET_CLASSID",
> +		"CONFIG_BPF_EVENTS",
> +		"CONFIG_LWTUNNEL_BPF",
> +		"CONFIG_NET_ACT_BPF",
> +		"CONFIG_NET_CLS_ACT",
> +		"CONFIG_NET_CLS_BPF",
> +		"CONFIG_NET_SCH_INGRESS",
> +		"CONFIG_XFRM",
> +		"CONFIG_SOCK_CGROUP_DATA",
> +		"CONFIG_IP_ROUTE_CLASSID",
> +		"CONFIG_IPV6_SEG6_BPF",
> +		"CONFIG_FUNCTION_ERROR_INJECTION",
> +		"CONFIG_BPF_KPROBE_OVERRIDE",
> +		"CONFIG_BPF_LIRC_MODE2",
> +		"CONFIG_NETFILTER_XT_MATCH_BPF",
> +		"CONFIG_TEST_BPF",
> +		"CONFIG_BPFILTER",
> +		"CONFIG_BPFILTER_UMH",
> +		"CONFIG_BPF_STREAM_PARSER",
> +	};
> +	char *value, *buf = NULL;
> +	struct utsname utsn;
> +	char path[PATH_MAX];
> +	size_t i, n;
> +	ssize_t ret;
> +	FILE *fd;
> +
> +	if (uname(&utsn))
> +		goto no_config;
> +
[..]
> +	snprintf(path, sizeof(path), "/boot/config-%s", utsn.release);
> +
> +	fd = fopen(path, "r");
> +	if (!fd && errno == ENOENT) {
> +		/* Sometimes config is at /proc/config */
> +		fd = fopen("/proc/config", "r");
I wonder whether the order here should be reversed?
1. try /proc/config.gz (CONFIG_IKCONFIG_PROC)
2. if not avail, try /boot/config-$(uname -r)

Because, at the end, /proc/config.gz is the real source of truth (if
available).

What is /proc/config btw? I can see only /proc/config.gz being exported.

> +	}
> +	if (!fd) {
> +		p_err("can't open kernel config file: %s", strerror(errno));
> +		goto no_config;
> +	}
> +	/* Sanity checks */
> +	ret = getline(&buf, &n, fd);
> +	ret = getline(&buf, &n, fd);
> +	if (!buf || !ret) {
> +		p_err("can't read from kernel config file: %s",
> +		      strerror(errno));
> +		free(buf);
> +		goto no_config;
> +	}
> +	if (strcmp(buf, "# Automatically generated file; DO NOT EDIT.\n")) {
> +		p_err("can't find correct kernel config file");
> +		free(buf);
> +		goto no_config;
> +	}
> +	free(buf);
> +
> +	for (i = 0; i < ARRAY_SIZE(options); i++) {
> +		value = get_kernel_config_option(fd, options[i]);
> +		print_kernel_option(options[i], value);
> +		free(value);
> +	}
> +	fclose(fd);
> +	return;
> +
> +no_config:
> +	for (i = 0; i < ARRAY_SIZE(options); i++)
> +		print_kernel_option(options[i], NULL);
> +}
> +
>  static int probe_kernel_version(void)
>  {
>  	int version, subversion, patchlevel, code = 0;
> @@ -270,6 +406,7 @@ static int do_probe(int argc, char **argv)
>  		} else {
>  			p_info("/* procfs not mounted, skipping related probes */");
>  		}
> +		probe_kernel_image_config();
>  		if (json_output)
>  			jsonw_end_object(json_wtr);
>  		else
> -- 
> 2.17.1
>