Re: [PATCH bpf-next v3] libbpf: fix probe code to return EPERM if encountered

[Yonghong Song <yhs@fb.com>]

On 5/11/20 5:40 AM, Eelco Chaudron wrote:
> When the probe code was failing for any reason ENOTSUP was returned, even
> if this was due to no having enough lock space. This patch fixes this by
> returning EPERM to the user application, so it can respond and increase
> the RLIMIT_MEMLOCK size.
> 
> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
> ---
> v3: Updated error message to be more specific as suggested by Andrii
> v2: Split bpf_object__probe_name() in two functions as suggested by Andrii
> 
>   tools/lib/bpf/libbpf.c |   31 ++++++++++++++++++++++++++-----
>   1 file changed, 26 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 8f480e29a6b0..ad3043c5db13 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -3149,7 +3149,7 @@ int bpf_map__resize(struct bpf_map *map, __u32 max_entries)
>   }
>   
>   static int
> -bpf_object__probe_name(struct bpf_object *obj)
> +bpf_object__probe_loading(struct bpf_object *obj)
>   {
>   	struct bpf_load_program_attr attr;
>   	char *cp, errmsg[STRERR_BUFSIZE];
> @@ -3170,14 +3170,34 @@ bpf_object__probe_name(struct bpf_object *obj)
>   	ret = bpf_load_program_xattr(&attr, NULL, 0);
>   	if (ret < 0) {
>   		cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
> -		pr_warn("Error in %s():%s(%d). Couldn't load basic 'r0 = 0' BPF program.\n",
> -			__func__, cp, errno);
> +		pr_warn("Error in %s():%s(%d). Couldn't load trivial BPF "
> +			"program. Make sure your kernel supports BPF "
> +			"(CONFIG_BPF_SYSCALL=y) and/or that RLIMIT_MEMLOCK is "
> +			"set to big enough value.\n", __func__, cp, errno);
>   		return -errno;

Just curious. Did "errno" always survive pr_warn() here? pr_warn() may 
call user supplied print function which it outside libbpf control.
Maybe should cache errno before calling pr_warn()?

>   	}
>   	close(ret);
>   
> -	/* now try the same program, but with the name */
> +	return 0;
> +}
> +
> +static int
> +bpf_object__probe_name(struct bpf_object *obj)
> +{
> +	struct bpf_load_program_attr attr;
> +	struct bpf_insn insns[] = {
> +		BPF_MOV64_IMM(BPF_REG_0, 0),
> +		BPF_EXIT_INSN(),
> +	};
> +	int ret;
> +
> +	/* make sure loading with name works */
>   
> +	memset(&attr, 0, sizeof(attr));
> +	attr.prog_type = BPF_PROG_TYPE_SOCKET_FILTER;
> +	attr.insns = insns;
> +	attr.insns_cnt = ARRAY_SIZE(insns);
> +	attr.license = "GPL";
>   	attr.name = "test";
>   	ret = bpf_load_program_xattr(&attr, NULL, 0);
>   	if (ret >= 0) {
> @@ -5386,7 +5406,8 @@ int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
>   
>   	obj->loaded = true;
>   
> -	err = bpf_object__probe_caps(obj);
> +	err = bpf_object__probe_loading(obj);
> +	err = err ? : bpf_object__probe_caps(obj);
>   	err = err ? : bpf_object__resolve_externs(obj, obj->kconfig);
>   	err = err ? : bpf_object__sanitize_and_load_btf(obj);
>   	err = err ? : bpf_object__sanitize_maps(obj);
>