Subject: Re: [PATCH bpf-next v3 4/5] libbpf: fix error when st-prefix_ops and ops from differ btf

From: Martin KaFai Lau <[email-masked]>
Date: Thu, 19 Dec 2024 14:43:30 -0800

On 12/17/24 6:44 PM, D. Wythe wrote:
> When a struct_ops named xxx_ops was registered by a module, and
> it will be used in both built-in modules and the module itself,
> so that the btf_type of xxx_ops will be present in btf_vmlinux
> instead of in btf_mod, which means that the btf_type of
> bpf_struct_ops_xxx_ops and xxx_ops will not be in the same btf.
> 
> Here are four possible case:
> 
> +--------+-------------+-------------+---------------------------------+
> |        | st_opx_xxx  | xxx         |                                 |
> +--------+-------------+-------------+---------------------------------+
> | case 0 | btf_vmlinux | bft_vmlinux | be used and reg only in vmlinux |
> +--------+-------------+-------------+---------------------------------+
> | case 1 | btf_vmlinux | bpf_mod     | INVALID                         |
> +--------+-------------+-------------+---------------------------------+
> | case 2 | btf_mod     | btf_vmlinux | reg in mod but be used both in  |
> |        |             |             | vmlinux and mod.                |
> +--------+-------------+-------------+---------------------------------+
> | case 3 | btf_mod     | btf_mod     | be used and reg only in mod     |
> +--------+-------------+-------------+---------------------------------+
> 
> At present, cases 0, 1, and 3 can be correctly identified, because
> st_ops_xxx is searched from the same btf with xxx. In order to
> handle case 2 correctly without affecting other cases, we cannot simply
> change the search method for st_ops_xxx from find_btf_by_prefix_kind()
> to find_ksym_btf_id(), because in this way, case 1 will not be
> recognized anymore.
> 
> To address this issue, if st_ops_xxx cannot be found in the btf with xxx
> and mod_btf does not exist, do find_ksym_btf_id() again to
> avoid such issue.
> 
> Fixes: 590a00888250 ("bpf: libbpf: Add STRUCT_OPS support")
> Signed-off-by: D. Wythe <[email-masked]>
> ---
>   tools/lib/bpf/libbpf.c | 25 +++++++++++++++++--------
>   1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 66173ddb5a2d..56bf74894110 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1005,7 +1005,8 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
>   	const struct btf_member *kern_data_member;
>   	struct btf *btf = NULL;
>   	__s32 kern_vtype_id, kern_type_id;
> -	char tname[256];
> +	char tname[256], stname[256];
> +	int ret;
>   	__u32 i;
>   
>   	snprintf(tname, sizeof(tname), "%.*s",
> @@ -1020,17 +1021,25 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
>   	}
>   	kern_type = btf__type_by_id(btf, kern_type_id);
>   
> +	ret = snprintf(stname, sizeof(stname), "%s%s", STRUCT_OPS_VALUE_PREFIX, tname);

How about always look for "struct bpf_struct_ops_smc_ops" first, figure out the 
btf, and then look for "struct smc_ops", would it work?

If CONFIG_SMC=y instead of =m, this change cannot be tested?

> +	if (ret < 0 || ret >= sizeof(stname))
> +		return -ENAMETOOLONG;
> +
>   	/* Find the corresponding "map_value" type that will be used
>   	 * in map_update(BPF_MAP_TYPE_STRUCT_OPS).  For example,
>   	 * find "struct bpf_struct_ops_tcp_congestion_ops" from the
>   	 * btf_vmlinux.
>   	 */
> -	kern_vtype_id = find_btf_by_prefix_kind(btf, STRUCT_OPS_VALUE_PREFIX,
> -						tname, BTF_KIND_STRUCT);
> +	kern_vtype_id = btf__find_by_name_kind(btf, stname, BTF_KIND_STRUCT);
>   	if (kern_vtype_id < 0) {
> -		pr_warn("struct_ops init_kern: struct %s%s is not found in kernel BTF\n",
> -			STRUCT_OPS_VALUE_PREFIX, tname);
> -		return kern_vtype_id;
> +		if (kern_vtype_id == -ENOENT && !*mod_btf)
> +			kern_vtype_id = find_ksym_btf_id(obj, stname, BTF_KIND_STRUCT,
> +							 &btf, mod_btf);
> +		if (kern_vtype_id < 0) {
> +			pr_warn("struct_ops init_kern: struct %s is not found in kernel BTF\n",
> +				stname);
> +			return kern_vtype_id;
> +		}
>   	}
>   	kern_vtype = btf__type_by_id(btf, kern_vtype_id);
>   
> @@ -1046,8 +1055,8 @@ find_struct_ops_kern_types(struct bpf_object *obj, const char *tname_raw,
>   			break;
>   	}
>   	if (i == btf_vlen(kern_vtype)) {
> -		pr_warn("struct_ops init_kern: struct %s data is not found in struct %s%s\n",
> -			tname, STRUCT_OPS_VALUE_PREFIX, tname);
> +		pr_warn("struct_ops init_kern: struct %s data is not found in struct %s\n",
> +			tname, stname);
>   		return -EINVAL;
>   	}
>