libbpf: further decouple feature checking logic from bpf_object

Add feat_supported() helper that accepts feature cache instead of
bpf_object. This allows low-level code in bpf.c to not know or care
about higher-level concept of bpf_object, yet it will be able to utilize
custom feature checking in cases where BPF token might influence the
outcome.
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231213190842.3844987-4-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/bpf.c

@@ -146,7 +146,7 @@ int bump_rlimit_memlock(void)
 	struct rlimit rlim;
 
 	/* if kernel supports memcg-based accounting, skip bumping RLIMIT_MEMLOCK */
-	if (memlock_bumped || kernel_supports(NULL, FEAT_MEMCG_ACCOUNT))
+	if (memlock_bumped || feat_supported(NULL, FEAT_MEMCG_ACCOUNT))
 		return 0;
 
 	memlock_bumped = true;
@@ -181,7 +181,7 @@ int bpf_map_create(enum bpf_map_type map_type,
 		return libbpf_err(-EINVAL);
 
 	attr.map_type = map_type;
-	if (map_name && kernel_supports(NULL, FEAT_PROG_NAME))
+	if (map_name && feat_supported(NULL, FEAT_PROG_NAME))
 		libbpf_strlcpy(attr.map_name, map_name, sizeof(attr.map_name));
 	attr.key_size = key_size;
 	attr.value_size = value_size;
@@ -265,7 +265,7 @@ int bpf_prog_load(enum bpf_prog_type prog_type,
 	attr.kern_version = OPTS_GET(opts, kern_version, 0);
 	attr.prog_token_fd = OPTS_GET(opts, token_fd, 0);
 
-	if (prog_name && kernel_supports(NULL, FEAT_PROG_NAME))
+	if (prog_name && feat_supported(NULL, FEAT_PROG_NAME))
 		libbpf_strlcpy(attr.prog_name, prog_name, sizeof(attr.prog_name));
 	attr.license = ptr_to_u64(license);
 

tools/lib/bpf/libbpf.c

@@ -5074,17 +5074,14 @@ static struct kern_feature_desc {
 	},
 };
 
-bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)
+bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)
 {
 	struct kern_feature_desc *feat = &feature_probes[feat_id];
-	struct kern_feature_cache *cache = &feature_cache;
 	int ret;
 
-	if (obj && obj->gen_loader)
-		/* To generate loader program assume the latest kernel
-		 * to avoid doing extra prog_load, map_create syscalls.
-		 */
-		return true;
+	/* assume global feature cache, unless custom one is provided */
+	if (!cache)
+		cache = &feature_cache;
 
 	if (READ_ONCE(cache->res[feat_id]) == FEAT_UNKNOWN) {
 		ret = feat->probe();
@@ -5101,6 +5098,17 @@ bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)
 	return READ_ONCE(cache->res[feat_id]) == FEAT_SUPPORTED;
 }
 
+bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)
+{
+	if (obj && obj->gen_loader)
+		/* To generate loader program assume the latest kernel
+		 * to avoid doing extra prog_load, map_create syscalls.
+		 */
+		return true;
+
+	return feat_supported(NULL, feat_id);
+}
+
 static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
 {
 	struct bpf_map_info map_info;

tools/lib/bpf/libbpf_internal.h

@@ -360,8 +360,11 @@ enum kern_feature_id {
 	__FEAT_CNT,
 };
 
-int probe_memcg_account(void);
+struct kern_feature_cache;
+bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id);
 bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id);
+
+int probe_memcg_account(void);
 int bump_rlimit_memlock(void);
 
 int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz);