[PATCH v2 dwarves 3/4] pahole: introduce --btf_feature=attributes

[Ihor Solodrai <ihor.solodrai@linux.dev>]

Add a feature flag "attributes" (default: false) controlling whether
pahole is allowed to generate BTF attributes: type tags and decl tags
with kind_flag = 1.

This is necessary for backward compatibility, as BPF verifier does not
recognize tags with kind_flag = 1 prior to (at least) 6.14-rc1 [1].

[1] https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
---
 btf_encoder.c |  6 ++++--
 dwarves.h     |  1 +
 pahole.c      | 11 +++++++++++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 3cec106..fc99ad2 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -136,7 +136,8 @@ struct btf_encoder {
 			  gen_floats,
 			  skip_encoding_decl_tag,
 			  tag_kfuncs,
-			  gen_distilled_base;
+			  gen_distilled_base,
+			  encode_attributes;
 	uint32_t	  array_index_id;
 	struct elf_secinfo *secinfo;
 	size_t             seccnt;
@@ -823,7 +824,7 @@ static int32_t btf_encoder__add_func_proto(struct btf_encoder *encoder, struct f
 	assert(ftype != NULL || state != NULL);
 
 #if LIBBPF_MAJOR_VERSION >= 1 && LIBBPF_MINOR_VERSION >= 6
-	if (is_kfunc_state(state) && encoder->tag_kfuncs)
+	if (is_kfunc_state(state) && encoder->tag_kfuncs && encoder->encode_attributes)
 		if (btf__add_bpf_arena_type_tags(encoder->btf, state) < 0)
 			return -1;
 #endif
@@ -2417,6 +2418,7 @@ struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filenam
 		encoder->skip_encoding_decl_tag	 = conf_load->skip_encoding_btf_decl_tag;
 		encoder->tag_kfuncs	 = conf_load->btf_decl_tag_kfuncs;
 		encoder->gen_distilled_base = conf_load->btf_gen_distilled_base;
+		encoder->encode_attributes = conf_load->btf_attributes;
 		encoder->verbose	 = verbose;
 		encoder->has_index_type  = false;
 		encoder->need_index_type = false;
diff --git a/dwarves.h b/dwarves.h
index 8234e1a..99ed783 100644
--- a/dwarves.h
+++ b/dwarves.h
@@ -89,6 +89,7 @@ struct conf_load {
 	bool			reproducible_build;
 	bool			btf_decl_tag_kfuncs;
 	bool			btf_gen_distilled_base;
+	bool			btf_attributes;
 	uint8_t			hashtable_bits;
 	uint8_t			max_hashtable_bits;
 	uint16_t		kabi_prefix_len;
diff --git a/pahole.c b/pahole.c
index af3e1cf..0bda249 100644
--- a/pahole.c
+++ b/pahole.c
@@ -1152,6 +1152,7 @@ ARGP_PROGRAM_VERSION_HOOK_DEF = dwarves_print_version;
 #define ARG_padding_ge		   347
 #define ARG_padding		   348
 #define ARGP_with_embedded_flexible_array 349
+#define ARGP_btf_attributes	   350
 
 /* --btf_features=feature1[,feature2,..] allows us to specify
  * a list of requested BTF features or "default" to enable all default
@@ -1210,6 +1211,9 @@ struct btf_feature {
 	BTF_NON_DEFAULT_FEATURE(distilled_base, btf_gen_distilled_base, false),
 #endif
 	BTF_NON_DEFAULT_FEATURE(global_var, encode_btf_global_vars, false),
+#if LIBBPF_MAJOR_VERSION >= 1 && LIBBPF_MINOR_VERSION >= 6
+	BTF_NON_DEFAULT_FEATURE(attributes, btf_attributes, false),
+#endif
 };
 
 #define BTF_MAX_FEATURE_STR	1024
@@ -1785,6 +1789,11 @@ static const struct argp_option pahole__options[] = {
 		.key = ARGP_running_kernel_vmlinux,
 		.doc = "Search for, possibly getting from a debuginfo server, a vmlinux matching the running kernel build-id (from /sys/kernel/notes)"
 	},
+	{
+		.name = "btf_attributes",
+		.key  = ARGP_btf_attributes,
+		.doc  = "Allow generation of attributes in BTF. Attributes are the type tags and decl tags with the kind_flag set to 1.",
+	},
 	{
 		.name = NULL,
 	}
@@ -1979,6 +1988,8 @@ static error_t pahole__options_parser(int key, char *arg,
 		show_supported_btf_features(stdout);	exit(0);
 	case ARGP_btf_features_strict:
 		parse_btf_features(arg, true);		break;
+	case ARGP_btf_attributes:
+		conf_load.btf_attributes = true;	break;
 	default:
 		return ARGP_ERR_UNKNOWN;
 	}
-- 
2.48.1