[PATCH v2 dwarves 3/5] pahole: add --btf_features support

[Alan Maguire <alan.maguire@oracle.com>]

This allows consumers to specify an opt-in set of features
they want to use in BTF encoding.

Supported features are a comma-separated combination of

	encode_force    Ignore invalid symbols when encoding BTF.
	var             Encode variables using BTF_KIND_VAR in BTF.
	float           Encode floating-point types in BTF.
	decl_tag        Encode declaration tags using BTF_KIND_DECL_TAG.
	type_tag        Encode type tags using BTF_KIND_TYPE_TAG.
	enum64          Encode enum64 values with BTF_KIND_ENUM64.
	optimized_func  Encode representations of optimized functions
	                with suffixes like ".isra.0" etc
	consistent_func Avoid encoding inconsistent static functions.
	                These occur when a parameter is optimized out
	                in some CUs and not others, or when the same
	                function name has inconsistent BTF descriptions
	                in different CUs.

Specifying "--btf_features=all" is the equivalent to setting
all of the above.  If pahole does not know about a feature
specified in --btf_features it silently ignores it.

The --btf_features can either be specified via a single comma-separated
list
	--btf_features=enum64,float

...or via multiple --btf_features values

	--btf_features=enum64 --btf_features=float

These properties allow us to use the --btf_features option in
the kernel scripts/pahole_flags.sh script to specify the desired
set of BTF features.

If a feature named in --btf_features is not present in the version
of pahole used, BTF encoding will not complain.  This is desired
because it means we no longer have to tie new features to a specific
pahole version.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 man-pages/pahole.1 |  24 ++++++++++
 pahole.c           | 114 ++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 137 insertions(+), 1 deletion(-)

diff --git a/man-pages/pahole.1 b/man-pages/pahole.1
index c1b48de..a09885f 100644
--- a/man-pages/pahole.1
+++ b/man-pages/pahole.1
@@ -273,6 +273,30 @@ Generate BTF for functions with optimization-related suffixes (.isra, .constprop
 .B \-\-btf_gen_all
 Allow using all the BTF features supported by pahole.
 
+.TP
+.B \-\-btf_features=FEATURE_LIST
+Encode BTF using the specified feature list, or specify 'all' for all features supported.  This option can be used as an alternative to unsing multiple BTF-related options. Supported features are
+
+.nf
+	encode_force       Ignore invalid symbols when encoding BTF; for example
+	                   if a symbol has an invalid name, it will be ignored
+	                   and BTF encoding will continue.
+	var                Encode variables using BTF_KIND_VAR in BTF.
+	float              Encode floating-point types in BTF.
+	decl_tag           Encode declaration tags using BTF_KIND_DECL_TAG.
+	type_tag           Encode type tags using BTF_KIND_TYPE_TAG.
+	enum64             Encode enum64 values with BTF_KIND_ENUM64.
+	optimized_func     Encode representations of optimized functions
+	                   with suffixes like ".isra.0".
+	consistent_func    Avoid encoding inconsistent static functions.
+	                   These occur when a parameter is optimized out
+	                   in some CUs and not others, or when the same
+	                   function name has inconsistent BTF descriptions
+	                   in different CUs.
+.fi
+
+So for example, specifying \-\-btf_encode=var,enum64 will result in a BTF encoding that (as well as encoding basic BTF information) will contain variables and enum64 values.
+
 .TP
 .B \-l, \-\-show_first_biggest_size_base_type_member
 Show first biggest size base_type member.
diff --git a/pahole.c b/pahole.c
index 7a41dc3..0d0f6c6 100644
--- a/pahole.c
+++ b/pahole.c
@@ -1229,6 +1229,110 @@ ARGP_PROGRAM_VERSION_HOOK_DEF = dwarves_print_version;
 #define ARGP_skip_emitting_atomic_typedefs 338
 #define ARGP_btf_gen_optimized  339
 #define ARGP_skip_encoding_btf_inconsistent_proto 340
+#define ARGP_btf_features	341
+
+/* --btf_features=feature1[,feature2,..] allows us to specify
+ * a list of requested BTF features or "all" to enable all features.
+ * These are translated into the appropriate conf_load values via a
+ * struct btf_feature which specifies the associated conf_load
+ * boolean field and whether its default (representing the feature being
+ * off) is false or true.
+ *
+ * btf_features is for opting _into_ features so for a case like
+ * conf_load->btf_gen_floats, the translation is simple; the presence
+ * of the "float" feature in --btf_features sets conf_load->btf_gen_floats
+ * to true.
+ *
+ * The more confusing case is for features that are enabled unless
+ * skipping them is specified; for example
+ * conf_load->skip_encoding_btf_type_tag.  By default - to support
+ * the opt-in model of only enabling features the user asks for -
+ * conf_load->skip_encoding_btf_type_tag is set to true (meaning no
+ * type_tags) and it is only set to false if --btf_features contains
+ * the "type_tag" keyword.
+ *
+ * So from the user perspective, all features specified via
+ * --btf_features are enabled, and if a feature is not specified,
+ * it is disabled.
+ *
+ * If --btf_features is not used, the usual pahole defaults for
+ * BTF encoding apply; we encode type/decl tags, do not encode
+ * floats, etc.  This ensures backwards compatibility.
+ */
+#define BTF_FEATURE(name, alias, default_value)			\
+	{ #name, #alias, &conf_load.alias, default_value }
+
+struct btf_feature {
+	const char      *name;
+	const char      *option_alias;
+	bool		*conf_value;
+	bool		default_value;
+} btf_features[] = {
+	BTF_FEATURE(encode_force, btf_encode_force, false),
+	BTF_FEATURE(var, skip_encoding_btf_vars, true),
+	BTF_FEATURE(float, btf_gen_floats, false),
+	BTF_FEATURE(decl_tag, skip_encoding_btf_decl_tag, true),
+	BTF_FEATURE(type_tag, skip_encoding_btf_type_tag, true),
+	BTF_FEATURE(enum64, skip_encoding_btf_enum64, true),
+	BTF_FEATURE(optimized_func, btf_gen_optimized, false),
+	BTF_FEATURE(consistent_func, skip_encoding_btf_inconsistent_proto, false),
+};
+
+#define BTF_MAX_FEATURES	32
+#define BTF_MAX_FEATURE_STR	1024
+
+bool set_btf_features_defaults;
+
+/* Translate --btf_features=feature1[,feature2] into conf_load values.
+ * Explicitly ignores unrecognized features to allow future specification
+ * of new opt-in features.
+ */
+static void parse_btf_features(const char *features)
+{
+	char *feature_list[BTF_MAX_FEATURES] = {};
+	char f[BTF_MAX_FEATURE_STR];
+	bool encode_all = false;
+	int i, j, n = 0;
+
+	strncpy(f, features, sizeof(f));
+
+	if (strcmp(features, "all") == 0) {
+		encode_all = true;
+	} else {
+		char *saveptr = NULL, *s = f, *t;
+
+		while ((t = strtok_r(s, ",", &saveptr)) != NULL && n < BTF_MAX_FEATURES) {
+			s = NULL;
+			feature_list[n++] = t;
+		}
+	}
+
+	for (i = 0; i < ARRAY_SIZE(btf_features); i++) {
+		bool match = encode_all;
+
+		/* Only set default values once, as multiple --btf_features=
+		 * may be specified on command-line, and setting defaults
+		 * again could clobber values.   The aim is to enable
+		 * all features set across all --btf_features options.
+		 */
+		if (!set_btf_features_defaults)
+			*(btf_features[i].conf_value) = btf_features[i].default_value;
+		if (!match) {
+			for (j = 0; j < n; j++) {
+				if (strcmp(feature_list[j], btf_features[i].name) == 0) {
+					match = true;
+					break;
+				}
+			}
+		}
+		/* switch "default-off" features on, and "default-on" features
+		 * off; i.e. negate the default value.
+		 */
+		if (match)
+			*(btf_features[i].conf_value) = !btf_features[i].default_value;
+	}
+	set_btf_features_defaults = true;
+}
 
 static const struct argp_option pahole__options[] = {
 	{
@@ -1651,6 +1755,12 @@ static const struct argp_option pahole__options[] = {
 		.key = ARGP_skip_encoding_btf_inconsistent_proto,
 		.doc = "Skip functions that have multiple inconsistent function prototypes sharing the same name, or that use unexpected registers for parameter values."
 	},
+	{
+		.name = "btf_features",
+		.key = ARGP_btf_features,
+		.arg = "FEATURE_LIST",
+		.doc = "Specify supported BTF features in FEATURE_LIST or 'all' for all supported features. See the pahole manual page for the list of supported features."
+	},
 	{
 		.name = NULL,
 	}
@@ -1796,7 +1906,7 @@ static error_t pahole__options_parser(int key, char *arg,
 	case ARGP_btf_gen_floats:
 		conf_load.btf_gen_floats = true;	break;
 	case ARGP_btf_gen_all:
-		conf_load.btf_gen_floats = true;	break;
+		parse_btf_features("all");		break;
 	case ARGP_with_flexible_array:
 		show_with_flexible_array = true;	break;
 	case ARGP_prettify_input_filename:
@@ -1826,6 +1936,8 @@ static error_t pahole__options_parser(int key, char *arg,
 		conf_load.btf_gen_optimized = true;		break;
 	case ARGP_skip_encoding_btf_inconsistent_proto:
 		conf_load.skip_encoding_btf_inconsistent_proto = true; break;
+	case ARGP_btf_features:
+		parse_btf_features(arg);		break;
 	default:
 		return ARGP_ERR_UNKNOWN;
 	}
-- 
2.31.1