[PATCH v2 dwarves 4/5] pahole: add --supported_btf_features

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

By design --btf_features=FEATURE1[,FEATURE2,...] will not complain
if an unrecognized feature is specified.  This allows the kernel
build process to specify new features regardless of whether they
are supported by the version of pahole used; in such cases we do
not wish for every invocation of pahole to complain.  However it is
still valuable to have a way of knowing which BTF features pahole
supports; this could be logged as part of the build process for
example.  By specifying --supported_btf_features a comma-separated
list is returned; for example:

 $ pahole --supported_btf_features
 encode_force,var,float,decl_tag,type_tag,enum64,optimized_func,consistent_func

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 man-pages/pahole.1 |  4 ++++
 pahole.c           | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/man-pages/pahole.1 b/man-pages/pahole.1
index a09885f..6148915 100644
--- a/man-pages/pahole.1
+++ b/man-pages/pahole.1
@@ -297,6 +297,10 @@ Encode BTF using the specified feature list, or specify 'all' for all features s
 
 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 \-\-supported_btf_features
+Show set of BTF features supported by \-\-btf_features option and exit.  Useful for checking which features are supported since \-\-btf_features will not emit an error if an unrecognized feature is specified.
+
 .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 0d0f6c6..816525a 100644
--- a/pahole.c
+++ b/pahole.c
@@ -1230,6 +1230,7 @@ ARGP_PROGRAM_VERSION_HOOK_DEF = dwarves_print_version;
 #define ARGP_btf_gen_optimized  339
 #define ARGP_skip_encoding_btf_inconsistent_proto 340
 #define ARGP_btf_features	341
+#define ARGP_supported_btf_features 342
 
 /* --btf_features=feature1[,feature2,..] allows us to specify
  * a list of requested BTF features or "all" to enable all features.
@@ -1334,6 +1335,18 @@ static void parse_btf_features(const char *features)
 	set_btf_features_defaults = true;
 }
 
+static void show_supported_btf_features(void)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(btf_features); i++) {
+		if (i > 0)
+			printf(",");
+		printf("%s", btf_features[i].name);
+	}
+	printf("\n");
+}
+
 static const struct argp_option pahole__options[] = {
 	{
 		.name = "bit_holes",
@@ -1761,6 +1774,11 @@ static const struct argp_option pahole__options[] = {
 		.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 = "supported_btf_features",
+		.key = ARGP_supported_btf_features,
+		.doc = "Show list of btf_features supported by pahole and exit."
+	},
 	{
 		.name = NULL,
 	}
@@ -1938,6 +1956,8 @@ static error_t pahole__options_parser(int key, char *arg,
 		conf_load.skip_encoding_btf_inconsistent_proto = true; break;
 	case ARGP_btf_features:
 		parse_btf_features(arg);		break;
+	case ARGP_supported_btf_features:
+		show_supported_btf_features();		exit(0);
 	default:
 		return ARGP_ERR_UNKNOWN;
 	}
-- 
2.31.1