pahole: Add global_var BTF feature

So far, pahole has only encoded type information for percpu variables. However, there are several reasons why type information for all global variables would be useful in the kernel: 1. Runtime kernel debuggers like drgn could use the BTF to introspect kernel data structures without needing to install heavyweight DWARF. 2. BPF programs using the "__ksym" annotation could declare the variables using the correct type, rather than "void". It makes sense to introduce a feature for this in pahole so that these capabilities can be explored in the kernel. The feature is non-default: when using "--btf-features=default", it is disabled. It must be explicitly requested, e.g. with "--btf-features=+global_var". Reviewed-by: Alan Maguire <[email protected]> Signed-off-by: Stephen Brennan <[email protected]> Tested-by: Alan Maguire <[email protected]> Acked-by: Jiri Olsa <[email protected]> Link: https://lore.kernel.org/r/[email protected] Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
Karinza38 · Oct 7, 2024 · d580a45 · d580a45
1 parent ff34e73
commit d580a45
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Showing 5 changed files with 14 additions and 3 deletions.
diff --git a/btf_encoder.c b/btf_encoder.c
@@ -2348,6 +2348,8 @@ struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filenam
 		encoder->encode_vars = 0;
 		if (!conf_load->skip_encoding_btf_vars)
 			encoder->encode_vars |= BTF_VAR_PERCPU;
+		if (conf_load->encode_btf_global_vars)
+			encoder->encode_vars |= BTF_VAR_GLOBAL;
 
 		GElf_Ehdr ehdr;
 
@@ -2400,6 +2402,9 @@ struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filenam
 			encoder->secinfo[shndx].name = secname;
 			encoder->secinfo[shndx].type = shdr.sh_type;
 
+			if (encoder->encode_vars & BTF_VAR_GLOBAL)
+				encoder->secinfo[shndx].include = true;
+
 			if (strcmp(secname, PERCPU_SECTION) == 0) {
 				found_percpu = true;
 				if (encoder->encode_vars & BTF_VAR_PERCPU)

diff --git a/btf_encoder.h b/btf_encoder.h
@@ -20,6 +20,7 @@ struct list_head;
 enum btf_var_option {
 	BTF_VAR_NONE = 0,
 	BTF_VAR_PERCPU = 1,
+	BTF_VAR_GLOBAL = 2,
 };
 
 struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filename, struct btf *base_btf, bool verbose, struct conf_load *conf_load);

diff --git a/dwarves.h b/dwarves.h
@@ -92,6 +92,7 @@ struct conf_load {
 	bool			btf_gen_optimized;
 	bool			skip_encoding_btf_inconsistent_proto;
 	bool			skip_encoding_btf_vars;
+	bool			encode_btf_global_vars;
 	bool			btf_gen_floats;
 	bool			btf_encode_force;
 	bool			reproducible_build;

diff --git a/man-pages/pahole.1 b/man-pages/pahole.1
@@ -238,7 +238,9 @@ the debugging information.
 
 .TP
 .B \-\-skip_encoding_btf_vars
-Do not encode VARs in BTF.
+By default, VARs are encoded only for percpu variables. When specified, this
+option prevents encoding any VARs. Note that this option can be overridden
+by the feature "global_var".
 
 .TP
 .B \-\-skip_encoding_btf_decl_tag
@@ -304,7 +306,7 @@ Encode BTF using the specified feature list, or specify 'default' for all standa
 	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.
+	var                Encode percpu 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.
@@ -329,6 +331,7 @@ Supported non-standard features (not enabled for 'default')
 	                   the associated base BTF to support later relocation
 	                   of split BTF with a possibly changed base, storing
 	                   it in a .BTF.base ELF section.
+	global_var         Encode all global variables using BTF_KIND_VAR in BTF.
 .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.

diff --git a/pahole.c b/pahole.c
@@ -1304,6 +1304,7 @@ struct btf_feature {
 	BTF_DEFAULT_FEATURE(decl_tag_kfuncs, btf_decl_tag_kfuncs, false),
 	BTF_NON_DEFAULT_FEATURE(reproducible_build, reproducible_build, false),
 	BTF_NON_DEFAULT_FEATURE(distilled_base, btf_gen_distilled_base, false),
+	BTF_NON_DEFAULT_FEATURE(global_var, encode_btf_global_vars, false),
 };
 
 #define BTF_MAX_FEATURE_STR	1024
@@ -1736,7 +1737,7 @@ static const struct argp_option pahole__options[] = {
 	{
 		.name = "skip_encoding_btf_vars",
 		.key  = ARGP_skip_encoding_btf_vars,
-		.doc  = "Do not encode VARs in BTF."
+		.doc  = "Do not encode any VARs in BTF [if this is not specified, only percpu variables are encoded. To encode global variables too, use --encode_btf_global_vars]."
 	},
 	{
 		.name = "btf_encode_force",