[dwarves,v4,4/4] pahole: add global_var BTF feature

Commit Message

Stephen Brennan Oct. 4, 2024, 5:26 p.m. UTC

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".

Signed-off-by: Stephen Brennan <[e-mail]>
Reviewed-by: Alan Maguire <[e-mail]>
Tested-by: Alan Maguire <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
---
 btf_encoder.c      | 5 +++++
 btf_encoder.h      | 1 +
 dwarves.h          | 1 +
 man-pages/pahole.1 | 7 +++++--
 pahole.c           | 3 ++-
 5 files changed, 14 insertions(+), 3 deletions(-)

Patch

diff --git a/btf_encoder.c b/btf_encoder.c
index 838a0b1..201a48c 100644
--- 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
index 91e7947..824963b 100644
--- 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
index 0fede91..fef881f 100644
--- 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
index b3e6632..7c1a69a 100644
--- 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
index b21a7f2..9f0dc59 100644
--- a/pahole.c
+++ b/pahole.c
@@ -1301,6 +1301,7 @@  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
@@ -1733,7 +1734,7 @@  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",