[PATCH] btf_encoder: Teach pahole to store percpu variables in vmlinux BTF.

[Hao Luo <haoluo-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>]

On SMP systems, the global percpu variables are placed in a speical
'.data..percpu' section, which is stored in a segment whose initial
address is set to 0, the addresses of per-CPU variables are relative
positive addresses[1].

This patch extracts these variables from vmlinux and places them with
their type information in BTF, so that they can be accessed in BPF.

In a v5.7-rc7 linux kernel, I was able to extract 291 such variables.
The space overhead is small.

Testing:

Before:
 $ readelf -SW vmlinux | grep BTF
 [25] .BTF              PROGBITS        ffffffff821a905c 13a905c 2d2bf8 00   A  0   0  1

After:
 $ pahole -J vmlinux
 $ readelf -SW vmlinux  | grep BTF
 [25] .BTF              PROGBITS        ffffffff821a905c 13a905c 2d4db8 00   A  0   0  1

Common percpu vars can be found in the BTF section.

 $ bpftool btf dump file vmlinux | grep runqueues
 [14098] VAR 'runqueues' type_id=13725, linkage=global-alloc

 $ bpftool btf dump file vmlinux | grep cpu_stopper
 [17589] STRUCT 'cpu_stopper' size=72 vlen=5
 [17609] VAR 'cpu_stopper' type_id=17589, linkage=global-alloc

References:
 [1] https://lwn.net/Articles/531148/
Signed-off-by: Hao Luo <haoluo-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
---
 btf_encoder.c | 27 +++++++++++++++++++++++++++
 libbtf.c      | 29 +++++++++++++++++++++++++++++
 libbtf.h      |  2 ++
 pahole.c      |  1 +
 4 files changed, 59 insertions(+)

diff --git a/btf_encoder.c b/btf_encoder.c
index df16ba0..f550f34 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -241,6 +241,33 @@ int cu__encode_btf(struct cu *cu, int verbose)
 		}
 	}
 
+	cu__for_each_variable(cu, core_id, pos) {
+		int btf_var_id;
+		struct variable *var;
+		uint32_t linkage;
+
+		var = tag__variable(pos);
+		/*
+		 * .data..percpu is stored in a segment whose initial address is
+		 * set to 0, the addresses of per-CPU variables are addresses
+		 * relative to 0.
+		 *
+		 * https://lwn.net/Articles/531148/
+		 */
+		if ((int64_t)var->ip.addr <= 0)
+			continue;
+		linkage = variable__scope(var) == VSCOPE_GLOBAL;
+		btf_var_id = btf_elf__add_var_type(btfe, var->ip.tag.type,
+						   var->name, linkage,
+						   type_id_off);
+		if (btf_var_id < 0) {
+			err = -1;
+			printf("error: failed to encode variable '%s'\n",
+			       variable__name(var, cu));
+			goto out;
+		}
+	}
+
 out:
 	if (err)
 		btf_elf__delete(btfe);
diff --git a/libbtf.c b/libbtf.c
index 2fbce40..de85666 100644
--- a/libbtf.c
+++ b/libbtf.c
@@ -613,6 +613,35 @@ int32_t btf_elf__add_func_proto(struct btf_elf *btfe, struct ftype *ftype, uint3
 	return type_id;
 }
 
+int32_t btf_elf__add_var_type(struct btf_elf *btfe, uint32_t type,
+			      uint32_t name_off, uint32_t linkage,
+			      uint32_t type_id_off)
+{
+	struct {
+		struct btf_type type;
+		struct btf_var var;
+	} t;
+
+	t.type.name_off = name_off;
+	t.type.info = BTF_INFO_ENCODE(BTF_KIND_VAR, 0, 0);
+	t.type.type = type_id_off + type;
+
+	t.var.linkage = linkage;
+
+	++btfe->type_index;
+	if (gobuffer__add(&btfe->types, &t.type, sizeof(t)) < 0) {
+		btf_elf__log_type(btfe, &t.type, true, true,
+				  "type=%u name=%s Error in adding gobuffer",
+				  t.type.type, btf_elf__name_in_gobuf(btfe, t.type.name_off));
+		return -1;
+	}
+
+	btf_elf__log_type(btfe, &t.type, false, false, "type=%u name=%s",
+			  t.type.type, btf_elf__name_in_gobuf(btfe, t.type.name_off));
+
+	return btfe->type_index;
+}
+
 static int btf_elf__write(const char *filename, struct btf *btf)
 {
 	GElf_Shdr shdr_mem, *shdr;
diff --git a/libbtf.h b/libbtf.h
index f3c8500..d9e723a 100644
--- a/libbtf.h
+++ b/libbtf.h
@@ -55,6 +55,8 @@ int32_t btf_elf__add_enum(struct btf_elf *btf, uint32_t name, uint32_t size,
 int btf_elf__add_enum_val(struct btf_elf *btf, uint32_t name, int32_t value);
 int32_t btf_elf__add_func_proto(struct btf_elf *btf, struct ftype *ftype,
 				uint32_t type_id_off);
+int32_t btf_elf__add_var_type(struct btf_elf *btfe, uint32_t type, uint32_t name_off,
+			      uint32_t linkage, uint32_t type_id_off);
 void btf_elf__set_strings(struct btf_elf *btf, struct gobuffer *strings);
 int  btf_elf__encode(struct btf_elf *btf, uint8_t flags);
 
diff --git a/pahole.c b/pahole.c
index e2a081b..8407db9 100644
--- a/pahole.c
+++ b/pahole.c
@@ -1084,6 +1084,7 @@ static error_t pahole__options_parser(int key, char *arg,
 	case 'i': find_containers = 1;
 		  class_name = arg;			break;
 	case 'J': btf_encode = 1;
+		  conf_load.get_addr_info = true;
 		  no_bitfield_type_recode = true;	break;
 	case 'l': conf.show_first_biggest_size_base_type_member = 1;	break;
 	case 'M': conf.show_only_data_members = 1;	break;
-- 
2.27.0.rc2.251.g90737beb825-goog