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 <stephen.s.brennan@xxxxxxxxxx>
---
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(-)
diff --git a/btf_encoder.c b/btf_encoder.c
index 2fd1648..2730ea8 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",
--
2.43.5
