[PATCH dwarves v3 0/3] btf_encoder: refactor emission of BTF funcs

[PATCH dwarves v3 0/3] btf_encoder: refactor emission of BTF funcs

[Ihor Solodrai]

This series refactors a few functions that handle how BTF functions
are emitted.

While addressing comments from Eduard, I noticed that we don't
actually need to carry the encoder in btf_encoder_func_state, because
only one encoder exists at all times. So in v3 I added a patch
removing encoder from the state struct, which resolves the ambiguity
of where state->encoder should be used.

v2: https://lore.kernel.org/dwarves/20251104233532.196287-1-ihor.solodrai@linux.dev/
v1: https://lore.kernel.org/dwarves/20251029190249.3323752-2-ihor.solodrai@linux.dev/

Ihor Solodrai (3):
  btf_encoder: Remove encoder pointer from btf_encoder_func_state
  btf_encoder: Refactor btf_encoder__add_func_proto
  btf_encoder: Factor out BPF kfunc emission

 btf_encoder.c | 204 +++++++++++++++++++++++++++++---------------------
 1 file changed, 117 insertions(+), 87 deletions(-)

-- 
2.51.1

[PATCH dwarves v3 1/3] btf_encoder: Remove encoder pointer from btf_encoder_func_state

[Ihor Solodrai]

Since multithreading was moved to dwarf_loader [1], pahole maintains a
single global btf_encoder instance. However parts of btf_encoder.c
exist to manage mulitple encoders, which is not necessary anymore.

This patch removes encoder pointer from struct btf_encoder_func_state.
We create a state for every encountered instance of a function in
DWARF, and currently they carry around the encoder unnecessarily.

[1] https://lore.kernel.org/all/20250109185950.653110-9-ihor.solodrai@pm.me/

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 btf_encoder.c | 34 +++++++++++++++-------------------
 1 file changed, 15 insertions(+), 19 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 03bc3c7..a6fdc58 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -79,7 +79,6 @@ struct btf_encoder_func_annot {
 
 /* state used to do later encoding of saved functions */
 struct btf_encoder_func_state {
-	struct btf_encoder *encoder;
 	struct elf_function *elf;
 	uint32_t type_id_off;
 	uint16_t nr_parms;
@@ -819,7 +818,7 @@ static int32_t btf_encoder__add_func_proto(struct btf_encoder *encoder, struct f
 					   struct btf_encoder_func_state *state)
 {
 	const struct btf_type *t;
-	struct btf *btf;
+	struct btf *btf = encoder->btf;
 	struct parameter *param;
 	uint16_t nr_params, param_idx;
 	int32_t id, type_id;
@@ -834,12 +833,9 @@ static int32_t btf_encoder__add_func_proto(struct btf_encoder *encoder, struct f
 
 	/* add btf_type for func_proto */
 	if (ftype) {
-		btf = encoder->btf;
 		nr_params = ftype->nr_parms + (ftype->unspec_parms ? 1 : 0);
 		type_id = btf_encoder__tag_type(encoder, ftype->tag.type);
 	} else if (state) {
-		encoder = state->encoder;
-		btf = state->encoder->btf;
 		nr_params = state->nr_parms;
 		type_id = state->ret_type_id;
 	} else {
@@ -1123,13 +1119,12 @@ static bool types__match(struct btf_encoder *encoder,
 	return false;
 }
 
-static bool funcs__match(struct btf_encoder_func_state *s1,
+static bool funcs__match(struct btf_encoder *encoder,
+			 struct btf_encoder_func_state *s1,
 			 struct btf_encoder_func_state *s2)
 {
-	struct btf_encoder *encoder = s1->encoder;
 	struct elf_function *func = s1->elf;
-	struct btf *btf1 = s1->encoder->btf;
-	struct btf *btf2 = s2->encoder->btf;
+	struct btf *btf = encoder->btf;
 	uint8_t i;
 
 	if (s1->nr_parms != s2->nr_parms) {
@@ -1138,7 +1133,7 @@ static bool funcs__match(struct btf_encoder_func_state *s1,
 					   s1->nr_parms, s2->nr_parms);
 		return false;
 	}
-	if (!types__match(encoder, btf1, s1->ret_type_id, btf2, s2->ret_type_id)) {
+	if (!types__match(encoder, btf, s1->ret_type_id, btf, s2->ret_type_id)) {
 		btf_encoder__log_func_skip(encoder, func, "return type mismatch\n");
 		return false;
 	}
@@ -1146,11 +1141,11 @@ static bool funcs__match(struct btf_encoder_func_state *s1,
 		return true;
 
 	for (i = 0; i < s1->nr_parms; i++) {
-		if (!types__match(encoder, btf1, s1->parms[i].type_id,
-				  btf2, s2->parms[i].type_id)) {
+		if (!types__match(encoder, btf, s1->parms[i].type_id,
+				  btf, s2->parms[i].type_id)) {
 			if (encoder->verbose) {
-				const char *p1 = btf__name_by_offset(btf1, s1->parms[i].name_off);
-				const char *p2 = btf__name_by_offset(btf2, s2->parms[i].name_off);
+				const char *p1 = btf__name_by_offset(btf, s1->parms[i].name_off);
+				const char *p2 = btf__name_by_offset(btf, s2->parms[i].name_off);
 
 				btf_encoder__log_func_skip(encoder, func,
 							   "param type mismatch for param#%d %s %s %s\n",
@@ -1244,7 +1239,6 @@ static int32_t btf_encoder__save_func(struct btf_encoder *encoder, struct functi
 	if (!state)
 		return -ENOMEM;
 
-	state->encoder = encoder;
 	state->elf = func;
 	state->nr_parms = ftype->nr_parms + (ftype->unspec_parms ? 1 : 0);
 	state->ret_type_id = ftype->tag.type == 0 ? 0 : encoder->type_id_off + ftype->tag.type;
@@ -1410,7 +1404,9 @@ static int saved_functions_cmp(const void *_a, const void *_b)
 	return elf_function__name_cmp(a->elf, b->elf);
 }
 
-static int saved_functions_combine(struct btf_encoder_func_state *a, struct btf_encoder_func_state *b)
+static int saved_functions_combine(struct btf_encoder *encoder,
+				   struct btf_encoder_func_state *a,
+				   struct btf_encoder_func_state *b)
 {
 	uint8_t optimized, unexpected, inconsistent, uncertain_parm_loc;
 
@@ -1421,7 +1417,7 @@ static int saved_functions_combine(struct btf_encoder_func_state *a, struct btf_
 	unexpected = a->unexpected_reg | b->unexpected_reg;
 	inconsistent = a->inconsistent_proto | b->inconsistent_proto;
 	uncertain_parm_loc = a->uncertain_parm_loc | b->uncertain_parm_loc;
-	if (!unexpected && !inconsistent && !funcs__match(a, b))
+	if (!unexpected && !inconsistent && !funcs__match(encoder, a, b))
 		inconsistent = 1;
 	a->optimized_parms = b->optimized_parms = optimized;
 	a->unexpected_reg = b->unexpected_reg = unexpected;
@@ -1471,7 +1467,7 @@ static int btf_encoder__add_saved_funcs(struct btf_encoder *encoder, bool skip_e
 		 */
 		j = i + 1;
 
-		while (j < nr_saved_fns && saved_functions_combine(&saved_fns[i], &saved_fns[j]) == 0)
+		while (j < nr_saved_fns && saved_functions_combine(encoder, &saved_fns[i], &saved_fns[j]) == 0)
 			j++;
 
 		/* do not exclude functions with optimized-out parameters; they
@@ -1488,7 +1484,7 @@ static int btf_encoder__add_saved_funcs(struct btf_encoder *encoder, bool skip_e
 					0, 0);
 
 		if (add_to_btf) {
-			err = btf_encoder__add_func(state->encoder, state);
+			err = btf_encoder__add_func(encoder, state);
 			if (err < 0)
 				goto out;
 		}
-- 
2.51.1

[PATCH dwarves v3 2/3] btf_encoder: Refactor btf_encoder__add_func_proto

[Ihor Solodrai]

btf_encoder__add_func_proto() essentially implements two independent
code paths depending on input arguments: one for struct ftype and the
other for struct btf_encoder_func_state.

Split btf_encoder__add_func_proto() into two variants:
  * btf_encoder__add_func_proto_for_ftype()
  * btf_encoder__add_func_proto_for_state()

And factor out common btf_encoder__emit_func_proto() subroutine.

No functional changes.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 btf_encoder.c | 135 +++++++++++++++++++++++++++++---------------------
 1 file changed, 79 insertions(+), 56 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index a6fdc58..bdda7d0 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -814,79 +814,102 @@ static inline bool is_kfunc_state(struct btf_encoder_func_state *state)
 	return state && state->elf && state->elf->kfunc;
 }
 
-static int32_t btf_encoder__add_func_proto(struct btf_encoder *encoder, struct ftype *ftype,
-					   struct btf_encoder_func_state *state)
+static int32_t btf_encoder__emit_func_proto(struct btf_encoder *encoder,
+					    uint32_t type_id,
+					    uint16_t nr_params)
 {
 	const struct btf_type *t;
-	struct btf *btf = encoder->btf;
-	struct parameter *param;
+	uint32_t ret;
+
+	ret = btf__add_func_proto(encoder->btf, type_id);
+	if (ret > 0) {
+		t = btf__type_by_id(encoder->btf, ret);
+		btf_encoder__log_type(encoder, t, false, false,
+			"return=%u args=(%s", t->type, !nr_params ? "void)\n" : "");
+	} else {
+		btf__log_err(encoder->btf, BTF_KIND_FUNC_PROTO, NULL, true, ret,
+			     "return=%u vlen=%u Error emitting BTF type",
+			     type_id, nr_params);
+	}
+
+	return ret;
+}
+
+static int32_t btf_encoder__add_func_proto_for_ftype(struct btf_encoder *encoder,
+						     struct ftype *ftype)
+{
 	uint16_t nr_params, param_idx;
+	struct parameter *param;
 	int32_t id, type_id;
-	char tmp_name[KSYM_NAME_LEN];
 	const char *name;
 
-	assert(ftype != NULL || state != NULL);
-
-	if (is_kfunc_state(state) && encoder->tag_kfuncs && encoder->encode_attributes)
-		if (btf__add_bpf_arena_type_tags(encoder->btf, state) < 0)
-			return -1;
+	assert(ftype != NULL);
 
 	/* add btf_type for func_proto */
-	if (ftype) {
-		nr_params = ftype->nr_parms + (ftype->unspec_parms ? 1 : 0);
-		type_id = btf_encoder__tag_type(encoder, ftype->tag.type);
-	} else if (state) {
-		nr_params = state->nr_parms;
-		type_id = state->ret_type_id;
-	} else {
-		return 0;
-	}
+	nr_params = ftype->nr_parms + (ftype->unspec_parms ? 1 : 0);
+	type_id = btf_encoder__tag_type(encoder, ftype->tag.type);
 
-	id = btf__add_func_proto(btf, type_id);
-	if (id > 0) {
-		t = btf__type_by_id(btf, id);
-		btf_encoder__log_type(encoder, t, false, false, "return=%u args=(%s", t->type, !nr_params ? "void)\n" : "");
-	} else {
-		btf__log_err(btf, BTF_KIND_FUNC_PROTO, NULL, true, id,
-			     "return=%u vlen=%u Error emitting BTF type",
-			     type_id, nr_params);
+	id = btf_encoder__emit_func_proto(encoder, type_id, nr_params);
+	if (id < 0)
 		return id;
-	}
 
 	/* add parameters */
 	param_idx = 0;
-	if (ftype) {
-		ftype__for_each_parameter(ftype, param) {
-			const char *name = parameter__name(param);
-
-			type_id = param->tag.type == 0 ? 0 : encoder->type_id_off + param->tag.type;
-			++param_idx;
-			if (btf_encoder__add_func_param(encoder, name, type_id,
-							param_idx == nr_params))
-				return -1;
-		}
 
+	ftype__for_each_parameter(ftype, param) {
+		name = parameter__name(param);
+		type_id = param->tag.type == 0 ? 0 : encoder->type_id_off + param->tag.type;
 		++param_idx;
-		if (ftype->unspec_parms)
-			if (btf_encoder__add_func_param(encoder, NULL, 0,
-							param_idx == nr_params))
-				return -1;
-	} else {
-		for (param_idx = 0; param_idx < nr_params; param_idx++) {
-			struct btf_encoder_func_parm *p = &state->parms[param_idx];
+		if (btf_encoder__add_func_param(encoder, name, type_id, param_idx == nr_params))
+			return -1;
+	}
 
-			name = btf__name_by_offset(btf, p->name_off);
+	++param_idx;
+	if (ftype->unspec_parms)
+		if (btf_encoder__add_func_param(encoder, NULL, 0, param_idx == nr_params))
+			return -1;
 
-			/* adding BTF data may result in a move of the
-			 * name string memory, so make a temporary copy.
-			 */
-			strncpy(tmp_name, name, sizeof(tmp_name) - 1);
+	return id;
+}
 
-			if (btf_encoder__add_func_param(encoder, tmp_name, p->type_id,
-							param_idx == nr_params))
-				return -1;
-		}
+static int32_t btf_encoder__add_func_proto_for_state(struct btf_encoder *encoder,
+						     struct btf_encoder_func_state *state)
+{
+	const struct btf *btf = encoder->btf;
+	struct btf_encoder_func_parm *p;
+	uint16_t nr_params, param_idx;
+	char tmp_name[KSYM_NAME_LEN];
+	int32_t id, type_id;
+	const char *name;
+	bool is_last;
+
+	/* Beware: btf__add_bpf_arena_type_tags may change some members of the state */
+	if (is_kfunc_state(state) && encoder->tag_kfuncs && encoder->encode_attributes)
+		if (btf__add_bpf_arena_type_tags(encoder->btf, state) < 0)
+			return -1;
+
+	type_id = state->ret_type_id;
+	nr_params = state->nr_parms;
+
+	id = btf_encoder__emit_func_proto(encoder, type_id, nr_params);
+	if (id < 0)
+		return id;
+
+	/* add parameters */
+	for (param_idx = 0; param_idx < nr_params; param_idx++) {
+		p = &state->parms[param_idx];
+		name = btf__name_by_offset(btf, p->name_off);
+		is_last = param_idx == nr_params;
+
+		/* adding BTF data may result in a move of the
+		 * name string memory, so make a temporary copy.
+		 */
+		strncpy(tmp_name, name, sizeof(tmp_name) - 1);
+
+		if (btf_encoder__add_func_param(encoder, tmp_name, p->type_id, is_last))
+			return -1;
 	}
+
 	return id;
 }
 
@@ -1343,7 +1366,7 @@ static int32_t btf_encoder__add_func(struct btf_encoder *encoder,
 	uint16_t idx;
 	int err;
 
-	btf_fnproto_id = btf_encoder__add_func_proto(encoder, NULL, state);
+	btf_fnproto_id = btf_encoder__add_func_proto_for_state(encoder, state);
 	name = func->name;
 	if (btf_fnproto_id >= 0)
 		btf_fn_id = btf_encoder__add_ref_type(encoder, BTF_KIND_FUNC, btf_fnproto_id,
@@ -1682,7 +1705,7 @@ static int btf_encoder__encode_tag(struct btf_encoder *encoder, struct tag *tag,
 	case DW_TAG_enumeration_type:
 		return btf_encoder__add_enum_type(encoder, tag, conf_load);
 	case DW_TAG_subroutine_type:
-		return btf_encoder__add_func_proto(encoder, tag__ftype(tag), NULL);
+		return btf_encoder__add_func_proto_for_ftype(encoder, tag__ftype(tag));
         case DW_TAG_unspecified_type:
 		/* Just don't encode this for now, converting anything with this type to void (0) instead.
 		 *
-- 
2.51.1

[PATCH dwarves v3 3/3] btf_encoder: Factor out BPF kfunc emission

[Ihor Solodrai]

Generating BTF for BPF kernel functions requires special
handling. Consolidate this behavior into btf_encoder__add_bpf_kfunc(),
which uses simplified btf_encoder_add_func().

No functional changes.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 btf_encoder.c | 49 ++++++++++++++++++++++++++++++-------------------
 1 file changed, 30 insertions(+), 19 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index bdda7d0..0bb7831 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -773,6 +773,7 @@ static int btf__tag_bpf_arena_arg(struct btf *btf, struct btf_encoder_func_state
 	return id;
 }
 
+/* Modifies state->ret_type_id and state->parms[i].type_id for flagged kfuncs */
 static int btf__add_bpf_arena_type_tags(struct btf *btf, struct btf_encoder_func_state *state)
 {
 	uint32_t flags = state->elf->kfunc_flags;
@@ -883,11 +884,6 @@ static int32_t btf_encoder__add_func_proto_for_state(struct btf_encoder *encoder
 	const char *name;
 	bool is_last;
 
-	/* Beware: btf__add_bpf_arena_type_tags may change some members of the state */
-	if (is_kfunc_state(state) && encoder->tag_kfuncs && encoder->encode_attributes)
-		if (btf__add_bpf_arena_type_tags(encoder->btf, state) < 0)
-			return -1;
-
 	type_id = state->ret_type_id;
 	nr_params = state->nr_parms;
 
@@ -1357,14 +1353,13 @@ static int btf__tag_kfunc(struct btf *btf, struct elf_function *kfunc, __u32 btf
 static int32_t btf_encoder__add_func(struct btf_encoder *encoder,
 				     struct btf_encoder_func_state *state)
 {
-	struct elf_function *func = state->elf;
 	int btf_fnproto_id, btf_fn_id, tag_type_id = 0;
+	struct elf_function *func = state->elf;
+	char tmp_value[KSYM_NAME_LEN];
 	int16_t component_idx = -1;
-	const char *name;
 	const char *value;
-	char tmp_value[KSYM_NAME_LEN];
+	const char *name;
 	uint16_t idx;
-	int err;
 
 	btf_fnproto_id = btf_encoder__add_func_proto_for_state(encoder, state);
 	name = func->name;
@@ -1377,15 +1372,6 @@ static int32_t btf_encoder__add_func(struct btf_encoder *encoder,
 		return -1;
 	}
 
-	if (func->kfunc && encoder->tag_kfuncs && !encoder->skip_encoding_decl_tag) {
-		err = btf__tag_kfunc(encoder->btf, func, btf_fn_id);
-		if (err < 0)
-			return err;
-	}
-
-	if (state->nr_annots == 0)
-		return 0;
-
 	for (idx = 0; idx < state->nr_annots; idx++) {
 		struct btf_encoder_func_annot *a = &state->annots[idx];
 
@@ -1408,6 +1394,28 @@ static int32_t btf_encoder__add_func(struct btf_encoder *encoder,
 		return -1;
 	}
 
+	return btf_fn_id;
+}
+
+static int btf_encoder__add_bpf_kfunc(struct btf_encoder *encoder,
+				      struct btf_encoder_func_state *state)
+{
+	int btf_fn_id, err;
+
+	if (encoder->tag_kfuncs && encoder->encode_attributes)
+		if (btf__add_bpf_arena_type_tags(encoder->btf, state) < 0)
+			return -1;
+
+	btf_fn_id = btf_encoder__add_func(encoder, state);
+	if (btf_fn_id < 0)
+		return -1;
+
+	if (encoder->tag_kfuncs && !encoder->skip_encoding_decl_tag) {
+		err = btf__tag_kfunc(encoder->btf, state->elf, btf_fn_id);
+		if (err < 0)
+			return -1;
+	}
+
 	return 0;
 }
 
@@ -1507,7 +1515,10 @@ static int btf_encoder__add_saved_funcs(struct btf_encoder *encoder, bool skip_e
 					0, 0);
 
 		if (add_to_btf) {
-			err = btf_encoder__add_func(encoder, state);
+			if (is_kfunc_state(state))
+				err = btf_encoder__add_bpf_kfunc(encoder, state);
+			else
+				err = btf_encoder__add_func(encoder, state);
 			if (err < 0)
 				goto out;
 		}
-- 
2.51.1

Re: [PATCH dwarves v3 3/3] btf_encoder: Factor out BPF kfunc emission

[Eduard Zingerman]

On Wed, 2025-11-05 at 10:59 -0800, Ihor Solodrai wrote:

[...]

> @@ -1408,6 +1394,28 @@ static int32_t btf_encoder__add_func(struct btf_encoder *encoder,
>  		return -1;
>  	}
>  
> +	return btf_fn_id;
> +}
> +
> +static int btf_encoder__add_bpf_kfunc(struct btf_encoder *encoder,
> +				      struct btf_encoder_func_state *state)
> +{
> +	int btf_fn_id, err;
> +
> +	if (encoder->tag_kfuncs && encoder->encode_attributes)
> +		if (btf__add_bpf_arena_type_tags(encoder->btf, state) < 0)
> +			return -1;

Nit: maybe keep the error check consistent with the one below? E.g.:
       err = btf__add_bpf_arena_type_tags(...);
       if (err)
          return err;
     ?
> +
> +	btf_fn_id = btf_encoder__add_func(encoder, state);
> +	if (btf_fn_id < 0)
> +		return -1;
> +
> +	if (encoder->tag_kfuncs && !encoder->skip_encoding_decl_tag) {
> +		err = btf__tag_kfunc(encoder->btf, state->elf, btf_fn_id);
> +		if (err < 0)
> +			return -1;
> +	}
> +
>  	return 0;
>  }
>  

[...]

Re: [PATCH dwarves v3 0/3] btf_encoder: refactor emission of BTF funcs

[Eduard Zingerman]

On Wed, 2025-11-05 at 10:59 -0800, Ihor Solodrai wrote:
> This series refactors a few functions that handle how BTF functions
> are emitted.
> 
> While addressing comments from Eduard, I noticed that we don't
> actually need to carry the encoder in btf_encoder_func_state, because
> only one encoder exists at all times. So in v3 I added a patch
> removing encoder from the state struct, which resolves the ambiguity
> of where state->encoder should be used.
> 
> v2: https://lore.kernel.org/dwarves/20251104233532.196287-1-ihor.solodrai@linux.dev/
> v1: https://lore.kernel.org/dwarves/20251029190249.3323752-2-ihor.solodrai@linux.dev/
> 
> Ihor Solodrai (3):
>   btf_encoder: Remove encoder pointer from btf_encoder_func_state
>   btf_encoder: Refactor btf_encoder__add_func_proto
>   btf_encoder: Factor out BPF kfunc emission
> 
>  btf_encoder.c | 204 +++++++++++++++++++++++++++++---------------------
>  1 file changed, 117 insertions(+), 87 deletions(-)

No difference in generated BTF with my test kernel config.

Tested-by: Eduard Zingerman <eddyz87@gmail.com>