[dwarves] btf_encoder: add "distilled_base" BTF feature to split BTF generation

Commit Message

Alan Maguire April 24, 2024, 3:54 p.m. UTC

Adding "distilled_base" to --btf_features when generating split BTF will
create split and .BTF.base BTF - the latter allows us to map references
from split BTF to base BTF, even if that base BTF has changed.  It does
this by providing just enough information about the base types in the
.BTF.base section.

Patch is applicable on the "next" branch of dwarves, and requires the
libbpf from the series in [1]

Signed-off-by: Alan Maguire <[e-mail]>

[1] https://lore.kernel.org/bpf/[e-mail]/
---
 btf_encoder.c      | 40 ++++++++++++++++++++++++++++------------
 dwarves.h          |  1 +
 man-pages/pahole.1 |  3 +++
 pahole.c           |  1 +
 4 files changed, 33 insertions(+), 12 deletions(-)

Patch

diff --git a/btf_encoder.c b/btf_encoder.c
index e1e3529..cc76f62 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -75,7 +75,8 @@  struct btf_encoder {
 			  verbose,
 			  force,
 			  gen_floats,
-			  is_rel;
+			  is_rel,
+			  gen_distilled_base;
 	uint32_t	  array_index_id;
 	struct {
 		struct var_info *vars;
@@ -1255,9 +1256,9 @@  static int btf_encoder__write_raw_file(struct btf_encoder *encoder)
 	return err;
 }
 
-static int btf_encoder__write_elf(struct btf_encoder *encoder)
+static int btf_encoder__write_elf(struct btf_encoder *encoder, const struct btf *btf,
+				  const char *btf_secname)
 {
-	struct btf *btf = encoder->btf;
 	const char *filename = encoder->filename;
 	GElf_Shdr shdr_mem, *shdr;
 	Elf_Data *btf_data = NULL;
@@ -1297,7 +1298,7 @@  static int btf_encoder__write_elf(struct btf_encoder *encoder)
 		if (shdr == NULL)
 			continue;
 		char *secname = elf_strptr(elf, strndx, shdr->sh_name);
-		if (strcmp(secname, ".BTF") == 0) {
+		if (strcmp(secname, btf_secname) == 0) {
 			btf_data = elf_getdata(scn, btf_data);
 			break;
 		}
@@ -1341,11 +1342,11 @@  static int btf_encoder__write_elf(struct btf_encoder *encoder)
 			goto unlink;
 		}
 
-		snprintf(cmd, sizeof(cmd), "%s --add-section .BTF=%s %s",
-			 llvm_objcopy, tmp_fn, filename);
+		snprintf(cmd, sizeof(cmd), "%s --add-section %s=%s %s",
+			 llvm_objcopy, btf_secname, tmp_fn, filename);
 		if (system(cmd)) {
-			fprintf(stderr, "%s: failed to add .BTF section to '%s': %d!\n",
-				__func__, filename, errno);
+			fprintf(stderr, "%s: failed to add %s section to '%s': %d!\n",
+				__func__, btf_secname, filename, errno);
 			goto unlink;
 		}
 
@@ -1380,12 +1381,26 @@  int btf_encoder__encode(struct btf_encoder *encoder)
 		fprintf(stderr, "%s: btf__dedup failed!\n", __func__);
 		return -1;
 	}
-
-	if (encoder->raw_output)
+	if (encoder->raw_output) {
 		err = btf_encoder__write_raw_file(encoder);
-	else
-		err = btf_encoder__write_elf(encoder);
+	} else {
+		struct btf *btf = encoder->btf, *distilled_base;
 
+		if (encoder->gen_distilled_base) {
+			if (btf__distill_base(encoder->btf, &distilled_base, &btf) < 0) {
+				fprintf(stderr, "could not generate distilled base BTF: %s\n",
+					strerror(errno));
+				return -1;
+			}
+		}
+		err = btf_encoder__write_elf(encoder, btf, BTF_ELF_SEC);
+		if (!err && encoder->gen_distilled_base)
+			err = btf_encoder__write_elf(encoder, distilled_base, BTF_BASE_ELF_SEC);
+		if (btf != encoder->btf) {
+			btf__free((struct btf *)btf__base_btf(btf));
+			btf__free(btf);
+		}
+	}
 	return err;
 }
 
@@ -1659,6 +1674,7 @@  struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filenam
 		encoder->force		 = conf_load->btf_encode_force;
 		encoder->gen_floats	 = conf_load->btf_gen_floats;
 		encoder->skip_encoding_vars = conf_load->skip_encoding_btf_vars;
+		encoder->gen_distilled_base = conf_load->btf_gen_distilled_base;
 		encoder->verbose	 = verbose;
 		encoder->has_index_type  = false;
 		encoder->need_index_type = false;
diff --git a/dwarves.h b/dwarves.h
index dd35a4e..5f5e2b6 100644
--- a/dwarves.h
+++ b/dwarves.h
@@ -94,6 +94,7 @@  struct conf_load {
 	bool			btf_gen_floats;
 	bool			btf_encode_force;
 	bool			reproducible_build;
+	bool			btf_gen_distilled_base;
 	uint8_t			hashtable_bits;
 	uint8_t			max_hashtable_bits;
 	uint16_t		kabi_prefix_len;
diff --git a/man-pages/pahole.1 b/man-pages/pahole.1
index e3c58e0..bfeb0ff 100644
--- a/man-pages/pahole.1
+++ b/man-pages/pahole.1
@@ -316,6 +316,9 @@  Supported non-standard features (not enabled for 'default')
 	reproducible_build Ensure generated BTF is consistent every time;
 	                   without this parallel BTF encoding can result in
 	                   inconsistent BTF ids.
+	distilled_base     For split BTF, generate a distilled version of
+	                   the associated base BTF to support later relocation
+	                   of split BTF with a possibly changed base.
 .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 750b847..79d01dc 100644
--- a/pahole.c
+++ b/pahole.c
@@ -1290,6 +1290,7 @@  struct btf_feature {
 	BTF_DEFAULT_FEATURE(optimized_func, btf_gen_optimized, false),
 	BTF_DEFAULT_FEATURE(consistent_func, skip_encoding_btf_inconsistent_proto, false),
 	BTF_NON_DEFAULT_FEATURE(reproducible_build, reproducible_build, false),
+	BTF_NON_DEFAULT_FEATURE(distilled_base, btf_gen_distilled_base, false),
 };
 
 #define BTF_MAX_FEATURE_STR	1024