[PATCH v2 dwarves 3/9] btf_encoder: cache all ELF section info

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To handle outputting all variables generally, we'll need to store more
section data. Create a table of ELF sections so we can refer to all the
cached data.

Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---
 btf_encoder.c | 68 +++++++++++++++++++++++++++++++++------------------
 1 file changed, 44 insertions(+), 24 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index c914647..e0e038b 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -38,6 +38,7 @@ struct elf_function {
 };
 
 #define MAX_VAR_CNT 4096
+#define MAX_ELF_SEC_CNT    128
 
 struct var_info {
 	uint64_t    addr;
@@ -45,6 +46,13 @@ struct var_info {
 	uint32_t    sz;
 };
 
+struct elf_secinfo {
+	uint64_t	addr;
+	const char	*name;
+	uint64_t	sz;
+	bool		include;
+};
+
 /*
  * cu: cu being processed.
  */
@@ -64,12 +72,12 @@ struct btf_encoder {
 			  gen_floats,
 			  is_rel;
 	uint32_t	  array_index_id;
+	struct elf_secinfo secinfo[MAX_ELF_SEC_CNT];
+	size_t		  seccnt;
 	struct {
 		struct var_info vars[MAX_VAR_CNT];
 		int		var_cnt;
 		uint32_t	percpu_shndx;
-		uint64_t	percpu_base_addr;
-		uint64_t	percpu_sec_sz;
 	} variables;
 	struct {
 		struct elf_function *entries;
@@ -1216,12 +1224,13 @@ static int btf_encoder__collect_percpu_var(struct btf_encoder *encoder, GElf_Sym
 	if (encoder->verbose)
 		printf("Found per-CPU symbol '%s' at address 0x%" PRIx64 "\n", sym_name, addr);
 
-	/* Make sure addr is section-relative. For kernel modules (which are
-	 * ET_REL files) this is already the case. For vmlinux (which is an
-	 * ET_EXEC file) we need to subtract the section address.
+	/* Make sure addr is absolute, so that we can compare it to DWARF
+	 * absolute addresses. We can compute to section-relative addresses
+	 * when necessary.
 	 */
-	if (!encoder->is_rel)
-		addr -= encoder->variables.percpu_base_addr;
+	if (encoder->is_rel && sym->st_shndx < encoder->seccnt) {
+		addr += encoder->secinfo[sym->st_shndx].addr;
+	}
 
 	if (encoder->variables.var_cnt == MAX_VAR_CNT) {
 		fprintf(stderr, "Reached the limit of per-CPU variables: %d\n",
@@ -1288,6 +1297,7 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder, uint32_
 	uint32_t core_id;
 	struct tag *pos;
 	int err = -1;
+	struct elf_secinfo *pcpu_scn = &encoder->secinfo[encoder->variables.percpu_shndx];
 
 	if (encoder->variables.percpu_shndx == 0 || !encoder->symtab)
 		return 0;
@@ -1315,14 +1325,9 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder, uint32_
 		addr = var->ip.addr;
 		dwarf_name = variable__name(var);
 
-		/* Make sure addr is section-relative. DWARF, unlike ELF,
-		 * always contains virtual symbol addresses, so subtract
-		 * the section address unconditionally.
-		 */
-		if (addr < encoder->variables.percpu_base_addr ||
-		    addr >= encoder->variables.percpu_base_addr + encoder->variables.percpu_sec_sz)
+		/* Make sure addr is in the percpu section */
+		if (addr < pcpu_scn->addr || addr >= pcpu_scn->addr + pcpu_scn->sz)
 			continue;
-		addr -= encoder->variables.percpu_base_addr;
 
 		if (!btf_encoder__percpu_var_exists(encoder, addr, &size, &name))
 			continue; /* not a per-CPU variable */
@@ -1397,7 +1402,7 @@ static int btf_encoder__encode_cu_variables(struct btf_encoder *encoder, uint32_
 		 * add a BTF_VAR_SECINFO in encoder->percpu_secinfo, which will be added into
 		 * encoder->types later when we add BTF_VAR_DATASEC.
 		 */
-		id = btf_encoder__add_var_secinfo(encoder, id, addr, size);
+		id = btf_encoder__add_var_secinfo(encoder, id, addr - pcpu_scn->addr, size);
 		if (id < 0) {
 			fprintf(stderr, "error: failed to encode section info for variable '%s' at addr 0x%" PRIx64 "\n",
 			        name, addr);
@@ -1461,20 +1466,35 @@ struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filenam
 			goto out;
 		}
 
-		/* find percpu section's shndx */
+		/* index the ELF sections for later lookup */
 
 		GElf_Shdr shdr;
-		Elf_Scn *sec = elf_section_by_name(cu->elf, &shdr, PERCPU_SECTION, NULL);
+		size_t shndx;
+		if (elf_getshdrnum(cu->elf, &encoder->seccnt))
+			goto out_delete;
+		if (encoder->seccnt >= MAX_ELF_SEC_CNT) {
+			fprintf(stderr, "%s: reached limit of ELF sections\n", __func__);
+			goto out_delete;
+		}
 
-		if (!sec) {
-			if (encoder->verbose)
-				printf("%s: '%s' doesn't have '%s' section\n", __func__, cu->filename, PERCPU_SECTION);
-		} else {
-			encoder->variables.percpu_shndx     = elf_ndxscn(sec);
-			encoder->variables.percpu_base_addr = shdr.sh_addr;
-			encoder->variables.percpu_sec_sz    = shdr.sh_size;
+		for (shndx = 0; shndx < encoder->seccnt; shndx++) {
+			const char *secname = NULL;
+			Elf_Scn *sec = elf_section_by_idx(cu->elf, &shdr, shndx, &secname);
+			if (!sec)
+				goto out_delete;
+			encoder->secinfo[shndx].addr = shdr.sh_addr;
+			encoder->secinfo[shndx].sz = shdr.sh_size;
+			encoder->secinfo[shndx].name = secname;
+
+			if (strcmp(secname, PERCPU_SECTION) == 0) {
+				encoder->variables.percpu_shndx = shndx;
+				encoder->secinfo[shndx].include = true;
+			}
 		}
 
+		if (!encoder->variables.percpu_shndx && encoder->verbose)
+			printf("%s: '%s' doesn't have '%s' section\n", __func__, cu->filename, PERCPU_SECTION);
+
 		if (btf_encoder__collect_symbols(encoder, !encoder->skip_encoding_vars))
 			goto out_delete;
 
-- 
2.31.1




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