linux-kernel - [RFC][PATCH 3/5] objtool: Add support for relocations without addends

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Message-ID: <17ee3f6f2a246008aaae70f92df24ae92fa0e21e.1588888003.git.mhelsley@vmware.com>
Date:   Mon, 11 May 2020 10:35:11 -0700
From:   Matt Helsley <mhelsley@...are.com>
To:     <linux-kernel@...r.kernel.org>
CC:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Julien Thierry <jthierry@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Steven Rostedt <rostedt@...dmis.org>,
        Matt Helsley <mhelsley@...are.com>
Subject: [RFC][PATCH 3/5] objtool: Add support for relocations without addends

Currently objtool only collects information about relocations with
addends. In recordmcount, which we are about to merge into objtool,
some supported architectures do not use rela relocations. Since
object files use one or the other the list can be reused.

Signed-off-by: Matt Helsley <mhelsley@...are.com>
---
 tools/objtool/elf.c | 55 ++++++++++++++++++++++++++++++++++++---------
 tools/objtool/elf.h |  5 ++++-
 2 files changed, 49 insertions(+), 11 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index c4857fa3f1d1..cd841e3df87d 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -465,12 +465,14 @@ static int read_relas(struct elf *elf)
 	unsigned long nr_rela, max_rela = 0, tot_rela = 0;
 
 	list_for_each_entry(sec, &elf->sections, list) {
-		if (sec->sh.sh_type != SHT_RELA)
+		if ((sec->sh.sh_type != SHT_RELA) &&
+		     (sec->sh.sh_type != SHT_REL))
 			continue;
 
-		sec->base = find_section_by_name(elf, sec->name + 5);
+		sec->base = find_section_by_name(elf, sec->name +
+				((sec->sh.sh_type != SHT_REL) ? 5 : 4));
 		if (!sec->base) {
-			WARN("can't find base section for rela section %s",
+			WARN("can't find base section for relocation section %s",
 			     sec->name);
 			return -1;
 		}
@@ -486,13 +488,26 @@ static int read_relas(struct elf *elf)
 			}
 			memset(rela, 0, sizeof(*rela));
 
-			if (!gelf_getrela(sec->data, i, &rela->rela)) {
-				WARN_ELF("gelf_getrela");
-				return -1;
+			switch(sec->sh.sh_type) {
+			case SHT_REL:
+				if (!gelf_getrel(sec->data, i, &rela->rel)) {
+					WARN_ELF("gelf_getrel");
+					return -1;
+				}
+				rela->addend = 0;
+				break;
+			case SHT_RELA:
+				if (!gelf_getrela(sec->data, i, &rela->rela)) {
+					WARN_ELF("gelf_getrela");
+					return -1;
+				}
+				rela->addend = rela->rela.r_addend;
+				break;
+			default:
+				break;
 			}
 
 			rela->type = GELF_R_TYPE(rela->rela.r_info);
-			rela->addend = rela->rela.r_addend;
 			rela->offset = rela->rela.r_offset;
 			symndx = GELF_R_SYM(rela->rela.r_info);
 			rela->sym = find_symbol_by_index(elf, symndx);
@@ -717,17 +732,27 @@ int elf_rebuild_rela_section(struct section *sec)
 	struct rela *rela;
 	int nr, idx = 0, size;
 	GElf_Rela *relas;
+	GElf_Rel *rels;
 
 	nr = 0;
 	list_for_each_entry(rela, &sec->rela_list, list)
 		nr++;
 
+	/*
+	 * Allocate a buffer for relocations with addends but also use
+	 * it for other relocations too. The section type determines
+	 * the size of the section, the buffer used, and the entries.
+	 */
 	size = nr * sizeof(*relas);
 	relas = malloc(size);
 	if (!relas) {
 		perror("malloc");
 		return -1;
 	}
+	rels = (void *)relas;
+	if (sec->sh.sh_type == SHT_REL) {
+		size = nr * sizeof(*rels);
+	}
 
 	sec->data->d_buf = relas;
 	sec->data->d_size = size;
@@ -736,9 +761,19 @@ int elf_rebuild_rela_section(struct section *sec)
 
 	idx = 0;
 	list_for_each_entry(rela, &sec->rela_list, list) {
-		relas[idx].r_offset = rela->offset;
-		relas[idx].r_addend = rela->addend;
-		relas[idx].r_info = GELF_R_INFO(rela->sym->idx, rela->type);
+		switch(sec->sh.sh_type) {
+		case SHT_REL:
+			rels[idx].r_offset = rela->offset;
+			rels[idx].r_info = GELF_R_INFO(rela->sym->idx, rela->type);
+			break;
+		case SHT_RELA:
+			relas[idx].r_addend = rela->addend;
+			relas[idx].r_offset = rela->offset;
+			relas[idx].r_info = GELF_R_INFO(rela->sym->idx, rela->type);
+			break;
+		default:
+			break;
+		}
 		idx++;
 	}
 
diff --git a/tools/objtool/elf.h b/tools/objtool/elf.h
index 0b79c2353a21..71bd47055963 100644
--- a/tools/objtool/elf.h
+++ b/tools/objtool/elf.h
@@ -61,7 +61,10 @@ struct symbol {
 struct rela {
 	struct list_head list;
 	struct hlist_node hash;
-	GElf_Rela rela;
+	union {
+		GElf_Rela rela;
+		GElf_Rel  rel;
+	};
 	struct section *sec;
 	struct symbol *sym;
 	unsigned int type;
-- 
2.20.1