objtool: Update section header before relocations

The libelf implementation from elftoolchain has a safety check in
gelf_update_rel[a] to check that the data corresponds to a section
that has type SHT_REL[A] [0]. If the relocation is updated before
the section header is updated with the proper type, this check
fails.

To fix this, update the section header first, before the relocations.
Previously, the section size was calculated in elf_rebuild_reloc_section
by counting the number of entries in the reloc_list. However, we
now need the size during elf_write so instead keep a running total
and add to it for every new relocation.

[0] https://sourceforge.net/p/elftoolchain/mailman/elftoolchain-developers/thread/CAGw6cBtkZro-8wZMD2ULkwJ39J+tHtTtAWXufMjnd3cQ7XG54g@mail.gmail.com/Signed-off-by: Michael Forney <mforney@mforney.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20210509000103.11008-2-mforney@mforney.org

tools/objtool/elf.c

@@ -508,6 +508,7 @@ int elf_add_reloc(struct elf *elf, struct section *sec, unsigned long offset,
 	list_add_tail(&reloc->list, &sec->reloc->reloc_list);
 	elf_hash_add(reloc, &reloc->hash, reloc_hash(reloc));
 
+	sec->reloc->sh.sh_size += sec->reloc->sh.sh_entsize;
 	sec->reloc->changed = true;
 
 	return 0;
@@ -977,26 +978,23 @@ static struct section *elf_create_reloc_section(struct elf *elf,
 	}
 }
 
-static int elf_rebuild_rel_reloc_section(struct section *sec, int nr)
+static int elf_rebuild_rel_reloc_section(struct section *sec)
 {
 	struct reloc *reloc;
-	int idx = 0, size;
+	int idx = 0;
 	void *buf;
 
 	/* Allocate a buffer for relocations */
-	size = nr * sizeof(GElf_Rel);
-	buf = malloc(size);
+	buf = malloc(sec->sh.sh_size);
 	if (!buf) {
 		perror("malloc");
 		return -1;
 	}
 
 	sec->data->d_buf = buf;
-	sec->data->d_size = size;
+	sec->data->d_size = sec->sh.sh_size;
 	sec->data->d_type = ELF_T_REL;
 
-	sec->sh.sh_size = size;
-
 	idx = 0;
 	list_for_each_entry(reloc, &sec->reloc_list, list) {
 		reloc->rel.r_offset = reloc->offset;
@@ -1011,26 +1009,23 @@ static int elf_rebuild_rel_reloc_section(struct section *sec, int nr)
 	return 0;
 }
 
-static int elf_rebuild_rela_reloc_section(struct section *sec, int nr)
+static int elf_rebuild_rela_reloc_section(struct section *sec)
 {
 	struct reloc *reloc;
-	int idx = 0, size;
+	int idx = 0;
 	void *buf;
 
 	/* Allocate a buffer for relocations with addends */
-	size = nr * sizeof(GElf_Rela);
-	buf = malloc(size);
+	buf = malloc(sec->sh.sh_size);
 	if (!buf) {
 		perror("malloc");
 		return -1;
 	}
 
 	sec->data->d_buf = buf;
-	sec->data->d_size = size;
+	sec->data->d_size = sec->sh.sh_size;
 	sec->data->d_type = ELF_T_RELA;
 
-	sec->sh.sh_size = size;
-
 	idx = 0;
 	list_for_each_entry(reloc, &sec->reloc_list, list) {
 		reloc->rela.r_offset = reloc->offset;
@@ -1048,16 +1043,9 @@ static int elf_rebuild_rela_reloc_section(struct section *sec, int nr)
 
 static int elf_rebuild_reloc_section(struct elf *elf, struct section *sec)
 {
-	struct reloc *reloc;
-	int nr;
-
-	nr = 0;
-	list_for_each_entry(reloc, &sec->reloc_list, list)
-		nr++;
-
 	switch (sec->sh.sh_type) {
-	case SHT_REL:  return elf_rebuild_rel_reloc_section(sec, nr);
-	case SHT_RELA: return elf_rebuild_rela_reloc_section(sec, nr);
+	case SHT_REL:  return elf_rebuild_rel_reloc_section(sec);
+	case SHT_RELA: return elf_rebuild_rela_reloc_section(sec);
 	default:       return -1;
 	}
 }
@@ -1117,12 +1105,6 @@ int elf_write(struct elf *elf)
 	/* Update changed relocation sections and section headers: */
 	list_for_each_entry(sec, &elf->sections, list) {
 		if (sec->changed) {
-			if (sec->base &&
-			    elf_rebuild_reloc_section(elf, sec)) {
-				WARN("elf_rebuild_reloc_section");
-				return -1;
-			}
-
 			s = elf_getscn(elf->elf, sec->idx);
 			if (!s) {
 				WARN_ELF("elf_getscn");
@@ -1133,6 +1115,12 @@ int elf_write(struct elf *elf)
 				return -1;
 			}
 
+			if (sec->base &&
+			    elf_rebuild_reloc_section(elf, sec)) {
+				WARN("elf_rebuild_reloc_section");
+				return -1;
+			}
+
 			sec->changed = false;
 			elf->changed = true;
 		}