[PATCH 6/7] objtool: Provide elf_write_{insn,reloc}()

Peter Zijlstra
2020-06-18T14:51:11Z

This provides infrastructure to rewrite instructions; this is
immediately useful for helping out with KCOV-vs-noinstr, but will
also come in handy for a bunch of variable sized jump-label patches
that are still on ice.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
tools/objtool/elf.c | 50 +++++++++++++++++++++++++++++++++++++++++++++++++-
tools/objtool/elf.h | 7 ++++++-
2 files changed, 55 insertions(+), 2 deletions(-)

--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -114,6 +114,53 @@ static int symbol_to_offset(struct rb_no
return 0;
}

+int elf_write_insn(struct elf *elf, struct section *sec,
+ unsigned long offset, unsigned int len,
+ const char *insn)
+{
+ Elf_Data *data = sec->data;
+
+ if (data->d_type != ELF_T_BYTE || data->d_off) {
+ WARN("write to unexpected data for section: %s", sec->name);
+ return -1;
+ }
+
+ memcpy(data->d_buf + offset, insn, len);
+ elf_flagdata(data, ELF_C_SET, ELF_F_DIRTY);
+
+ elf->changed = true;
+
+ return 0;
+}
+
+int elf_write_reloc(struct elf *elf, struct reloc *reloc)
+{
+ struct section *sec = reloc->sec;
+
+ if (sec->sh.sh_type == SHT_REL) {
+ reloc->rel.r_info = GELF_R_INFO(reloc->sym->idx, reloc->type);
+ reloc->rel.r_offset = reloc->offset;
+
+ if (!gelf_update_rel(sec->data, reloc->idx, &reloc->rel)) {
+ WARN_ELF("gelf_update_rel");
+ return -1;
+ }
+ } else {
+ reloc->rela.r_info = GELF_R_INFO(reloc->sym->idx, reloc->type);
+ reloc->rela.r_addend = reloc->addend;
+ reloc->rela.r_offset = reloc->offset;
+
+ if (!gelf_update_rela(sec->data, reloc->idx, &reloc->rela)) {
+ WARN_ELF("gelf_update_rela");
+ return -1;
+ }
+ }
+
+ elf->changed = true;
+
+ return 0;
+}
+
static int symbol_by_offset(const void *key, const struct rb_node *node)
{
const struct symbol *s = rb_entry(node, struct symbol, node);
@@ -563,8 +610,9 @@ static int read_relocs(struct elf *elf)
break;
default: return -1;
}
- reloc->sym = find_symbol_by_index(elf, symndx);
reloc->sec = sec;
+ reloc->idx = i;
+ reloc->sym = find_symbol_by_index(elf, symndx);
if (!reloc->sym) {
WARN("can't find reloc entry symbol %d for %s",
symndx, sec->name);
--- a/tools/objtool/elf.h
+++ b/tools/objtool/elf.h
@@ -67,9 +67,10 @@ struct reloc {
};
struct section *sec;
struct symbol *sym;
- unsigned int type;
unsigned long offset;
+ unsigned int type;
int addend;
+ int idx;
bool jump_table_start;
};

@@ -122,6 +123,10 @@ struct elf *elf_open_read(const char *na
struct section *elf_create_section(struct elf *elf, const char *name, size_t entsize, int nr);
struct section *elf_create_reloc_section(struct elf *elf, struct section *base, int reltype);
void elf_add_reloc(struct elf *elf, struct reloc *reloc);
+int elf_write_insn(struct elf *elf, struct section *sec,
+ unsigned long offset, unsigned int len,
+ const char *insn);
+int elf_write_reloc(struct elf *elf, struct reloc *reloc);
int elf_write(struct elf *elf);
void elf_close(struct elf *elf);