linux-kernel - [RFC][PATCH 2/3] objtool: Provide elf_write

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Message-ID: <20200612143554.012293047@infradead.org>
Date:   Fri, 12 Jun 2020 16:30:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, dvyukov@...gle.com,
        elver@...gle.com, andreyknvl@...gle.com, mark.rutland@....com,
        mhelsley@...are.com, rostedt@...dmis.org, jthierry@...hat.com,
        mbenes@...e.cz, peterz@...radead.org
Subject: [RFC][PATCH 2/3] objtool: Provide elf_write_{insn,reloc}()

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@...radead.org>
---
 tools/objtool/elf.c |   52 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 tools/objtool/elf.h |    7 ++++++-
 2 files changed, 57 insertions(+), 2 deletions(-)

--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -562,8 +562,9 @@ static int read_relocs(struct elf *elf)
 					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);
@@ -848,6 +849,55 @@ static int elf_rebuild_rela_section(stru
 
 	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);
+
+	sec->changed = true;
+	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;
+		}
+	}
+
+	sec->changed = true;
+	elf->changed = true;
+
+	return 0;
+}
 
 int elf_rebuild_reloc_section(struct section *sec)
 {
--- 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);