[PATCH v3 13/21] objtool: Reuse string references

[Josh Poimboeuf]

For duplicate strings, elf_add_string() just blindly adds duplicates.

That can be a problem for arm64 which often uses two consecutive
instructions (and corresponding relocations) to put an address into a
register, like:

  d8:   90000001        adrp    x1, 0 <meminfo_proc_show>       d8: R_AARCH64_ADR_PREL_PG_HI21  .rodata.meminfo_proc_show.str1.8
  dc:   91000021        add     x1, x1, #0x0    dc: R_AARCH64_ADD_ABS_LO12_NC   .rodata.meminfo_proc_show.str1.8

Referencing two different addresses in the ADRP+ADD pair would corrupt
the memory access.  Avoid that by detecting and reusing duplicates when
cloning string relocs.

Signed-off-by: Josh Poimboeuf <jpoimboe at kernel.org>
---
 tools/objtool/elf.c                 | 29 +++++++++++++++++++++++------
 tools/objtool/include/objtool/elf.h |  3 ++-
 tools/objtool/klp-diff.c            |  4 +++-
 3 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index e09bb0a63be35..065ccfeb98288 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -1366,9 +1366,27 @@ struct elf *elf_create_file(GElf_Ehdr *ehdr, const char *name)
 	return elf;
 }
 
-unsigned int elf_add_string(struct elf *elf, struct section *strtab, const char *str)
+int elf_find_string(struct elf *elf, struct section *strtab, const char *str)
 {
-	unsigned int offset;
+	char *d_buf;
+	int i;
+
+	if (!strtab->data)
+		return -1;
+
+	d_buf = strtab->data->d_buf;
+
+	for (i = 0; i < strtab->data->d_size; i += strlen(d_buf + i) + 1) {
+		if (!strcmp(d_buf + i, str))
+			return i;
+	}
+
+	return -1;
+}
+
+int elf_add_string(struct elf *elf, struct section *strtab, const char *str)
+{
+	void *data;
 
 	if (!strtab)
 		strtab = find_section_by_name(elf, ".strtab");
@@ -1382,12 +1400,11 @@ unsigned int elf_add_string(struct elf *elf, struct section *strtab, const char
 		return -1;
 	}
 
-	offset = ALIGN(sec_size(strtab), strtab->sh.sh_addralign);
-
-	if (!elf_add_data(elf, strtab, str, strlen(str) + 1))
+	data = elf_add_data(elf, strtab, str, strlen(str) + 1);
+	if (!data)
 		return -1;
 
-	return offset;
+	return data - strtab->data->d_buf;
 }
 
 void *elf_add_data(struct elf *elf, struct section *sec, const void *data, size_t size)
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index 0801fcad516bb..d895023674673 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -187,7 +187,8 @@ struct symbol *elf_create_section_symbol(struct elf *elf, struct section *sec);
 void *elf_add_data(struct elf *elf, struct section *sec, const void *data,
 		   size_t size);
 
-unsigned int elf_add_string(struct elf *elf, struct section *strtab, const char *str);
+int elf_find_string(struct elf *elf, struct section *strtab, const char *str);
+int elf_add_string(struct elf *elf, struct section *strtab, const char *str);
 
 struct reloc *elf_create_reloc(struct elf *elf, struct section *sec,
 			       unsigned long offset, struct symbol *sym,
diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index 6a1cec57dc6a3..6957292e455e4 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -1509,7 +1509,9 @@ static int clone_reloc(struct elfs *e, struct reloc *patched_reloc,
 
 		__dbg_clone("\"%s\"", escape_str(str));
 
-		addend = elf_add_string(e->out, out_sym->sec, str);
+		addend = elf_find_string(e->out, out_sym->sec, str);
+		if (addend == -1)
+			addend = elf_add_string(e->out, out_sym->sec, str);
 		if (addend == -1)
 			return -1;
 	}
-- 
2.53.0