[PATCH v4 01/15] objtool: Add CONFIG_CFI_CLANG support

Sami Tolvanen
2021-09-30T18:05:41Z

With CONFIG_CFI_CLANG, the compiler replaces function references with
references to the CFI jump table, which confuses objtool. This change,
based on Josh's initial patch [1], goes through the list of relocations
and replaces jump table symbols with the actual function symbols.

[1] https://lore.kernel.org/r/d743f4b36e120c06506567a9f87a062ae03da47f.1611263462.git.jpoimboe@redhat.com/

Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
tools/objtool/arch/x86/decode.c | 17 ++++++++++
tools/objtool/elf.c | 51 ++++++++++++++++++++++++++++
tools/objtool/include/objtool/arch.h | 3 ++
tools/objtool/include/objtool/elf.h | 2 +-
4 files changed, 72 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 3172983bf808..9b043220b0af 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -63,6 +63,23 @@ bool arch_callee_saved_reg(unsigned char reg)
}
}

+unsigned long arch_cfi_section_reloc_offset(struct reloc *reloc)
+{
+ if (!reloc->addend)
+ return 0;
+
+ if (reloc->type == R_X86_64_PC32 || reloc->type == R_X86_64_PLT32)
+ return reloc->addend + 4;
+
+ return reloc->addend;
+}
+
+unsigned long arch_cfi_jump_reloc_offset(unsigned long offset)
+{
+ /* offset to the relocation in a jmp instruction */
+ return offset + 1;
+}
+
unsigned long arch_dest_reloc_offset(int addend)
{
return addend + 4;
diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 8676c7598728..05a5f51aad2c 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -18,6 +18,7 @@
#include <errno.h>
#include <objtool/builtin.h>

+#include <objtool/arch.h>
#include <objtool/elf.h>
#include <objtool/warn.h>

@@ -291,6 +292,10 @@ static int read_sections(struct elf *elf)
if (sec->sh.sh_flags & SHF_EXECINSTR)
elf->text_size += sec->len;

+ /* Detect -fsanitize=cfi jump table sections */
+ if (!strncmp(sec->name, ".text..L.cfi.jumptable", 22))
+ sec->cfi_jt = true;
+
list_add_tail(&sec->list, &elf->sections);
elf_hash_add(section, &sec->hash, sec->idx);
elf_hash_add(section_name, &sec->name_hash, str_hash(sec->name));
@@ -576,6 +581,49 @@ static int read_rela_reloc(struct section *sec, int i, struct reloc *reloc, unsi
return 0;
}

+/*
+ * CONFIG_CFI_CLANG replaces function relocations to refer to an intermediate
+ * jump table. Undo the conversion so objtool can make sense of things.
+ */
+static int fix_cfi_relocs(const struct elf *elf)
+{
+ struct section *sec;
+ struct reloc *reloc;
+
+ list_for_each_entry(sec, &elf->sections, list) {
+ list_for_each_entry(reloc, &sec->reloc_list, list) {
+ struct reloc *cfi_reloc;
+ unsigned long offset;
+
+ if (!reloc->sym->sec->cfi_jt)
+ continue;
+
+ if (reloc->sym->type == STT_SECTION)
+ offset = arch_cfi_section_reloc_offset(reloc);
+ else
+ offset = reloc->sym->offset;
+
+ /*
+ * The jump table immediately jumps to the actual function,
+ * so look up the relocation there.
+ */
+ offset = arch_cfi_jump_reloc_offset(offset);
+ cfi_reloc = find_reloc_by_dest(elf, reloc->sym->sec, offset);
+
+ if (!cfi_reloc || !cfi_reloc->sym) {
+ WARN("can't find a CFI jump table relocation at %s+0x%lx",
+ reloc->sym->sec->name, offset);
+ return -1;
+ }
+
+ reloc->sym = cfi_reloc->sym;
+ reloc->addend = 0;
+ }
+ }
+
+ return 0;
+}
+
static int read_relocs(struct elf *elf)
{
struct section *sec;
@@ -639,6 +687,9 @@ static int read_relocs(struct elf *elf)
tot_reloc += nr_reloc;
}

+ if (fix_cfi_relocs(elf))
+ return -1;
+
if (stats) {
printf("max_reloc: %lu\n", max_reloc);
printf("tot_reloc: %lu\n", tot_reloc);
diff --git a/tools/objtool/include/objtool/arch.h b/tools/objtool/include/objtool/arch.h
index 589ff58426ab..93bde8aaf2e3 100644
--- a/tools/objtool/include/objtool/arch.h
+++ b/tools/objtool/include/objtool/arch.h
@@ -81,6 +81,9 @@ unsigned long arch_jump_destination(struct instruction *insn);

unsigned long arch_dest_reloc_offset(int addend);

+unsigned long arch_cfi_section_reloc_offset(struct reloc *reloc);
+unsigned long arch_cfi_jump_reloc_offset(unsigned long offset);
+
const char *arch_nop_insn(int len);
const char *arch_ret_insn(int len);

diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index c3857fadee7a..e8d838217c77 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -39,7 +39,7 @@ struct section {
char *name;
int idx;
unsigned int len;
- bool changed, text, rodata, noinstr;
+ bool changed, text, rodata, noinstr, cfi_jt;
};

struct symbol {
--
2.33.0.800.g4c38ced690-goog