[PATCH 02/15] objtool: Add CONFIG_CFI_CLANG support

From: Sami Tolvanen
Date: Fri Apr 16 2021 - 16:38:56 EST


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@xxxxxxxxxx/

Reported-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
Suggested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
---
tools/objtool/elf.c | 48 +++++++++++++++++++++++++++++
tools/objtool/include/objtool/elf.h | 2 +-
2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index d08f5f3670f8..5cf2c61ce7b8 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -273,6 +273,10 @@ static int read_sections(struct elf *elf)
}
sec->len = sec->sh.sh_size;

+ /* 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(elf->section_hash, &sec->hash, sec->idx);
elf_hash_add(elf->section_name_hash, &sec->name_hash, str_hash(sec->name));
@@ -548,6 +552,48 @@ static int read_rela_reloc(struct section *sec, int i, struct reloc *reloc, unsi
return 0;
}

+static int fix_cfi_relocs(const struct elf *elf)
+{
+ struct section *sec, *tmpsec;
+ struct reloc *reloc, *tmpreloc;
+
+ list_for_each_entry_safe(sec, tmpsec, &elf->sections, list) {
+ list_for_each_entry_safe(reloc, tmpreloc, &sec->reloc_list, list) {
+ struct symbol *sym;
+ struct reloc *func_reloc;
+
+ /*
+ * CONFIG_CFI_CLANG replaces function relocations to refer
+ * to an intermediate jump table. Undo the conversion so
+ * objtool can make sense of things.
+ */
+ if (!reloc->sym->sec->cfi_jt)
+ continue;
+
+ if (reloc->sym->type == STT_SECTION)
+ sym = find_func_by_offset(reloc->sym->sec,
+ reloc->addend);
+ else
+ sym = reloc->sym;
+
+ if (!sym || !sym->sec)
+ continue;
+
+ /*
+ * The jump table immediately jumps to the actual function,
+ * so look up the relocation there.
+ */
+ func_reloc = find_reloc_by_dest_range(elf, sym->sec, sym->offset, 5);
+ if (!func_reloc || !func_reloc->sym)
+ continue;
+
+ reloc->sym = func_reloc->sym;
+ }
+ }
+
+ return 0;
+}
+
static int read_relocs(struct elf *elf)
{
struct section *sec;
@@ -608,6 +654,8 @@ static int read_relocs(struct elf *elf)
tot_reloc += nr_reloc;
}

+ fix_cfi_relocs(elf);
+
if (stats) {
printf("max_reloc: %lu\n", max_reloc);
printf("tot_reloc: %lu\n", tot_reloc);
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index 45e5ede363b0..ef19578fc5e4 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 {
--
2.31.1.368.gbe11c130af-goog