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Date:   Mon, 23 Aug 2021 10:13:05 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     x86@...nel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH v2 01/14] objtool: Add CONFIG_CFI_CLANG support

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@...il.com>
Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
---
 tools/objtool/arch/x86/decode.c      | 16 +++++++++
 tools/objtool/elf.c                  | 51 ++++++++++++++++++++++++++++
 tools/objtool/include/objtool/arch.h |  3 ++
 tools/objtool/include/objtool/elf.h  |  2 +-
 4 files changed, 71 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index bc821056aba9..318189c8065e 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -62,6 +62,22 @@ 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)
+{
+	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 062bb6e9b865..2205b2b08268 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);
 
 int arch_decode_hint_reg(struct instruction *insn, u8 sp_reg);
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index e34395047530..d9c1dacc6572 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.rc2.250.ged5fa647cd-goog

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