1 files changed, 145 insertions, 0 deletions
diff --git a/tools/objtool/arch/x86/special.c b/tools/objtool/arch/x86/special.c
new file mode 100644
index 000000000..7c97b7391
--- /dev/null
+++ b/tools/objtool/arch/x86/special.c
@@ -0,0 +1,145 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include <string.h>
+
+#include <objtool/special.h>
+#include <objtool/builtin.h>
+
+#define X86_FEATURE_POPCNT (4 * 32 + 23)
+#define X86_FEATURE_SMAP   (9 * 32 + 20)
+
+void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
+{
+	switch (feature) {
+	case X86_FEATURE_SMAP:
+		/*
+		 * If UACCESS validation is enabled; force that alternative;
+		 * otherwise force it the other way.
+		 *
+		 * What we want to avoid is having both the original and the
+		 * alternative code flow at the same time, in that case we can
+		 * find paths that see the STAC but take the NOP instead of
+		 * CLAC and the other way around.
+		 */
+		if (opts.uaccess)
+			alt->skip_orig = true;
+		else
+			alt->skip_alt = true;
+		break;
+	case X86_FEATURE_POPCNT:
+		/*
+		 * It has been requested that we don't validate the !POPCNT
+		 * feature path which is a "very very small percentage of
+		 * machines".
+		 */
+		alt->skip_orig = true;
+		break;
+	default:
+		break;
+	}
+}
+
+bool arch_support_alt_relocation(struct special_alt *special_alt,
+				 struct instruction *insn,
+				 struct reloc *reloc)
+{
+	/*
+	 * The x86 alternatives code adjusts the offsets only when it
+	 * encounters a branch instruction at the very beginning of the
+	 * replacement group.
+	 */
+	return insn->offset == special_alt->new_off &&
+	       (insn->type == INSN_CALL || is_jump(insn));
+}
+
+/*
+ * There are 3 basic jump table patterns:
+ *
+ * 1. jmpq *[rodata addr](,%reg,8)
+ *
+ *    This is the most common case by far.  It jumps to an address in a simple
+ *    jump table which is stored in .rodata.
+ *
+ * 2. jmpq *[rodata addr](%rip)
+ *
+ *    This is caused by a rare GCC quirk, currently only seen in three driver
+ *    functions in the kernel, only with certain obscure non-distro configs.
+ *
+ *    As part of an optimization, GCC makes a copy of an existing switch jump
+ *    table, modifies it, and then hard-codes the jump (albeit with an indirect
+ *    jump) to use a single entry in the table.  The rest of the jump table and
+ *    some of its jump targets remain as dead code.
+ *
+ *    In such a case we can just crudely ignore all unreachable instruction
+ *    warnings for the entire object file.  Ideally we would just ignore them
+ *    for the function, but that would require redesigning the code quite a
+ *    bit.  And honestly that's just not worth doing: unreachable instruction
+ *    warnings are of questionable value anyway, and this is such a rare issue.
+ *
+ * 3. mov [rodata addr],%reg1
+ *    ... some instructions ...
+ *    jmpq *(%reg1,%reg2,8)
+ *
+ *    This is a fairly uncommon pattern which is new for GCC 6.  As of this
+ *    writing, there are 11 occurrences of it in the allmodconfig kernel.
+ *
+ *    As of GCC 7 there are quite a few more of these and the 'in between' code
+ *    is significant. Esp. with KASAN enabled some of the code between the mov
+ *    and jmpq uses .rodata itself, which can confuse things.
+ *
+ *    TODO: Once we have DWARF CFI and smarter instruction decoding logic,
+ *    ensure the same register is used in the mov and jump instructions.
+ *
+ *    NOTE: RETPOLINE made it harder still to decode dynamic jumps.
+ */
+struct reloc *arch_find_switch_table(struct objtool_file *file,
+				    struct instruction *insn)
+{
+	struct reloc  *text_reloc, *rodata_reloc;
+	struct section *table_sec;
+	unsigned long table_offset;
+
+	/* look for a relocation which references .rodata */
+	text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
+					      insn->offset, insn->len);
+	if (!text_reloc || text_reloc->sym->type != STT_SECTION ||
+	    !text_reloc->sym->sec->rodata)
+		return NULL;
+
+	table_offset = text_reloc->addend;
+	table_sec = text_reloc->sym->sec;
+
+	if (text_reloc->type == R_X86_64_PC32)
+		table_offset += 4;
+
+	/*
+	 * Make sure the .rodata address isn't associated with a
+	 * symbol.  GCC jump tables are anonymous data.
+	 *
+	 * Also support C jump tables which are in the same format as
+	 * switch jump tables.  For objtool to recognize them, they
+	 * need to be placed in the C_JUMP_TABLE_SECTION section.  They
+	 * have symbols associated with them.
+	 */
+	if (find_symbol_containing(table_sec, table_offset) &&
+	    strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
+		return NULL;
+
+	/*
+	 * Each table entry has a rela associated with it.  The rela
+	 * should reference text in the same function as the original
+	 * instruction.
+	 */
+	rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset);
+	if (!rodata_reloc)
+		return NULL;
+
+	/*
+	 * Use of RIP-relative switch jumps is quite rare, and
+	 * indicates a rare GCC quirk/bug which can leave dead
+	 * code behind.
+	 */
+	if (text_reloc->type == R_X86_64_PC32)
+		file->ignore_unreachables = true;
+
+	return rodata_reloc;
+}