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-rw-r--r--tools/objtool/arch/x86/special.c145
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..151b13d0a
--- /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 "../../special.h"
+#include "../../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 (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;
+}