Commit b7460462 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

objtool: Better handle IRET 



Teach objtool a little more about IRET so that we can avoid using the
SAVE/RESTORE annotation. In particular, make the weird corner case in
insn->restore go away.

The purpose of that corner case is to deal with the fact that
UNWIND_HINT_RESTORE lands on the instruction after IRET, but that
instruction can end up being outside the basic block, consider:

	if (cond)
		sync_core()
	foo();

Then the hint will land on foo(), and we'll encounter the restore
hint without ever having seen the save hint.

By teaching objtool about the arch specific exception frame size, and
assuming that any IRET in an STT_FUNC symbol is an exception frame
sized POP, we can remove the use of save/restore hints for this code.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarMiroslav Benes <mbenes@suse.cz>
Reviewed-by: default avatarAlexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20200416115118.631224674@infradead.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 65ea47dc

arch/x86/include/asm/processor.h

+0 −2
Original line number Diff line number Diff line
@@ -727,7 +727,6 @@ static inline void sync_core(void) 
	unsigned int tmp;



	asm volatile (

		UNWIND_HINT_SAVE

		"mov %%ss, %0\n\t"

		"pushq %q0\n\t"

		"pushq %%rsp\n\t"
@@ -737,7 +736,6 @@ static inline void sync_core(void) 
		"pushq %q0\n\t"

		"pushq $1f\n\t"

		"iretq\n\t"

		UNWIND_HINT_RESTORE

		"1:"

		: "=&r" (tmp), ASM_CALL_CONSTRAINT : : "cc", "memory");

#endif

tools/objtool/arch.h

+1 −0
Original line number Diff line number Diff line
@@ -19,6 +19,7 @@ enum insn_type { 
	INSN_CALL,

	INSN_CALL_DYNAMIC,

	INSN_RETURN,

	INSN_EXCEPTION_RETURN,

	INSN_CONTEXT_SWITCH,

	INSN_STACK,

	INSN_BUG,

tools/objtool/arch/x86/decode.c

+12 −2
Original line number Diff line number Diff line
@@ -446,9 +446,19 @@ int arch_decode_instruction(struct elf *elf, struct section *sec, 
		*type = INSN_RETURN;

		break;



	case 0xcf: /* iret */

		*type = INSN_EXCEPTION_RETURN;



		/* add $40, %rsp */

		op->src.type = OP_SRC_ADD;

		op->src.reg = CFI_SP;

		op->src.offset = 5*8;

		op->dest.type = OP_DEST_REG;

		op->dest.reg = CFI_SP;

		break;



	case 0xca: /* retf */

	case 0xcb: /* retf */

	case 0xcf: /* iret */

		*type = INSN_CONTEXT_SWITCH;

		break;
@@ -494,7 +504,7 @@ int arch_decode_instruction(struct elf *elf, struct section *sec, 


	*immediate = insn.immediate.nbytes ? insn.immediate.value : 0;



	if (*type == INSN_STACK)

	if (*type == INSN_STACK || *type == INSN_EXCEPTION_RETURN)

		list_add_tail(&op->list, ops_list);

	else

		free(op);

tools/objtool/check.c

+16 −13
Original line number Diff line number Diff line
@@ -2065,15 +2065,14 @@ static int validate_return(struct symbol *func, struct instruction *insn, struct 
 * tools/objtool/Documentation/stack-validation.txt.

 */

static int validate_branch(struct objtool_file *file, struct symbol *func,

			   struct instruction *first, struct insn_state state)

			   struct instruction *insn, struct insn_state state)

{

	struct alternative *alt;

	struct instruction *insn, *next_insn;

	struct instruction *next_insn;

	struct section *sec;

	u8 visited;

	int ret;



	insn = first;

	sec = insn->sec;



	if (insn->alt_group && list_empty(&insn->alts)) {
@@ -2126,16 +2125,6 @@ static int validate_branch(struct objtool_file *file, struct symbol *func, 
				}



				if (!save_insn->visited) {

					/*

					 * Oops, no state to copy yet.

					 * Hopefully we can reach this

					 * instruction from another branch

					 * after the save insn has been

					 * visited.

					 */

					if (insn == first)

						return 0;



					WARN_FUNC("objtool isn't smart enough to handle this CFI save/restore combo",

						  sec, insn->offset);

					return 1;
@@ -2228,6 +2217,20 @@ static int validate_branch(struct objtool_file *file, struct symbol *func, 


			break;



		case INSN_EXCEPTION_RETURN:

			if (handle_insn_ops(insn, &state))

				return 1;



			/*

			 * This handles x86's sync_core() case, where we use an

			 * IRET to self. All 'normal' IRET instructions are in

			 * STT_NOTYPE entry symbols.

			 */

			if (func)

				break;



			return 0;



		case INSN_CONTEXT_SWITCH:

			if (func && (!next_insn || !next_insn->hint)) {

				WARN_FUNC("unsupported instruction in callable function",

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