Commit d54dba41 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

objtool: Allow UNWIND_HINT to suppress dodgy stack modifications 



rewind_stack_do_exit()
	UNWIND_HINT_FUNC
	/* Prevent any naive code from trying to unwind to our caller. */

	xorl	%ebp, %ebp
	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rax
	leaq	-PTREGS_SIZE(%rax), %rsp
	UNWIND_HINT_REGS

	call	do_exit

Does unspeakable things to the stack, which objtool currently fails to
detect due to a limitation in instruction decoding. This will be
rectified after which the above will result in:

arch/x86/entry/entry_64.o: warning: objtool: .text+0xab: unsupported stack register modification

Allow the UNWIND_HINT on the next instruction to suppress this, it
will overwrite the state anyway.

Suggested-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Acked-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Tested-by: default avatarNick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/20210211173626.918498579@infradead.org
parent a38fd874

tools/objtool/check.c

+9 −6
Original line number Diff line number Diff line
@@ -1959,8 +1959,9 @@ static void restore_reg(struct cfi_state *cfi, unsigned char reg) 
 *   41 5d			pop    %r13

 *   c3				retq

 */

static int update_cfi_state(struct instruction *insn, struct cfi_state *cfi,

			     struct stack_op *op)

static int update_cfi_state(struct instruction *insn,

			    struct instruction *next_insn,

			    struct cfi_state *cfi, struct stack_op *op)

{

	struct cfi_reg *cfa = &cfi->cfa;

	struct cfi_reg *regs = cfi->regs;
@@ -2161,7 +2162,7 @@ static int update_cfi_state(struct instruction *insn, struct cfi_state *cfi, 
				break;

			}



			if (op->dest.reg == cfi->cfa.base) {

			if (op->dest.reg == cfi->cfa.base && !(next_insn && next_insn->hint)) {

				WARN_FUNC("unsupported stack register modification",

					  insn->sec, insn->offset);

				return -1;
@@ -2433,13 +2434,15 @@ static int propagate_alt_cfi(struct objtool_file *file, struct instruction *insn 
	return 0;

}



static int handle_insn_ops(struct instruction *insn, struct insn_state *state)

static int handle_insn_ops(struct instruction *insn,

			   struct instruction *next_insn,

			   struct insn_state *state)

{

	struct stack_op *op;



	list_for_each_entry(op, &insn->stack_ops, list) {



		if (update_cfi_state(insn, &state->cfi, op))

		if (update_cfi_state(insn, next_insn, &state->cfi, op))

			return 1;



		if (op->dest.type == OP_DEST_PUSHF) {
@@ -2719,7 +2722,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func, 
				return 0;

		}



		if (handle_insn_ops(insn, &state))

		if (handle_insn_ops(insn, next_insn, &state))

			return 1;



		switch (insn->type) {