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objtool: Allow UNWIND_HINT to suppress dodgy stack modifications

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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: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/20210211173626.918498579@infradead.org
This commit is contained in:
Peter Zijlstra 2021-02-11 13:03:28 +01:00 committed by Ingo Molnar
parent a38fd87484
commit d54dba4199
1 changed files with 9 additions and 6 deletions
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15
tools/objtool/check.c
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@ -1959,8 +1959,9 @@ static void restore_reg(struct cfi_state *cfi, unsigned char reg)
* 41 5d pop %r13 * 41 5d pop %r13
* c3 retq * c3 retq
*/ */
static int update_cfi_state(struct instruction *insn, struct cfi_state *cfi, static int update_cfi_state(struct instruction *insn,
struct stack_op *op) struct instruction *next_insn,
struct cfi_state *cfi, struct stack_op *op)
{ {
struct cfi_reg *cfa = &cfi->cfa; struct cfi_reg *cfa = &cfi->cfa;
struct cfi_reg *regs = cfi->regs; struct cfi_reg *regs = cfi->regs;
@ -2161,7 +2162,7 @@ static int update_cfi_state(struct instruction *insn, struct cfi_state *cfi,
break; 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", WARN_FUNC("unsupported stack register modification",
insn->sec, insn->offset); insn->sec, insn->offset);
return -1; return -1;
@ -2433,13 +2434,15 @@ static int propagate_alt_cfi(struct objtool_file *file, struct instruction *insn
return 0; 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; struct stack_op *op;
list_for_each_entry(op, &insn->stack_ops, list) { 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; return 1;
if (op->dest.type == OP_DEST_PUSHF) { if (op->dest.type == OP_DEST_PUSHF) {
@ -2719,7 +2722,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
return 0; return 0;
} }
if (handle_insn_ops(insn, &state)) if (handle_insn_ops(insn, next_insn, &state))
return 1; return 1;
switch (insn->type) { switch (insn->type) {